updated inflight calibration of hayabusa2’s optical ...1 department of earth and planetary...
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Updated Inflight Calibration of Hayabusa2’s Optical Navigation Camera (ONC) forScientificObservationsduringtheCruisePhaseEriTatsumi1
ToruKouyama2
HidehikoSuzuki3
ManabuYamada4
NaoyaSakatani5
ShingoKameda6
YasuhiroYokota5,7
RieHonda7
TomokatsuMorota8
KeiichiMoroi6
NaoyaTanabe1
HiroakiKamiyoshihara1
MarikaIshida6
KazuoYoshioka9
HiroyukiSato5
ChikatoshiHonda10
MasahikoHayakawa5
KoheiKitazato10
HirotakaSawada5
SeijiSugita1,11
1DepartmentofEarthandPlanetaryScience,TheUniversityofTokyo,Tokyo,Japan
2NationalInstituteofAdvancedIndustrialScienceandTechnology,Ibaraki,Japan
3MeijiUniversity,Kanagawa,Japan
4PlanetaryExplorationResearchCenter,ChibaInstituteofTechnology,Chiba,Japan
5InstituteofSpaceandAstronauticalScience,JapanAerospaceExplorationAgency,Kanagawa,Japan
6RikkyoUniversity,Tokyo,Japan
7KochiUniversity,Kochi,Japan
8NagoyaUniversity,Aichi,Japan
9DepartmentofComplexityScienceandEngineering,TheUniversityofTokyo,Chiba,Japan
10TheUniversityofAizu,Fukushima,Japan
11ResearchCenteroftheEarlyUniverse,TheUniversityofTokyo,Tokyo,Japan
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Abstract
TheOpticalNavigationCamera(ONC-T,ONC-W1,ONC-W2)onboardHayabusa2arealsobeingusedforscientific
observationsofthemissiontarget,C-complexasteroid162173Ryugu.Scienceobservationsandanalysesrequire
rigorous instrument calibration. In order to meet this requirement, we have conducted extensive inflight
observationsduringthe3.5yearsofcruiseafterthelaunchofHayabusa2on3December2014.Inadditiontothe
firstinflightcalibrationsbySuzukietal.(2018),weconductedanadditionalseriesofcalibrations,includingread-
outsmear,electronic-interferencenoise,bias,darkcurrent,hotpixels,sensitivity,linearity,flat-field,andstraylight
measurementsfortheONC.Moreover,thecalibrations,especiallyflat-fieldsandsensitivities,ofONC-W1and-W2
areupdatedfortheanalysisofthelow-altitude(i.e.,high-resolution)observations,suchasthegravitymeasurement,
touchdowns,andthedescentsforMASCOTandMINERVA-IIpayloadreleases.TheradiometriccalibrationforONC-
TisalsoupdatedinthisstudybasedonstarandMoonobservations.Ourupdatedinflightsensitivitymeasurements
suggesttheaccuracyoftheabsoluteradiometriccalibrationcontainslessthan1.8%errorfortheul-,b-,v-,Na-,w-,
andx-bandsbasedonstarcalibrationobservationsand~5%forthep-bandbasedonlunarcalibrationobservations.
TheradiancespectraoftheMoon,Jupiter,andSaturnfromtheONC-Tshowgoodagreementwiththespacecraft-
basedobservationsoftheMoonfromSP/SELENEandWAC/LROCandwithground-basedtelescopicobservations
forJupiterandSaturn.Ourcalibrationresultssuggestthatthe0.7-µmabsorptionbandtypicallyobservedonChand
Cgh asteroids at the ~3-4% level can be detected with the ONC’s signal-to-noise ratio (SNR) of ~2. We also
demonstrateadecreaseinSNRduetoCCDtemperatureincreasescausedbyradiantheatwhenthespacecraftis
closetothesurface,astheSNRismeasuredtobe150ataCCDtemperatureof20˚C(theworstcasescenario).Since
Ryugumaypossessasignificantamountofinternalvolatiles,asodiumatmospherearoundRyuguisconsideredto
behighlyplausible.WeevaluatedtheupperlimitofdetectabilityofasodiumatmospherearoundJupiterusingthe
Na-filteras100Rwith100images.ThisimpliesthattheONC-Tcandetectasodiumatmosphereofseveral10skR
basedonasingleimagesetofv-andNa-bandsandofseveral100sRbasedon100imagesets.Finally,wereportthe
first inflight observation of Ryugu by ONC-T from 1.3×106 km away on 26 February 2018. The ONC-T v-band
observationdisplaysconsistencywithground-basedobservation,whichconfirmsthecapabilityofONC-T.
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1.Introduction
TheHayabusa2spacecraftwaslaunchedbytheJapanAerospaceeXplorationAgency(JAXA)inDecember
2014andarrivedatitstarget,theC-complexasteroid162173Ryugu(formerly1999JU3)in27June2018.Its
mission is to rendezvouswithRyuguand to return samples from theasteroid’ssurface. Inadditionto the
samplingsystem,Hayabusa2alsocarriesremote-sensinginstruments,suchastheOpticalNavigationCamera
(ONC), the 3µm Near-Infrared Spectrometer (NIRS3), the Thermal Infrared Imager (TIR), and the Light
Detection andRanging laser (LIDAR). Before sampling, theasteroid surfacewill bemappedpreciselyand
accurately with the remote-sensing instrument payload, for example 0.5-2 m/pix for the whole asteroid
surfacebyONC-T,andcharacterizedfrombothamineralogicalandmorphologicalpointofview.Thispaper
describestheinflightcalibrationsofthecharge-coupledevice(CCD)forthetelescopiccamera(ONC-T)andthe
twowide-anglecameras(ONC-W1andONC-W2)onboardtheHayabusa2spacecraft.ONC’sdesignsareshown
inTable 1.1. Their exposure time settings of three cameras are different becauseONC-T engages optical
navigationsbyobserving starsandRyuguwith longexposure time,ONC-W1and-W2aremainlyused for
observingduringcloseencountertotheRyugusurfacewithshortexposuretime.Especially,ONC-W1which
needs to capture target markers with flash light, are set to have shortest exposure times. More detailed
specificationssuchasaperformanceofCCDandhardwareweightandsizeareshowninAppendixA.
Anumberofmissionsareplannedtovisitspectrallydifferenttypesofasteroids,suchasHayabusa2toa
C-typeasteroid,OSIRIS-RExtoaB-typeasteroid(Laurettaetal.,2011),PsychetoaM-typeasteroid(Elkins-
Tanton, 2018), andLucy toa set of Jupiter Trojanasteroids (Levisonet al., 2017). The comparison of the
absolute reflectance from the remote sensing observations across these missions is important for
understanding the difference between the various target asteroids, including differences in composition,
aqueous alteration, the degree of space weathering, and thermal metamorphism. Furthermore, the
compositional constraints and connections to meteorite analogs are important for understanding the
propertiesofprimitivematerialsintheearlySolarSystem.Absolutereflectancespectra,sometimesreferred
toasspectralalbedo,isoneofthemostpowerfulindicesforthemineralogicalclassification(e.g.,Johnsonand
Fanale,1973;Gaffy,1976)ofsolarsystemobjects.Also,reflectancevariationsacrossanobject’ssurfacecan
beusedtodeterminelevelsofaqueousalteration(Fornasieretal.,2014),thermalmetamorphism(Hiroietal.,
1996),grain-sizevariation(Cloutisetal.,2013),andspaceweathering(e.g.,Matsuokaetal.,2015;Lantzetal.,
2017).
Another important feature of theHayabusa2mission is theMobile Asteroid Surface Scout (MASCOT)
lander,whichhasan instrumentpayloadwithoverlappingwavelengthcoveragewiththemainspacecraft’s
payload, both of which will be observing the asteroid’s surface (e.g., Ho et al., 2017) at different spatial
resolutions. The counterpart of theONC is theMASCOTCamera (MasCam),which hasa 1024x1024 pixel
complementarymetaloxidesemiconductor(CMOS)imagesensorsensitiveinthe400-1000nmwavelength
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rangewithfourcolorLEDs.InordertocompareandconnectthespectrafromMasCamandONC-T,arobust
spectralradiometriccalibrationisnecessary.
Themainobjectiveofthisstudyistodescribetheinflightcalibrationofthreecameras,ONC-T,ONC-W1,
and ONC-W2, during the 3.5 year cruise phase from December 2014 to May 2017, and to describe the
calibrationpipelinewhichwill begeneratingproducts for JAXA’sDataArchives andTransmission System
(DARTS; https://www.darts.isas.jaxa.jp/planet/project/hayabusa2/) and NASA’s Planetary Data System
(PDS).TheresultsofpreflightcalibrationsofONC-Tandtheresultsof inflightgeometriccalibrationswere
described indetailbyKamedaet al. (2017)andSuzuki et al. (2018), respectively.However,differences in
temperature and environment between the laboratory and inflight calibration measurements requires a
validation and update to thepreflight radiometric calibration. Thus,we conduct re-calibration of theCCD
sensitivity based on inflight star observations obtained at the reference temperature (~-30℃ ). We also
measured the sensitivity temperature dependence based on inflight Jupiter observations under varying
hardware temperatures.Furthermore,we summarize the sensitivities ofONC-W1and -W2 based on both
preflightandinflightmeasurements.Thegoalofthispaperistoprovidereliablesensitivitymeasurementsfor
theONCsystemandevaluateitsuseinmineralogicalmapping.
Firstwewilldescribe the calibration flowofONC images inSec.2, andeachdetailedanalysis in the
calibrationprocessesisshowninSec.3forONC-TandSec.5forONC-W1and-W2.Detectabilityofsodium
atmospherewillbediscussedinSec.4.ONCsystemalignmentwiththespacecraftandNIRS3willbeanalyzed
inSec.6and7,respectively.Finally,applicationsinscientificanalyseswillbediscussedinSec.8,includingthe
reportofthefirstinflightobservationofRyuguinSec.8.4.
Table1.1.DesignsofONC.
ONC-T ONC-W1 ONC-W2
Effectivelensaperture 15.1mm 1.08mm 1.08mm
Focallength 120.50±0.01mmfor
wide-filter*(Suzukiet
al.,2018)
10.22mm 10.38mm
Fieldofview 6.27˚(Nadirview) 69.71˚(Nadirview) 68.89˚(Slanted~30˚
fromnadir)
Fnumber(F#) 9.05 9.6 9.6
Colorfilters 7colorbandpassfilters
(ul:0.40µm,b:0.48µm,
v:0.55µm,Na:0.59µm,
w:0.70µm,x:0.86µm,
Clearfilter Clearfilter
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p:0.95µm)and1clear
filter(wide).
Exposuretime 5.44ms,8.20ms,10.9
ms,16.4ms,21.8ms,
32.8ms,43.5ms,65.6
ms,87.0ms,131ms,
174ms,262ms,348ms,
525ms,696ms,1.05s,
1.39s,2.10s,2.79s,
4.20s,5.57s,8.40s,
11.1s,16.8s,22.3s,
33.6s,44.6s,67.2s,
89.1s,134s,178s,0s
(forsmear)
170µs,256µs,340µs,
513µm,680µs,1.03ms,
1.36ms,2.05ms,2.72
ms,4.1ms,5.44ms,8.20
ms,10.9ms,16.4ms,
21.8ms,32.8ms,43.5
ms,65.6ms,87.0ms,
131ms,174ms,262ms,
348ms,525ms,696ms,
1.05s,1.39s,2.10s,
2.79s,4.20s,5.57s,0s
(forsmear)
1.36ms,2.05ms,2.72
ms,4.1ms,5.44ms,8.20
ms,10.9ms,16.4ms,
21.8ms,32.8ms,43.5
ms,65.6ms,87.0ms,
131ms,174ms,262ms,
348ms,525ms,696ms,
1.05s,1.39s,2.10s,
2.79s,4.20s,5.57s,
8.40s,11.1s,16.8s,
22.3s,33.6s,44.6s,0s
(forsmear)
*Focallengthsforv-andNa-filtersofONC-TareinAppendix.A.
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2.ImageCalibrationFlow
Raw imagesacquiredby theONCsystemneedtobeprocessed ina sequenceof stepsto scientifically
calibratetheimagedata.Inthissection,wesummarizethecalibrationflowoftheONCimagesfromrawdata
(Level0)tohigher-level,radiometricallyandgeometricallycalibrateddata(Level2).Figure2.1displaysthe
datacalibrationflowchartfortheONCandshowstheproductlevelstobearchivedinbothDARTSandPDS
(L0:rawdata,L1:rawdatawithaheader,L2a:spacecraft informationaddeddata,L2b: flat-fieldcorrected
digitalunitdata,L2c:radiance,L2d:radiancefactor(I/F),L2e:reflectancefactorat(incidenceangle,emission
angle,phaseangle)=(30˚,0˚,30˚)).
Thedigitalnumberforeachpixelinarawimageisgivenasa8-,10-,or12-bitbinarynumber.Forhigher-
levelproducts,weusetheFITS(FlexibleImageTransportSystem)formattedfiles.AFITSformattedfileforthe
ONCsystemincludesaprimaryheaderdataunitandanimageextension.Headersincludehardwarestatus,
navigation,andotherancillaryinformation,suchasspacecraftpositionandattitudecalculatedfromtheSPICE
kernels. As described inFig. 2.1, the calibration flow requires inputparameters for each step, and those
parametersare included intheheaders.Thebiasandsmearsubtraction isconducted inflightduringhome
position (at alt. ~20 km) observations. However, for images acquired during the cruise phase and the
proximityoperationsduringtherendezvousphase,thebiasandsmearhastobecorrectedongroundthrough
thepipelineprocess.OnemorethingtonoteisthatsomestepsforONC-Thavenotyetbeenimplementedin
thecalibrationpipeline(grayboxesinFig.2.1).Darkcurrentandstraylightwillbenegligibleinmostasteroid
observations,exceptforsomeproximityobservations,suchasobservationsduringtheverycloseapproachto
thesurfaceorscanobservations.Whendelicateanalyses,suchasphotometricorspectralanalysesclosetothe
asteroid limbare conducted, point-spread-function (PSF) correctionsneed to be incorporated. However,
becausethesamePSFcorrectionmethodmaynotalwaysbeapplicable,dependingonwhereintheFOVthe
targetfallsorifitfillsoronlypartiallyfillstheFOV,wedonotapplythisstepforfirstpublicproductrelease.
Nevertheless,the innerpartof thetarget’sdiskwillbe littleaffectedbythePSF; thevaluewithoutthePSF
correctionissufficientlyaccurate.TheeffectofthePSFwillbediscussedmoreinSec.3.7.1.Thisprocesscould
beimplementedinafutureupdatetothepipelineprocess.
Each calibration step is described in detail in the following sections. Calibration campaigns were
conductedprefightandinflightduringthecruisephase.TheONC-Tincludes7colorbandpassfiltersandone
“wide”clearfilter,whereastheONC-W1andONC-W2includeasingle“wide”clearfilter.Wecarefullytestand
validatethespectralreconstructionoftheONC-Tusingmultiplespectrallywell-knownstarsandplanets,since
theabsolutesensitivityiscriticalforthespectralcharacterizationoftheRyugu’ssurface.
FileNamingRules
ONCimagingdata,Level2a–2dareproducedinFITSfileformat.ThefilenamingconventionsforONCimages
are
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“hyb2_onc_(Date)_(Time)_t(Filter)(i)_(Level).fit”forONC-T,
and
“hyb2_onc_(Date)_(Time)_(Cam)(i)_(Level).fit”forONC-W1and-W2,
where(Date)indicatestheobservationdatein8digitnumber,YYYYMMDD,(Time)indicatestheobservation
timein6digitnumber,hhmmss,(Filter)isthebandnameoffilter(u:ul-filter,b:b-filter,v:v-filter,n:Na-filter,
w:w-filter,x:x-filter,p:p-filter,andi:wide-filter),(i)is“f”for1024x1024pixelsfeardataand“b”for32x1024
pixels dark-reference data (Optical Black) (see Fig. 5 in Kameda et al. (2017)), (Cam) is “w1” or “w2”
correspondingtotheinstrumentsONC-W1andW2respectively,and(Level)indicatestheproductlevels.Note
thatthehigher-levelproductsmaybeprovideddifferentnamingrulesandformats.
Figure2.1.ONCdata calibration flowchart, starting fromraw images tohigher-levelproducts.Calibration
processes input parameters are acquired from the FITS file headers. Note that gray boxes have not been
implemented inthecurrentversionof thepipeline,butareexpected ina futureupdate.Products levels in
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bracketsareintermediateproductsandnotgoingtobearchived.SomeL2aproductshavebeensmearandbias
correctedonboard.(n:filter,t:exposuretime,TAE:electronicspackageofONCsystem(AE)temperature,TCCD:
CCDtemperatureofONC-T,TELE:theelectriccircuittemperatureofONC-T,(XPNL,YPNL):spacecraftattitude,Ssun:
solarirradiance)
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3.RadiometriccalibrationofONC-T
Forradiometriccalibration,imagedigitalnumber(DN)isconvertedtophysicalunitsofradiance(Wm-
2µm-1sr-1).Theradiometriccalibrationequationhastheform:
" =$%&(()*+,(-.*/01223,5,1678,5,196;,(/<=*)(()*+),(?*)@0A,1223,5;),(/B
C5,D∗FD(1223,5)∗A [Wm-2µm-1sr-1],
(n=ul,b,v,Na,w,x,p)
(3.1)
whereF[Wm-2µm-1sr-1]istheradiancefromthesurface, GHIJ [DN]isarawimage, GKLMN0OPPQ,1, OR$R,1; [DN]
isthebiaslevel, GSIHT0U, OPPQ,1; [DN]isthedarkcurrent, GVWXIH(GHIJ) [DN]isthesmearimage, GVY [DN]is
thestraylightlevel.L-1(I)isthelinearitycorrectionfunction,Z1,[ istheflat-fieldimageofONC-T, \[0OPPQ,1;
[(DN/s)/(Wm-2µm-1sr-1)]isthesensitivity,t[s]isexposuretime, OPPQ,1 [℃] istheCCDtemperatureofONC-
T, OR$R,1 [℃] istheelectriccircuit(ELE)temperatureoftheONC-T,O_R,1 [℃] istheAEtemperatureofthe
ONC system, and n indicates filter name. Each term is described in detailed in following subsections.
Calibrationsconductedso far forboth inflightandpreflightaresummarized inTable3.1.Notethatall the
calibrationimagesduringthecruisephasewereobtainedin12-bitformat,buttherendezvousphasedatawill
be12-bit,10-bit,or8-bitformatsfordifferentpurposes,suchas10-bitimagesforspectroscopicobservations
and8-bitimagesforgeometricobservations.Thebitdepthinformationisincludedintheheaderofeachimage.
InKamedaetal.(2017)theeffectivewavelengthswerecalculatedusingonlythetransmissionofband-
passfilters.Toobtainamoreprecisemeasurement,werecalculatetheeffectivewavelengthsusingthesystem
efficiencyΦ[(a) (Fig.3.1),includingthetransmittanceoftheband-passfilters,theneutraldensity(ND)filter,
lenses,CCDcoverglasses,thequantumefficiencyoftheCCD,andthespectrumofthelightsource.Theeffective
wavelength for each filter, using the solar spectrum (ASTM E-490) and the white light as a light source,
respectively,aretabulatedinTable3.2.Table3.2alsoshowsthebandwidth,theeffectivebandwidth,andthe
effectivesolarirradiancethrougheachfilter.ThebandwidthisdefinedinthesamewayasKamedaetal.(2017),
buthereweusedthesystemefficienciesinsteadofthetransmissionsthemselves.Theeffectivewavelengthis
thusdefinedas
aXbb,[ =∫de(d)fD(d)gd
∫e(d)fD(d)gd , (3.2)
where the system efficiency Φ[(a) = h[(a)hij(a)hk(a)hPl(a)m0a, OPPQ,1; ; h[(a) , hnQ(a) , h$(a) , and
hPl(a) are the transmissionsof theband-pass filters, theND filter, the lenses,and theCCDcoverglasses,
respectively, m0a, OPPQ,1; is the quantum efficiency, and o(a) is the light source spectrum. The effective
bandwidth ΔaXbb isdefinedas
ΔaXbb,[ ∙ Φ[0aXbb,[; = ∫Φ[(a)ra . (3.3)
Hereafterweusetheeffectivewavelengthwithrespecttothesolarspectrum.Notethatthequantumefficiency
usedhereisextrapolatedto-30℃ ofCCDtemperatureusingthetemperaturedependencefunctionprovided
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byitsmanufacturer,whichisdefinedforthetemperaturerangefrom-20℃ to40 ℃ andnormalizedat20
℃. Thequantumefficiencyat20℃ andother componentsof the systemefficiencyare shown indetail in
AppendixB.Exceptforsomeproximityoperations,theONCwillbeoperatedataCCDtemperaturearound-
30℃(thereferencetemperature).
Figure3.1.SystemefficiencyofONC-Tbandpassphotometricsystem.
Table3.1.Summaryofdatausedforcalibrationduringthecruisephase.
Preflightmeasurement Inflightmeasurement InflightObservationDate
Bias 0-sec exposures (Kameda
etal.,2017)
0-secexposures Continuouslyduringthecruisephase
EMI N/A 0-secexposures Continuouslyduringthecruisephase
Darkcurrentand
Hotpixels
Thermal vacuum test
(variationofCCD,ELE,and
AEtemperatures)
Deepskyobservationwith
variation of CCD and ELE
temperatures
17April2017,23May2017,25May2017
Sensitivity Integrating sphere
(Kameda et al., 2017),
temperature dependence
provided the
manufacturer (Suzuki et
al.,2018)
ESO standard and
Alekseeva (1996) stars.
Temperature dependence
byJupiterwithvariationof
CCDtemperature.
The sensitivities derived
fromstarsaretestedbythe
Moon, Mars, Jupiter, and
Saturn.
(Stars)19October2016,23May2017,10
October2017,12October2017
(Moon)5December2015
(Mars)24May2016,31May2016,
7June,2016
(Jupiter)16to18May2017
(Saturn)3November2017
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Sensitivity Time-
dependence
N/A Healthcheck(FFlamp) 11 December 2014, 16 April 2015, 10
September2015,8July2017,16October
2017,5December2017
Linearity Integratingsphere FF lampwith variation of
exposuretime
2December2017
Flat-Field Integrating spheres
(Kameda et al., 2017;
Suzukietal.,2018)
Star observation with
variation of spacecraft
attitude
12to14October2017
Geometric
distortion
N/A Stars(Suzukietal.,2018) 11December2017
Alignment to the
S/C
N/A Stars 11December2017
SharpPSFs Pinhole (Kameda et al.,
2017)
Stars(Suzukietal.,2018) 11December2017
BroadPSFs Smallextendedsources EarthandMars 4December2015,25May2016
Straylights N/A Radiator stray lights with
variationof thespacecraft
attitude.GhostwithMoon
andEarth.
11December2014,23June2015,12 to
17October 2015, 9November 2015, 14
November 2015, 22 December 2015, 9
February to 17 March 2016, 21 March
2016,28June to23July2016,13 to20
June2017,17to21October2017
Table3.2.CharacteristicsoftheONC-Tphotometricsystem.
ul b v Na w x p
asbbt) (nm) 397.5 479.8 548.9 589.9 700.1 857.3 945.1
aXbbu) (nm) 395.2 480.0 549.0 590.0 700.3 857.7 945.7
aXbb (nm)
Kamedaetal.(2017)390.4 479.8 549.0 590.2 700.4 858.9 949.7
Effectivebandwidth
ΔaXbb (nm)36.0 26.6 30.6 11.8 29.2 41.7 56.0
Bandwidth(nm) 34.7 26.6 27.9 11.6 28.8 42.4 57.2
Bandwidth(nm)
Kamedaetal.(2017)45 25 28 10 28 42 57
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Effective solar
irradiance(W/m2/um)1343.7 1969.1 1859.7 1788.0 1414.4 985.8 834.9
1)withrespecttosolarspectrum.2)withrespecttowhitelight.
3.1.Read-outsmearremoval
ONCisnotequippedwithamechanicalshutter, instead,eachexposure isdividedwithanelectronical
mechanism.Becauseof themechanicalshutter-lessdesignof theONC, theCCD is irradiatedduring frame
transferalong the vertical direction. So that, even 0-sec exposure images actually include read-out smear
duringthisveryshortexposuretime(<10ms).FortheglobalobservationsatthehomepositiontheONC-T
willtake0-secexposuresandproperexposuresalternately,andtheread-outsmearwillberemovedonboard
bysubtractingthe0-secexposuresfromtheproperexposures.However,duringthetouchdownsequencesthe
spatialcoverageofthefield-of-view(FOV)changesrapidlyovertheshorttimedurationoftheseoperations,
and appropriate 0-sec exposureswith the same spatial coverageas theproperexposure image cannot be
acquired.Inthisoperationalcase,theread-outsmearremovalprocesswillbeconductedaftertheimageshave
beendownlinkedusingthemethoddescribedbyIshiguroetal.(2010):
GNvsMw(x) =Ay25Ay25zA
∑ (G − GKLMN − GgMw})/�Äny,tÅÇÉ . (3.4)
The exposure duration during vertical charge transfer UÄP1 is 7.2µs × 1024 = 7.373ms for ONC-W2,
whichhasbeenderivedfromtheEarthobservationimagesacquiredduringtheEarth-Moonflyby.Becausethe
samemanufacturedCCDsareequippedonONC-T,-W1and-W2,thisverticaltransfertimesimilarbetweenthe
threecameras.Forexample,Fig.3.2showsanoriginalimageandthesmear-removedimagesofanONC-W2
(hyb2_onc_20151203_084458_w2f_l2a.fit)setofobservations.Theresidualof thesmear-removed image is
lessthan1%oftheintensityoftheEarth(~900DN),for95%ofpixels.ForthecaseofRyugu,theeffectofthe
smearwillbelessthanintheEarthobservationscaseduetothelongerexposuresneededforRyuguduetoits
correspondinglowerbrightness.ThisbiasremovalmethodworkssufficientlyfortheONC-W2image.Wewill
obtaintheverticaltransfertimesforONC-TandW1usingRyuguimagesduringtherendezvousphase.
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Figure3.2.An example of smear removal of anONC-W2 image (hyb2_onc_20151203_084458_w2f_l2a.fit)
usingEq.(3.4),showingtheread-outsmearwaswellremoved<1%intensityoftheobject.
3.2.BiasCurrentCorrection
TheONCsystemhasanelectronicbiascurrenttooffsetitszerolevel.Thetemperaturedependenceofthe
preflightbias levelwasassessedbyKamedaetal. (2017).Here,wealsoverify thebias levelusing inflight
observations.Weanalyzedimagesacquiredwithazero-lengthexposureandtargetedtoskyinordertoavoid
strong smear. As Kameda et al. (2017) reported, bias level changes depending on the CCD temperature
(OPPQ,1[℃])andtheAEtemperature(O_R,1[℃]).BiasleveldependenciesontheCCDandtheELEtemperatures
arealsoobserved inthe inflight imagedata(Fig.3.3). Itshouldbenotedthatdueto inflexibility intheAE
control,weinsteadchangedthetemperatureofELEthatresidesjustbehindtheCCD.Duringthepreflighttest,
theAEandtheELEtemperatureswerecontrolledsimultaneouslytoalmostsametemperature.Thus,because
theresultsofthepre-flighttestincludetheeffectofboththeELEandAEtemperatures,wecannotdirectly
compare the preflight and inflight testmeasurements. We plotted the preflight AE and ELE temperature
dependencetestwiththeinflightELEtemperaturedependencetestresultstogetherinFig.3.3.Thebiaslevel
is well described by fitting the inflight data with an empirical linear combination of the CCD and ELE
temperaturesas
GKLMN(OPPQ,1, OR$R,1) = 320.66 + 0.652OPPQ,1 − 0.953OR$R,1[ì�]. (3.5)
Figure3.4displayshowwelltheempiricalfunctiondescribesthebiasleveloftheinflightdataacquiredfrom
launchuntilDecember2017.Figure3.5showsthetime-dependentchangeofthebiaslevel,whichsuggests
anincreaseinthebiaslevelduringthefirsthalf-yearafterlaunch.Althoughtheempiricalrelationshipderived
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fromtheinflightimagedatadoesnotmatchthepre-flightexperiments,thisisascribedtotheeffectoftheAE
temperaturevariationinthepreflightdata.Thebiaslevelhasstayednearlyconstant,within1%change,inthe
3yearsofcruise.Moreover,theeffectoftheAEtemperaturecanbeestimatedfromthepreflightmeasurements
ofKamedaetal.(2017).AftercorrectingfortheeffectsofCCDandELEtemperatures,theremainingeffectis
ascribedtovariationsduetoAEtemperaturedifferences.Theempiricalcorrectionfactor î(O_R) fortheAE
temperature,derivedfromthepreflightmeasurements,isgivenby
î(O_R) = 0.987 − 0.00251O_R , for-30℃< O_R < 59℃ (3.6)
WhentheAEtemperaturechanges, thebias levelcanbeestimatedby GKLMN(OPPQ,1, OR$R,1)î(O_R).Notethat
thisrelationshipbasedonpreflightmeasurementsmayincludeerrorsof>4%,asthebiaslevelappearstohave
increasedby4%justafterthelaunch.
Figure3.3.Thebiaslevelofinflight(dots)andpreflight(triangles)0-secexposureimagesasafunctionofCCD
andELEtemperatures.TheAEtemperaturedeferencecontributesthebiaslevelsignificantly.
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Figure3.4. Bias level of thepreflight data (Kamedaet al., 2017) and the inflight data comparedwith the
empiricalfunction GKLMN(OPPQ,1, OR$R,1).
Figure3.5.BiaslevelvariationofONC-T.Redlinesindicatethelaunchdate.(a)Theobservedbiaslevelof0-
secexposuresduringthecruisephase.(b)ComparisonsoftheobservedbiasandthebiasmodelEq.(3.5).The
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biasmodelwellpredictstheinflightdatawithin1%changeoverthe3yearsofcruise.
3.3.DarkCurrentandHotPixelsCorrection
The CCD constantly responses to thermal electrons evenwhen the detector is not irradiated, which
producesadarkcurrentthatshouldbesubtractedfromobservedimagestoaccuratelyreproducereflectance
levels.However,becausetheONCdoesnothaveaphysicalshutter,wecannotdirectlyassessthedarkcurrent
atthetimeofeachobservation.Skyobservations,however,canprovideameasureofthesignalsfromthedark
current,hotpixels,EMI,stars,andstraylight.AsSuzukietal.(2018)reported,theradiatorstraylighthasa
gradualstructureoveranimage.Thestraylightcannotbecompletelyseparatedfromthedarkcurrent,which
alsohasa contributiontothe intensityoveran image.Thus,weobtainedupper limitsof thedark current
(<0.09DN/s)contributionatthereferencetemperature, OPPQ,1 = −30℃,basedonthepartsofimageswhere
thestray lightcontamination isminimized.Thisvalue iscomparabletothepreflightgroundmeasurement
(<0.05DN/sat OPPQ,1 = −30℃)byKamedaetal.(2017).Furthermore,thedarkcurrentlevelvariationasa
functionoftemperaturecanbemeasuredusingskyobservations,assumingthatthegradualstructureinthe
imagesareduetoradiatorstraylightasmeasuredatthereferencetemperature.Toobtaindarkcurrentlevels
athigherCCDtemperatures(-10℃,10℃,20℃,and25℃),weobtainedskyimageswithashortexposure
time(1.05s)foralltemperatureconditionsandlongexposuretimesof33.6sat-10℃,5.57sat10℃,and2.1
sat20℃and25℃.Wetook3imagesforeachexposurecondition,andremovedtheeffectsofcosmicraysby
takingamedianofthese3images.Afterthecosmicraycorrection,allimageswerestraylightcorrectedbased
on images taken at OPPQ,1 = −30℃, where the stray light contributionwas extracted using a fifth-order
polynomialregressionandneglectingoutlierswhicharecausedbystarsandhotpixels.Weobtainedthedark
currentdistributionexceptforstar-containingregionswithintheimageframe.Figure3.6(a)isahistogram
ofdarkcurrentlevelsfordifferentCCDtemperaturesintheONC-T.Theaveragesandthestandarddeviations
forthedistributionsarealsoshownintheFig.3.6(b).Thedarkcurrentgrowsexponentiallydependingon
theCCDtemperature,whichisconsistentwiththedarkcurrentbeingcausedbythermalnoise.Theempirical
relationshipoftheCCDtemperatureandthedarkcurrentlevelisgivenby
GgMw} = Uóòô(0.10OPPQ,1 + 0.52) [DN]. (3.7)
Carefullymonitoringthedarkimagesat-30℃,weobservedsomepixelsexhibitanomalyhighresponselevels
(hot pixels).We defined hot pixels asapixelwitha response larger than 30DN/s, or 0.3% the expected
intensityoftheasteroid.Hotpixelpositionson14October2017arelistedinTable3.3fortheCCDtemperature
conditioniscold(-30℃).ThenumberofhotpixelsincreasesasthefunctionoftheCCDtemperature(Fig.3.7).
Thisisespeciallyimportantduringthetouchdownsequences,astheCCDtemperaturecouldincreaseashigh
as20℃.Thus,theabsolutesignalatthehotpixellocationsshouldbeanalyzedcarefully.Notethat,however,
thenumberofhotpixelscanbeincreasedwithtimeduetocosmicrayirradiation.Thetemporalvariationin
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hotpixelsisshowninFig.3.8,indicatingthatthenumberofhotpixelsgraduallyincreases.Thus,weneedto
repeatthisanalysisduringtheapproachandrendezvousphasestomonitorthechangeinhotpixels.
Table3.3.Hotpixellistat OPPQ,1 = −30℃.HandVarethehorizontalandverticalpixelcoordinateinanimage.
H V Signallevel[DN/s]
24 653 47.0
52 276 89.3
296 939 55.8
407 239 37.1
505 29 31.4
582 204 75.1
Figure3.6.DarkcurrentleveldependenceontheCCDtemperatureofONC-T.(a)Darkcurrentdistribution.
The deviations of dark currentget largeras theCCD temperature increases. (b)Dark current level grows
exponentiallywithCCDtemperature,indicatingdarkcurrentlevelsarewelldescribedbythermalnoise.
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Figure3.7.TheamountofhotpixelsintheONCFOV.TheCCDtemperaturecouldbeashighas20℃ during
thetouchdowns.
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Figure3.8.Increasingnumberofhotpixels(sensitivepixels)inthebox{[492,118];[532,138]}.
3.4.CharacterizationofElectro-MagneticInterference
Figure3.9 shows thepresenceofelectro-magnetic interference (EMI)patternsembedded intheCCD
noise(stripepatterns).ThisiscausedbyinterferencewiththeactivecircuitsnearbytheONCsystemduring
thetransferoftheelectronsgeneratedattheCCDtothedigitalelectronics.Thus,theintensityoftheEMIis
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independent of the exposure time. The EMIpattern changeswith time depending onwhich systemswith
specificfrequencieswereactiveatthattimeofexposure.0-secexposureimageswereanalyzedline-by-line
basedonFourierFastTransforms (FFTs) toextract theperiodicpatterns.Figure3.10 (a) showsapower
spectrumof3imagestakenatdifferenttimes(16April2015,24May2016,16October2017),whichindicates
strongerpeaksforsomespecificfrequencies.WeevaluatedtheEMIfrominvertedFFTsoftheextracteddata
forfrequencies>0.001Hzandamplitudes>0.3DN.Figure3.10(b)showstheEMIstructurevariationduring
thecruisephase.AftertheEMIisextractedfromtheimages,theamplitudeoftheEMIisevaluatedasahalfof
thegapbetweenthemaximumandminimumsignalsintheline(Fig.3.11).ThissuggeststheEMIintensities
havenotdrasticallychangedduringthecruisephaseandthehighestEMIintensitywas~5DN,whichis<0.2%
oftheexpectedasteroiddiskintensity(~2500DN).However,wecontinuetomonitortheEMIcontributionto
accuratelyremovetheEMIfromimagesrequiredforsensitiveandadvancedspectralanalyses.
Figure3.9.Anexample imageof theEMIpattern(hyb2_onc_20171016_00056_tvf_l2a.fit).Thepresenceof
periodicalstripepatternsovertheimagewasobserved.
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Figure 3.10. (a) The results of FFT analysis of 3 images (hyb2_onc_20150416_074730_tvf_l2a.fit,
hyb2_onc_20160524_124508_tvf_l2a.fit, hyb2_onc_20171016_000056_tvf_l2a.fit). There are several strong
peakspossiblyassociatedwithothersystemcircuits.(b)TheextractedEMIsignalshavedifferentstructures
atdifferentobservations.Offsetsare5DNfromeachother.Samplingrateis3MHz.
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Figure3.11.VariationintheamplitudeoftheEMIwavesduringthecruisephase.Theamplitudeisdefinedas
ahalfofthegapbetweenmaximumandminimumsignalsofEMIwaves.Theredlineindicatesthelaunchdate.
3.5.Linearity
Inthissection,wereportonthelinearityoftheONC-TCCDresponsebasedoninflightobservationsof
Flat-Field(FF)lamp.TheCCDresponsetothenumberofphotonsshouldideallybelinear.Preflighttesting(Fig.
3.12) shows that the responseof theONC-TCCDwas linearupto3200DN,withinanerrorof0.6%.This
responsewasmeasuredusingan integratingsphereandtaking imagesoveravarietyofexposuretimesat
roomtemperature.ThelinearitywasmeasuredagaininflightbyobservingtheFFlampthroughthev-band
filteratexposuretimes0.131,0.348,0.525and0.696s.Wetook3shotsevery2secondsforeachexposure
time tomeasure the output stability of FF lamp.We alsomeasured the stray lightbefore and after these
observationsbyturningofftheFFlampinordertoexaminestraylighteffectsonthistest.Afterbeingcorrected
forbias levelsandradiatorstray light, thebright{[384,384] ; [415,415]}andthedark{[384,480] ; [415,
511]}boxes,showninFig.3.13,ineachimagewereaveraged.Inflight-testresultsareshowninFig.3.14.The
DNaccumulationrateissensitivetosmalldeparturesinlinearity.ThoughtheDNcountsupto3100DNappear
linearwithexposuretime(Fig3.14),smallchangesintheaccumulationrate(shownbythevariationsalong
thehorizontallinesinFig.3.14)indicatesmalldeviationsintheCCDlinearityresponse.Thistestshowsthat
stronglinearity(lessthan1%deviation)wasachievedupto3100DN,whereasa10-13%dropinlinearityis
seenatsignallevelsof3600DN.Thus,exposuretimesshouldbeselectedcarefullyinordernottoexceedthis
quantitative limitof3100DN.Thedecreaseof accumulation rate canbe fittedbya third-orderpolynomial
function,providing,theempiricalrelationshipbetweentheobservedintensityIobsandtheidealintensityI0of
GöKN = 1.0073GÉ − 2.9285 × 10,õGÉu − 3.6434 × 10,tÉGÉ
ú. (Iobs<3100DN) (3.8)
Usingthisrelationship,thelinearitycanbecorrectedwithin0.1%error(redsymbolsinFig.3.14).
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Figure3.12.Preflightgroundlinearitytestshowingthelinearrelationshipupto3200DN.(left)Theaveraged
intensityfromtheintegratingsphereimageswithdifferentexposuretimesisshownbyopencircles.Thesolid
lineshowslinearrelationshipof24.52DN/s.(right)Thesignalaccumulationrateovereachtimedurationis
shownbycrosses.Thehorizontallineindicatestheidealaccumulationrateof24.52DN/swithperfectlinearity.
Thedeviationfromtheidealaccumulationratewaswithin0.6%.
Figure3.13.Theredsquaresshowtheregionsofinterestintheinflightlinearitytest.Twodifferentintensity
regions,thebrightanddarkareas,oftheFFlampimagesaretested.
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Figure3.14. Inflight linearitytestdisplayingthe linearrelationshipupto3100DN.(left)Opencirclesand
trianglesareaveragedintensityinthebrightanddarkboxesoftheFFlampimageswithdifferentexposure
times,respectively.(right)Crossesandplussesrepresenttheaccumulationrateovereachtimedurationinthe
brightanddarkboxes.Bluesaretheobservedvaluesandredsarethelinearitycorrectedvalues.Beforethe
linearitycorrection,bothareasshowsimilartrendswithdecayinaccumulationrateforthelongerexposure
times.Thehorizontallinesindicatetheidealaccumulationrates6.05DN/msforbrightareaand5.82DN/ms
fordarkareawithperfectlinearity.Thedeviationsfromtheidealaccumulationratesare<1%forbeforeand
<0.1%forafterthelinearitycorrection.
3.6.CorrectionofScatteredandReflectedLightinOptics
3.6.1.ScatteredLight(BroadPSFs)
Thepresenceofscatteredlightaroundobjects(dimhalo)wasobserved(Fig.3.15)inallONCfilterbands.
Thatis,theskyclosetotheimagedobjectwasbrighterthanaccountableforfromthebiasanddarkcurrent
levelsduetothescatteringof light fromadjacentpixelscontainingtheobject image.This iscausedbythe
multiple scatteringofphotonswithin theoptical systembeforedetectionby theCCD.Thiseffectwasalso
observedinAMICA(AsteroidMultibandImagingCamera)imagesonboardHayabusa(Ishiguro,2014).This
scattered light affect has severe effects on the spectral analysis of regions near the object limb since the
scatteringbehaviorhasawavelengthdependency,whichisreportedanddescribedindetailbyIshiguro(2014)
fortheAMICAinstrument.TheyreportedthatfortheAMICAp-bandtheintensitychangedbymorethan10%
closetoobjectlimbinimagesacquiredattheHomePosition(altitudeof~7.5km).Thus,thescatteredlight
correction is especially critical for photometric and spectral analyses. Because the light is distributed
isotopicallyabouttheobject,wecanevaluatethescatteredlighteffectasapartofpointspreadfunction(PSF).
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However, to distinguish this scatteringeffect from the sharp peak of a PSF,which is usuallymeasured by
observingaroundstarsasinfinitelysmallpointlightsources,werefertothiswidespreadattenuationfunction
as the “broadPSF”.The sharppeakPSFhavebeenalreadyobtainedaspartof thepreflight calibrationby
Kamedaetal.(2017)andinflightcalibrationbySuzukietal.(2018).Therefore,hereweinvestigatethebroad
PSFbasedonthemethoddescribedinIshiguro(2014)andwillshowanapplicationtoanEarthimageobtained
duringtheEarth-Moonswing-by.
Methodology:
ThetotalPSF ZùFû isasummationofthesharppeakPSF, ZN ,andthebroadPSF, ZK:
ZùFû = ZN + ZK . (3.9)
Inprinciple,wedon’tobtainimageswithoutaPSFbutwhatweobtainareblurredimages.
GöKN = GÉ ∗ ZùFû = GÉ ∗ (ZN + ZK) (3.10)
where GÉ istheimagewithoutblurand GöKN istheobservedimage.Consideringwehavetostartfromblurred
images GöKN ,whatwedetermineisthefunction ZK′,soastosatisfythefollowingequationintheskyregion
wherecountsfrom ZN arenegligible:
GöKN ∗ Z†K = GÉ ∗ ZK (3.11)
ThenwecanevaluateblurredcountsduetoaPSFwith GöKN and ZK′. AsisdescribedinIshiguro(2014),we
alsoassumedthatthe ZK′ canbeexpressedasthesummationofGaussianfunctions:
Z′K(°) =¢£L
√2•¶Lexp ™−
°u
2¶Lu´
n
LÇt
(3.12)
where ° [pix]isthedistancefromapointsource, £L and ¶L areconstants(N=1to6),where ¶L = 2Lzu(� ≤
4),¶≠ = 110,and ¶õ = 710.Theeffectofscatteredlightcanthenbecorrectedas
GÉ ∗ ZN = GöKN − GöKN ∗ Z′K (3.13)
Assumingthatthe ZN issharpenoughtosatisfy ZN = ÆNØ(0,0),where ÆN isthecontributionduetothesharp
peakPSF,and
GÉ = (GöKN − GöKN ∗ Z†K)/ÆN where (3.14)
ÆN = ∫2•°ZNr°.
PreflightdiskobjectimageswereobtainedthroughallfilterswithdifferentsizeddisksplacedintheFOV(Fig.
3.16).We determined the £L Gaussian coefficient values sequentially, usinga bisectionmethod from the
broader components (beginningwith £õ ) to narrower component (endingwith £t ).We performed this
analysistoderivethecoefficients £L foreachfiltertoreducetheresidualsignalinskyregionfartherthan8
pixelsfromedgeoftheobjectto<1%oftheobject’ssignalbyusingEq.(3.14).SmalldiskobjectsintheFOV
wereusefulinderiving £L forsmallervaluesof ∞ andlargediskobjectsforlargervaluesof ∞.Thecoefficients
weobtainedarelistedinTable3.4andFig.3.17showsthebroadPSFonAMICAandONC-T.Thecontribution
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ofthebroadPSFonONC-TissmallerthanthatobservedwithintheAMICAimagedatasets.
Evaluation:
ThevalidityofthebroadPSFcorrectionisassessedwithapplicationstotheEarthimagesobtainedduring
theEarth-Moongravityswing-by.ThebroadPSFcorrected imagesare shown inFig.3.18withbrightness
variationsacrosshorizontalprofilesdisplayedtoshowthereduction inscattered light.Comparingthesky
closetotheobjectlimb,theresidualislessthan1%andnowtheghostcanberecognizedmoreclearlythan
thescatteredlight.
ThebroadPSFcorrectionisalsoverifiedbyusingasaturatedMarsimageobservedon25May2016.In
thistest,thestrategyisthatifapointsourceobjectisobservedwithalongexposuretime,thebroadPSFcan
beobservedmoreclearly.Marswasimagedalmostasthepointsource(~1.4pixelindiameterintheFOV).We
obtainedtheimagesusingthep-bandfilterwithtwodifferentexposuretimes,aproperexposuretimeof0.044
sandalongexposuretimeof178.26s.Thedarkcurrentandtheradiatorstraylightcorrectionswereapplied
beforetheintensityanalysisforthebroadPSF.Theproperexposureimagesareusedtomeasuretheradiation
fluxfromtheobjectbyfittingtheobservedintensitywithaGaussianfunction GvMwN ,whichisequivalentto
theMarssignalconvolvedbyonlythesharppeakPSF.Here,theFull-Maximum-Half-Width(FMHW)of the
Gaussianfunctionis1.53,whichisconsistentwiththesharppeakPSFobtainedinKamedaetal.(2017)and
Suzukietal.(2018).TheDNcountofthelongexposureimagewasestimatedbymultiplyingbytheratiooftwo
exposuretimes:
≤≥ö[¥ = ™U≥ö[¥Uµwöµ
´≤µwöµ (3.15).
Thus, the scattered light around Mars is estimated by ≤≥ö[¥ ∗ ZK′ . Figure 3.19 compares the observed
scatteredlightwiththelongexposureandtheestimationofourbroadPSF.Thus,ourbroadPSFobtainedbased
onpreflightimagesareconsistentwiththeobservedscatteredlightfromapointsource.
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Figure3.15.AnEarth image (hyb2_onc_20151204_040959_tpf_l2a.fit)before(top)andafter (bottom) the
scattered light correction shown by histogram equalization, which enhances contrast. The contour lines
correspondto0.5,1,2,4,8%ofthediskintensity(magenta:0.5%,red:1%,orange:2%,green:4%,blue:8%).
Figure3.16.ONC-Tpreflightimages(p-band)usedforfittingthebroadPSF.ThesizeofobjectsintheFOV
were225,468,and979pixelsindiameterfromlefttoright.Thecontourlinescorrespondto0.5,1,2,4,and
8%ofthediskintensity(magenta:0.5%,red:1%,orange:2%,green:4%,blue:8%).
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Figure3.17.ComparisonofthebroadPSFonONC-T(solidlines)andAMICA(dottedlines).
Table3.4.ThebroadPSFcoefficientsAi (10-4)andthebroadPSFcontribution ∫∂∑∏π′∫ª∏ onONC-Tand
AMICA by Ishiguro (2014). Note that the contribution of broad PSF on AMICA is evaluated by π∫† =
[∫∂∑∏πºª∏/∫∂∑∏πΩæøª∏] × π∫.
ONC-T A1 A2 A3 A4 A5 A6¿2•°Z′Kr°
ul(0.4µm) 12.0 0.7 0.7 0.5 0.7 0.3 0.11
b(0.48µm) 10.0 0.5 0.0 0.0 0.4 0.2 0.07
v(0.55µm) 9.0 0.0 0.2 0.1 0.4 0.2 0.07
Na(0.59µm) 10.0 0.3 0.4 0.0 0.3 0.2 0.08
w(0.70µm) 11.0 0.4 0.5 0.2 0.2 0.3 0.09
x(0.86µm) 10.0 1.3 0.6 0.1 0.6 0.5 0.14
p(0.95µm) 9.0 1.5 1.2 0.1 1.1 0.7 0.19
AMICA A1 A2 A3 A4 A5 A6 Contributionof ZK
ul(0.38µm) 11.4 7.6 1.1 1.0 0.8 0.7 0.23
b(0.43µm) 9.7 1.5 0.3 0.4 0.4 0.5 0.13
v(0.55µm) 9.6 1.3 0.3 0.4 0.4 0.5 0.13
w(0.70µm) 9.2 1.4 0.6 0.7 0.6 0.5 0.15
x(0.86µm) 7.9 3.1 1.8 2.4 1.9 0.4 0.20
p(0.96µm) 7.9 4.0 6.6 3.2 5.1 1.4 0.53
zs(1.01µm) 33.5 10.7 4.0 6.0 6.4 3.0 0.95
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Figure3.18.EnhancedEarthimagesbefore(thefirstrowimages)andafter(thesecondrowimages)the
broadPSFscatteredlightcorrection.Horizontalprofilesalongredlinesofthemareshowninthethirdrow.
Figure3.19.Observed scattered lightas a functionofdistance fromMars, comparingwithourbroadPSF
obtainedpreflight.NotethatclosepartfromMarsisaffectedbythesharp-peakPSF(grayhatch).
3.6.2.GhostEffect
ThereisnoisewithintheONCsystemduetoreflectionsbetweenoptics,suchasfilters,lens,andtheCCD,
thatcreateaneffectcapturedwithinthe imagehereafterreferredtoasthe“ghosteffect”.BecausetheONC
opticsareaxi-symmetric,aghostimageusuallyappearsatasymmetricallocationabouttheopticalaxis.Here,
weinvestigatedtheintensityoftheghosteffectandthepositionoftheghostimagewithintheimageframe.
Figure3.20displaysexamplesofghostsintheimagesobtainedduringtheEarth-Moonswing-by.Thedegree
ormagnitudeoftheghosteffectismeasuredtobeweakerthan0.1%oftheintensityoftheobservedobject
forallfilters,however,thereisawavelengthdependencytothemagnitudeoftheeffect.Forexample,v-,Na-,
andw-filtersdonothaveghosteffectslargerthanotherCCDnoisesources,suchasMEInoise,darknoise,and
shotnoise,whiletheul-,x-,p-,andwide-filtersallsufferfromasmall(<0.1%)butdetectableghosteffect.From
theEarth-Moonswing-by images,wedeterminethereflectioncoefficients(the intensityratiobetweenthe
ghostandobject)forthefiltersas0.0033,0.0012,<0.0001,<0.0001,<0.0001,0.0045,0.0065,and0.0028from
shorttolongwavelengthandthewidefilter,respectively.Thelightreflectssymmetricallyaboutthesub-pixel
positionat~(496.5,501).Usingthesevalues,examplesofghostremovalareshowninFig.3.21.Itshouldbe
notedthatwhentheobjectisfarfromthecenterofFOV,thiscorrectionmethodisinsufficient(Fig.3.22).The
ghosteffectonimagesthatincludetheobjectatacornerismuchweakerthanthatonimageswhichinclude
theobjectatthecenter.Thismayindicatethattheintensityoftheghosteffectisafunctionofthedistancefrom
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theopticalaxis.Thus,theintensityassessmentisvalidforthecenterregionofFOV,whereRyuguwillbelocated
on home position observations. Moreover, when the object fills the entire field of view, such as close
approachestotheasteroid<10km,thisghostremovalmethodmayworkonlyinthearea<500pixradius
fromthecenter.
Figure 3.20. Ghost effect captured in images of the Moon with multiband filters
(hyb2_onc_20151205_115044_tuf_l2a.fit, hyb2_onc_20151205_115024_tbf_l2a.fit,
hyb2_onc_20151205_114908_tvf_l2a.fit, hyb2_onc_20151205_114952_tnf_l2a.fit,
hyb2_onc_20151205_114920_twf_l2a.fit, hyb2_onc_20151205_114940_txf_l2a.fit,
hyb2_onc_20151205_115012_tpf_l2a.fit, hyb2_onc_20151205_115056_tif_l2a.fit). The centers of images,
200x200pix,areshown.GhostsoftheMoonappearpointsymmetricallyattheopticalaxis(atlowerleftin
thiscase).ColorbarsindicatethesignalnormalizedbythediskaveragesignaloftheMoon.
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Figure 3.21. Two examples of ghost removal for Earth images taken through the p-filter
(hyb2_onc_20151126_023805_tpf_l2a.fit, hyb2_onc_20151204_040959_tpf_l2a.fit). The centers of the FOV,
200x200pixand600x600pixrespectively,areshown.
Figure3.22. Over-correction of an imagewith the object observedwithin the corner of the image frame
(hyb2_onc_20151204_045401_tpf_l2p.fit).Theghosteffectismuchweakerfortheobjectimageatacorner.
3.7.RadiatorStrayLightRemoval
The radiator stray light contamination is caused by the reflection of sunlight by apart of theONC-T
radiatorwhichwasreported inSuzukietal. (2018).Figure3.23 summarizesthestray light intensitywith
respecttothespacecraftattitude,whichwascontinuouslymonitoredwithonboardattitudesensors(i.e.,star
trackers).TherearetwowaystodescribethespacecraftattitudewithrespecttotheSun,whichare(XPNL,YPNL)
and(¡, ¬).ThedefinitionofattitudesandspacecraftcoordinatesaredescribedinAppendixC.Theintensity
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ofthestraylightwasevaluatedatthehighestintensitypartoftheimageframe,inthesamewayasSuzukiet
al. (2018),andwenormalized the intensityby thedistance from theSun.Thedefinitionof thespacecraft
attitudewasreferredtoSec.8.1 inSuzukietal. (2018).Thereferenceattitudeof thespacecraft is that the
antennaispointingtotheEarth.Therebythevalueof ¡ correspondstothephaseangle,i.e.,theEarth-Ryugu-
Sunangle,andisautomaticallydeterminedbythepositionoftheEarth,RyuguandtheSunattheobservation
time.Weexpectthat ¡ willbechangefrom-20˚to-10˚beforethefirsttouchdown.Thetrendofthestraylight
didnotchangeduringthe3.5yearsofthecruisephase.Thatis,thestrongestpartisalwaysinthe(+H,+V)
corner,andthestraylightcomponentgraduallyweakenstowardstheopposite(-H,-V)corner(seeFig.21in
Suzuki et al. (2018)). Because attitudeswith ¬ < −7° do not have strong stray light component, we are
planningtoobtainimagesofthetargetasteroidatthoseattitudeswithnegligiblestraylight,<3.8DN/sat1
AU,forglobalmappings.Ontheotherhand,fortheproximityobservations,suchasthelanderreleases,the
SmallCarry-on Impactor (SCI) crater scan observations, and touchdowns,when the observationattitudes
allowstraylightatthe ~1000DN/slevelmaycontaminatetheimages.Here,wemodelthestraylightasa
functionofspacecraftattitudetowardstheSuntoprovideamethodforremovingthiscomponentfromthe
images.
Basedoninflightobservations,thespatialpatternsoftheradiatorstraylightdoesnotchangedrastically
with the spacecraft attitude.Moreover, the stray light component is reproducible for the same spacecraft
attitudewhenthe intensity isnormalizedbythesquareof theheliocentricdistanceof thespacecraft(Fig.
3.23).Thus,weseparatelymodeledthestraylightpatternwithintensityvariation(modeledatthecenterof
FOV)andthespatialdistribution.
Investigationsofastrongstraylightcomponentforpossibleattitudesduringtherendezvousphasewas
conductedon17,19,and21October2017,especiallywiththespacecrafttwistingangleof ¬ < −5°.First,we
empiricallymodeledtheintensityofthestraylightatthecenterofFOVwithapolynomialfunction:
GN≥ = (−0.0879¡ú − 5.616¡u − 119.4¡ − 603.8)
(−0.0037¬u + 0.113¬ + 0.956) [DN/s].
(3.16)
ThecomparisonofthestraylightintensitymodelandtheobservedintensityisshowninFig.3.24.Bydividing
the stray light images by the central intensity, the spatial distributions can be examined. We bin those
normalizedpatternsinto8×8(averageof64×64pixels)imagestosmoothoutthenoise.Afternormalization
andsmoothing,weappliedprincipalcomponentanalysistothoseimagesandextractedtheaveragepattern
and the principal components of the stray light patterns. The principal component analysis extracts the
orthogonalcomponentswithhighestvariancetolowervariance.Thecomponentwithhighest
variance,thefirstprincipalcomponent,mayexplainthestraylightpatternchangemost.Thecomponentswith
smallvariancecouldbenoise.Thus,thecomponentswithhighvarianceareenoughtoexpressthevariation
instraylightpattern.Inouranalysis,Thefirstprincipalcomponent(PC1)scorecontains81%ofvarianceand
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the second principal component (PC2) score contains 18 % of variance. Thus, 99% most of the spatial
distribution variation can be explained by the PC1 and PC2 scores. The stray light patterns ƒVY can be
decomposedlinearlybytheprincipalcomponentsas
ƒVY = ƒI≈X + ∆«»tƒ«»t + ∆«»uƒ«»u, (3.17)
Where ƒI≈X, ƒ«»t, andƒ«»u aretheaveragespatialdistributionimage,thefirstprincipalcomponentimage,
andthesecondprincipalcomponentimagewhichareshowninFig.3,25. ∆«»tand∆«»u arethePC1andPC2
scores,respectively.Moreover,wefoundthatthePC1scoreforstraylightcorrelatewiththevalueof ¡ (Fig.
3.26).Basedontheintensitymodelandthespatialdistributionmodel,wecanestimatethestraylightatan
arbitralattitudeintherangeof −30° ≤ ¡ ≤ −10° and −5° ≤ ¬ ≤ 10°.Applyingthisremovalmethodtothe
observed images, the residual of the stray light averaged over the FOV is smaller than 25 DN/s/pixel,
correspondingto0.25%oftheintensityoftheasteroiddisk.Notethat,theedgeofFOVtendstohavemore
errorthanthecenterwiththismethod.
Figure3.23.(a)Typicalpatternofradiatorstraylight.(b)Theintensityclassificationofradiatorstraylight.
RadiatorstraylightoccursasafunctionoftheangleofthesuntotheXPNLandYPNL(left),andthesolarphase
angle(¡)andthetwistinganglearoundZSCaxis(¬)(right).Duringtherendezvousphase,weareplanningto
twistthespacecraftto ¬ < −7° asmuchaspossibletoavoidstrongstraylightcontributions.
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Figure3.24.Intensitymodel(blackmesh)ofstraylightasafunctionof −30° ≤ ¡ ≤ −10° and −5˚ ≤ ¬ ≤
10˚,comparingwithobservations(red).
Figure3.25.Theaverageandtheprincipalcomponentsofthestray-lightspatialdistributions.
Figure3.26.ThePC1scoresandthephaseangle ¡ ofthestray-lightspatialdistributions.
3.8.Flat-FieldCorrection
Flat-fieldcorrectionisoneofthekeycalibrationsneededforderivingreliablebrightnesspropertiesat
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anypositionwithinan image frame. After deriving the preflight flat-field correction using the integrating
sphere measurements, the front hood position of ONC-T was changed slightly during disassembly and
assemblyatthelaunchingsite.Thus,allONC-Tbandsareexpectedtohavedifferentsensitivitydistributions
(flat-fields)inflightthanthoseobtainedpreflightusingtheintegratingsphereimages(Kamedaetal.,2017;
Suzukietal.,2018).Infact,~10%differencesinthelunarbrightnesswereconfirmedbetweenimageswith
theMoonlocatedinthecenterofanimageandthosewiththeMoonlocatedinthecornerforallfilterbands
(Suzukietal.,2018).ThelunarimageswereacquiredduringHayabusa2’sEarth-Moonflybyon3December
2015,whichprovidedthegravityassisttoreachitstarget,162173Ryugu.Thelunarimageswerecorrected
withanadditional flat-field correctionderived frommeasurements takenwithaportable flat light source
conductedafterthehoodpositionwasslightlychanged.Byapplyingtheadditionalflat-fieldcorrection,the
discrepancybetween lunarbrightnessvalueswasreducedto2%inallbandsexcept forul-band,while the
relativelylargediscrepancies(~3%)stillremainintheul-band(Suzukietal.,2018).
Forevaluatingthevalidityoftheflat-fieldcorrectionsandinvestigatingthedetailedstructureofthecurrent
sensitivitydistributioninul-band,astarobservationcampaignwasconductedfrom12to14October2017.
DuringthiscampaignfourbrightstarswereobservedunderfivedifferentattitudeconditionsoftheHayabusa2
spacecrafttoplacethestarswithindifferentlocationswithintheONC-TFOV.Thefourbrightstarsobserved
werePhi,Sigma,Tau,andZetaSagittariiwhoselocationsarecloseenoughtoeachothertobeobservableat
thesametimewithintheFOV.StarlocationsinONC-Timageframeduringtheobservationcampaigncover
almostthewholeFOVoftheimageframe(Fig3.27(a)).Starobservationswereconductedwithexposuretimes
of16.7and22.8secondsforv-band,and16.7and67.2secondsforul-band,toobtainsufficientcountsfrom
thedifferentstarbrightnesstomeasuretheCCDsensitivity.
TheexpectedfluxfromastarJstar,nthroughn-filtercanbedeterminedfrom
oVÕIH,[ =∫de/Œ*)(d)fD(d)gd
∫dfD(d)gd, (3.18)
whereJstar[W/m2/µm]istheirradiancefromthestar.Thisvalueshouldbeproportionaltothetotaldigital
countrateoftheobservedstarobtainedbyONC-T:
(∑œ–,—“–,—
–,— )/A
∫ aostar(a)Φ’(a)ra= Æ(÷◊’ÿU∆’U), (3.19)
whereIi,jistheDNcountsandfi,jisacoefficientofflatfieldcorrectionat(i,j)pixel.Thisindicatesthatlefthand
sideofEq.(3.19)shouldhavethesamevalue foranystarandanyexposuretime, if flat fieldcorrection is
perfectatalllocations.Inotherwords,ifastellarobservationhasasmallervaluefortherighthandsideofEq.
(3.19)thanotherstars,theflatfieldcorrectionatthepositionisinsufficient,andthusthecorrectedsensor
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sensitivityissmallerthanexpected.
To evaluate sensor sensitivities at the star positions, we performed flat field correction with those
proposed inSuzukietal(2018)afterperformingdarksignalsubtractionandhot-pixelreduction.Thenwe
measured the total DN counts for each star by integrating theDN counts in a 20-pixel radiusaround the
brightnesscenterofthestar.TheonlyexceptiontothiswasforTauSagittariilocatedat(H,V)=(86.7,619.7)
withanexposuretimeof67.2sec.Inthiscaseweuseda15-pixelradiusforintegratingtheDNcountstoavoid
includinghugecountsfromacosmicrayhitlocatednearbythestarintheFOV.Inthesecalculations,weused
the star flux from Alekseeva et al. (1996), which is available in the database VisieR (http://vizier.u-
strasbg.fr/viz-bin/VizieR).
Table3.5summarizesthevaluesofnormalizedCwhicharecalculatedfromasCvaluescalculatedfrom
Eq.(3.19)ineachobservationarenormalizedtothemeanCvalueforeachfilter.Examinationofthestandard
deviationsofCvaluesshowsthatthev-bandimageframeachievesuniformsensitivitywithadeviation<2%
withnosignificantoutliers.Consideringthatthestandarddeviationincludeserrorsinstarirradianceandin
noisereductionprocesses,the2%deviationcanbeconsideredasanupperlimitofthesensor’suniformityin
sensitivity.Inaddition,gradualsensitivityvariationsacrosstheimagecanbeobservedinthev-bandFOV(Fig.
3.27(b)),indicatingthatadditionalflatfieldcorrectionscouldbeperformed,thoughitishardtoderiveahigh-
accuracy flat-field correction only from this star observation campaign due to the sparseness of the star
positionsacrosstheFOV.
Theul-bandimageframehasalargersensitivitydeviation(3.9%)acrosstheimagescenecomparedto
thesensitivitydeviationinthev-band.AgradualsensitivityvariationacrosstheFOV(lowsensitivityarounda
lower-leftimagecornerandhighsensitivityinupperregion)canalsobeobservedinul-band(Fig.3.27(c)).
Especiallyatpositionsof(H,V)=(85.8,903.8)and(38.6,958.1),Cvalues(denotedbyunderlines)are6-9%
smallerthanameanvalue,indicatingadeclineinsensorsensitivityuniformityneartheimageedges.From
theseresults, thebrightness inconsistencyseen inthe lunarul-bandobservation,reportedbySuzukietal.
(2018),couldbeexplainedbytheworseuniformityneartheimagecorners.Thiscouldaffectmappingand
photometricanalysisespeciallyintheperipheralregionsintheFOV.Thisdecreaseduniformityintheul-band
flat-fieldmaybefundamentallycausedbythepreflightmeasurements.Duringthepreflightintegratingsphere
measurementsstraylightcontributionsfromthegapbetweenthefilterwheelandbaffleweredetected(see
“FWroundabout”straylightdescriptioninSuzukietal.,2018).TheFWroundaboutstraylighthasastronger
contributionattheshorterwavelengths,thuspreferentiallycontaminatingtheul-bandimages.Itshouldbe
notedthatifwefocusonlyonthestarsobservedinsideoftheradiusof400pixfromtheimagecenter,where
the standard deviation of C values is 2.2% in the ul-band, indicating we may have sufficient sensitivity
uniformitywhenweuseonlythecenterofFOV.
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Figure3.27.(a)Starpositionsobtainedduringthestarobservationcampaignfrom12to14October2017.
Bluecrosses,greencrosses,redtriangles,andorangediamondsindicateobservedpositionsofSigma,Phi,Tau,
andZetaSagittarii,respectively.(b)Deviationsofthemeasuredsensorsensitivitiesatstarpositionsfromtheir
meanvalueforv-bandafterperformingtheflat-fieldcorrectionproposedinSuzukietal(2018).(c)Samefor
ul-band.
Table3.5.ObtainedCvaluesfromEq.(3.19)andtheirstandarddeviationsfromstarobservationsforul-and
v-bands.EachCvalueisnormalizedbyanaveragedCvalueofallstarstogetherforeachband.
Starname
Position(pixel) NormalizedC(=normalizedsensitivity)
H Vv-band ul-band
16.8sec 22.3sec 16.7sec 67.2sec
Phi
836.7 430.3 0.968 0.997 1.043 1.019
837.6 146.3 0.995 0.965 0.985 1.003
837.1 713.5 0.974 1.000 1.019 1.003
594.3 430.2 0.989 0.984 1.046 1.031
Sigma
480.3 350.8 1.004 1.002 1.045 (saturated)
480.5 66.8 1.016 1.014 1.058 (saturated)
480.5 633.4 0.991 0.995 1.042 (saturated)
238.0 350.4 1.025 1.020 0.983 (saturated)
804.5 350.2 0.994 0.994 1.039 (saturated)
Tau
86.7 619.7 0.990 1.020 (toolow) 0.973
87.0 336.4 1.023 1.010 (toolow) 1.004
85.8 903.8 0.995 0.985 (toolow) 0.938
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3.9.RadiometricCalibration
Inthissection,theconversionfactorfromcountperseconds(DN/s)toradiance(W/m2/µm/sr),referredto
as“radiometriccalibration”,isdiscussed.First,thetemporalvariationinsensitivityisdiscussedinSec.3.9.1.Suzuki
etal.(2018)usedthesensitivitycorrectedbythetypicaltemperaturedependenceprovidedbythemanufacturer
(E2V). However, the temperature dependence had not been evaluated inflight at that time. Thus, we second
measuredtheeffectofCCDtemperatureonthesensitivitybasedoninflightfluxmeasurementstakenofJupiter
(Sec.3.9.2).Theobjectiveinthissectionistoupdatethesensitivityconversionbasedoninflightmeasurements.
Weevaluatethesensitivityconversionintwoways;1)basedonthehardwarespecificationsofcomponents,such
asCCD,filters,andlensoftheONC-T(Sec.3.9.3),and2)basedonstarobservations(Sec.3.9.4).Finally,theupdated
conversiontoradianceisvalidatedbasedonONCobservationsoftheMoon,JupiterandSaturnasdiscussedinSec.
3.9.5.
3.9.1.Time-DependentSensitivityChange
TheFFlampisdesignedtomonitorthesensitivity(absolutevalueandspatialpattern)oftheCCDduringthe
mission.Table3.6summarizestheFFlampobservationstakenduringthecruisephase.FFlampobservationsare
included in the default routine of the health-checkout sequence of operations. Health-checkouts have been
conducted four times over the cruise phase after the initial post-launch checkout. An extra observation on 5
December2017wasconductedtomonitorthelinearitypropertiesoftheCCD(seeSec.3.5).Onlyv-bandwasused
inthislinearitymeasurement,andonlyasection(128×128pixels)oftheFOVwasdownlinkedforexamination.
Figure3.28 showsanexample theONC-T images takenof theFF lampoutputon16October2017.The
brightness gradient in the vertical direction of the ul- and b-band images are characteristic of the FF lamp
properties,due to theFWroundaboutstray light.Figure3.29examines thebrightnessgradient in thevertical
directionintheFOVforallfiltersbydisplayingtheratiooftopquarteroftheimage(#1)andthebottomquarterof
theimage(#4).Thereasonforthestronggradationattheshortwavelengthscanbeexplainedasfollows.TheFF
lampradianceisclosetoa2000Kblackbodyradiatorthathaspeakemissionatlongwavelengths.Forexample,
411.8 618.9 1.022 1.030 (toolow) 1.008
Zeta
282.4 957.0 0.985 1.005 0.999 0.971
283.3 672.4 1.003 1.016 1.015 1.002
38.6 958.1 0.993 0.996 0.921 0.911
607.4 956.2 0.993 1.009 0.972 0.974
Standard
deviation(%) 1.6 3.9
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theenergyfluxfromtheFFlampatthep-bandwavelengthrangeisabout500timeslargerthanthefluxattheul-
bandwavelength. Therefore, the FW roundabout stray light is a negligible percentage of the transmitted light
throughthelongerwavelengthfiltersbutisameasurablepercentageofthetransmittedlightthroughtheshort
wavelength (e.g., ul- and b-band) filters. The difference between segments #1 and #4 are due to the spatial
distributionbiasoftheFWroundaboutstraylight.ThisFWroundaboutstraylight isnotexpectedtoaffectthe
observationsofRyugu,sincethereflectancespectrumofRyuguisrelativelyflatandthesolarradiancespectrum
hasapeakatthev-bandwavelengthrangeofonly~2timeslargerthanul-bandwavelengthrange.
AveragesignalvaluesoftheFFlampimagesareusedtomonitor thetimevariationsintheCCDresponse.
Figure3.30showsthetimevariationofthecountfluxoftheFFlampimagesobtainedduringthecruisephase.The
FFlamphastwomodesofvoltagesettingswhichisselecteddependingonthesensitivityoffiltersusedtoimage
thelamp.Figure3.30(a)showsthe“Highvoltagemode”data(usedfortheshorterwavelengthfilters:ul-toNa-
bands),andFig.3.30(b)showsthe“Lowvoltagemode”data(usedforthelongerwavelengthfilters:w-top-,and
wide-bands).Toshowrelativesensitivitychanges,themeasuredoutputoftheFFlampisnormalizedtotheoutput
measuredon16April2015.Notethatthedataacquiredon8July2017areexcludedfromtheplot linesdueto
possiblecontaminationbyradiatorstraylightaffects(Sec.3.7).
Figure3.30(a)indicatesthatthesensitivityoftheshorterwavelengthfilters(ul-toNa-bands)decreasedafter
theinitialcheckout.Figure3.30(b)alsoindicatesthatthesensitivityofthelongerwavelengthfilters(w-through
p-andwide-bands)decreasedaftertheinitialcheckout.However,wecannotconcludethedegradationisrealdue
to the unstable emission from the “Low voltagemode” of the FF lamp used for the preflight longwavelength
measurements.Sincethe“Highvoltagemode”wasmorestableduringthepre-flightmeasurements,wearelimiting
ourdiscussiontotheul-to-NabandswhichwerecharacterizedusingtheFFlamp“Highvoltagemode”.Thereare
twopossibleexplanationsforthedecreasingtrendinthe“Highvoltagemode”data;1)degradationoftheFFlamp,
e.g., low electric current and 2) degradation in the transmittance of the filters due to exposure to the space
environment. During the initial checkout, the instrument temperatures were slightly higher than those of
subsequentobservations(Table3.6).Duetothesetemperaturedifferences(FFlampandCCD),weinter-compare
theobservationsacquiredafter16April2015,whichwereobtainedundersimilartemperatureconditions.After16
April2015,theFFlampfluxismorestable,decreasing~1%fortheb-to-Nabandsanddecreasing~2.5%fortheul-
band. This demonstrates that the ul-bandhas a steeper decrease in sensitivity than the other three bands. To
investigatethereasonforthedecreaseinsensitivity,wecomparethelocalmeasuredoutputchange.Figure3.29
showsthemeasuredoutputchangeforsegments#1and#4.Althoughsegment#1ismorecontaminatedbytheFW
roundaboutstraylight,thedecreasingrateinthemeasuredoutputisalmostsame.Thus,thisdecreaseinsensitivity
ismorelikelycausedbydegradationoftheFFlamp.
Sincethemainpurposeofthis5December2017observationwastoexaminethelinearityresponse,onlythe
ONC-Tv-filterwasused.Additionally,toreducethedatadownlinksize,onlyaportion(128x128pixels)oftheFOV
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was downlinked, using the region of interest (ROI) function form the on-board processing algorithms. Time
variation in the FF lamp output in this area of the CCD are plotted in Fig. 3.31. Although the temperature
environment (Table 3.6) during this observation is very similar to the environment of 16 October 2017
observation, there isa small increase (~0.4%) fromthe16October2017observation. It is suspected that this
randomtrendmaybeduetoslightstabilitiesintheFFlampoutput.
Insummary,weconcludethatthecountfluxfromtheFFlamphavealong-termdecreasingtrend(~1–2.5%)
between16April2015to5December2017.Therearetwoplausiblecandidatesforthedecreasedcountflux:(1)
degradationoftheFFlamp,and(2)degradationofthefiltersunderthespaceenvironment.Toresolvewhetherthe
sensitivitydegradationintheCCDisrealornot,additionalobservationsareneeded;especially,repeatcalibration
observationsusingstarsthathavealreadybeenobservedbytheONC.
Table3.6.SummaryoftheFFlampobservationsheldafterlaunch.
DateObservation
purposeNote Temperature[℃]
ONC-AE ONC-TCCD
Start End Start End
11December2014Initial
checkout7band+wideband -4.49 -4.10 -14.79 -14.79
16April2015Health
checkout7band+wideband -6.46 -6.20 -29.56 -29.54
10September2015Health
checkout7band+wideband -6.95 -6.46 -29.46 -29.43
8July2017Health
checkout
7band+wideband.
SolarStraylightwas
contaminated
-7.05 -7.02 -28.77 -28.77
16October2017Health
checkoutONC-T(7band+wide) -5.77 -5.77 -28.59 -28.91
5December2017Linearity
observation
ONC-Tvband.Asmall
partoftheFOViscutout.-5.77 -5.77 -28.72 -28.72
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Figure3.28.ExamplesoftheFFlampimages.Theseobservationsweretakenon16October2017.
Figure3.29.VerticalbrightnessgradientsinanFFlampimage.Theobservationwastakenon16October2017.
Theimagewasdividedinto4segmentsintheverticaldirection,andtheaveragesignalratiobetweensegment#1
(topquarterofanimages)andsegment#4(bottomquarter)asafunctionofwavelengthisshown.TheONC-Twide
banddataisplottedat550nmforreference.
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Figure3.30.ThetimevariationintheFFlampcountfluxduringthecruisephase.(a)ul-toNa-banddatawere
obtainedinthe“Highvoltagemode”.(b)w-top-,andwide-banddatawereobtainedinthe“Lowvoltagemode”.
Figure3.31.TimevariationoftheFFlampcountfluxinthev-band.Square:FullFOVdata(sameasFig.3.30).
Cross:ROIsub-frame.Thedatapointsarenormalizedtothe16April2015observation.
3.9.2.SensitivityDependencefromCCDtemperatureBasedonJupiterObservations
ThespectralsensitivityoftheONC-Twasmeasuredusinganintegratingsphereatroomtemperaturepriorto
launch (Kameda et al., 2017). The sensitivity at cold temperatureswas estimated using the typical sensitivity
dependencefromCCDtemperatureprovidedbythemanufacturer(E2V),andwasdemonstratedtobeconsistent
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with the SP/SELENE lunar reflectance model (Suzuki et al., 2018). In this section, we examine the Jupiter
observationstakenbytheONC-TwithdifferentCCDandELEtemperaturestoprovideamodelofthesensitivity
dependencefromhardwaretemperatureseachfilterband.
WeobservedJupiterusingONC-TfromMay16–17,2017.TheCCDtemperaturewascontrolledbytheheater
torangebetween-29℃to+25℃andtheELEtemperaturefrom-10℃to0℃(Table3.7).BecauseofJupiter's
non-uniformity of reflectance, wemeasured the Jupiter’s brightness over 4 rotation periods under a different
temperatureforeachperiod,foratotalof4rotations.Figure3.32showsthenormalizedfluxdependencytothe
CCDtemperature.TheeffectoftheCCDtemperatureismuchlargerthanthatofELEtemperatureespeciallyforthe
longwavelengthfilterbands.AstheCCDtemperatureincreases,thesensitivitiesoftheul-tow-bandsdecrease,
whilethesensitivitiesofthex-andp-bandsincrease.Weassumedthatthesensitivity,Sn,canbeexpressedasa
functionoftheCCDtemperature,TCCD,T;
\1 = \É,[0∆PPQ,[0OPPQ,1 + 30; + 1;, (3.20)
whereS0,n is thesensitivityforn-bandat the referencetemperature OPPQ,1 = −30℃ andan is the temperature
dependencefactordeterminedbyχ2-fitting.ThesensitivityofeachbandatanytemperaturesofTCCD,T(-30℃ to
+25℃)canbecalculatedbyEq.(3.19)andthevalueslistedinTable3.8,wherethe2-σerrorisbasedondeviations
ofthemeasurementsfromEq.(3.19).Similarly,theELEtemperaturedependencecanbeexpressedas(Table3.9):
\′1 = \É,[0∆R$R,[(OR$R + 10) + 1;. (3.21)
ThereferenceELEtemperatureis-10℃.Notethat,theELEtemperaturedependencesforul-to-Nabandsarenearly
0withinthe2-σerrors,showingnodependenceontheELEtemperatureintheCCDandfilterresponse.However,
w-to-pbandsshownoticeabledependencesfromELEtemperature.TheCCDtemperaturehasthedominanteffect
ontheCCDandfilterresponses.Thus,weuseEq.(3.19)toquantifytheCCDsensitivityatCCDtemperatureranging
from-30℃to25℃.AlthoughthedependencefromELEtemperaturemightbeobservedforlongerwavelengths,we
do not apply ELE temperature correction for first products becausewe do not have physical explanations of
dependencedifferenceinfilters,whilethedependencefromCCDtemperatureforeachbandshouldbedifferent
duetothetemperaturedependenceofquantumefficiencyindifferentwavelengths.ToapplytheELEtemperature
correction,furtherinvestigationisneeded.Forexample,wewillbeabletotestthecorrectiontoimagesobtained
during close approaches. While this does not account for sensitivity variations caused by other hardware
temperatures,forexample,theerrorintroducedbytheELEtemperatureeffectissmall(<0.1%/℃fortheul-to-Na
bandsand<0.3%/℃forthew-to-p-band).
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Figure3.32.ObservationsofJupiterusingtheONC-Tforeachband(ul-p).Thesymbols,�,△,□,and×,indicate
thenormalizedsignalrate(DN/s)forrotationalperiods1,2,3,and4,respectively.Theblacklinesshowtheresults
ofχ2-fittingwiththegraylinesof1-σerrors.
Table3.7.CCDandELEtemperaturesduringtheONC-TJupiterobservations.
Phase# CCDTemp.
[℃]
ELETemp.
[℃]
DateinMay,2017 Starttime,UT Endtime,UT
1 21.04 -8.84 16 12:30 12:33
2 21.05 -8.32 16 15:00 15:03
3 -27.94 -9.89 16 17:30 17:33
4 -28.20 -9.89 16 20:00 20:03
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1 -27.94 -9.89 16 22:30 22:33
2 -28.06 -9.89 17 1:00 1:03
3 -8.66 -9.37 17 3:30 3:33
4 -9.18 -9.37 17 6:00 6:03
1 11.93 -8.84 17 8:30 8:33
2 11.41 -8.84 17 11:00 11:03
3 25.11 -8.32 17 13:30 13:33
4 25.22 -8.32 17 16:00 16:03
1 -29.24 -3.63 17 18:30 18:33
2 -28.46 -3.63 17 21:00 21:03
3 -28.72 0.19 17 23:30 23:33
4 -28.77 -0.03 18 2:00 2:03
Table3.8.SensitivitydependencefromCCDtemperatureforallband-pass-filters.Theerrorscitedarethe2-σ
errors.
band ∆PPQ,[[/℃]
ul(0.40µm) −0.001449 ± 0.000244
b(0.48µm) −0.000968 ± 0.000108
v(0.55µm) −0.000814 ± 0.000090
Na(0.59µm) −0.000866 ± 0.000154
w(0.70µm) −0.000355 ± 0.000112
x(0.86µm) 0.001771 ± 0.000158
p(0.95µm) 0.004201 ± 0.000202
Table3.9.SensitivitydependencefromELEtemperatureforallband-pass-filters.Theerrorscitedarethe2-σ
errors.
band ∆R$R,[[/℃]
ul(0.40µm) −0.00077 ± 0.00084
b(0.48µm) −0.00055 ± 0.0078
v(0.55µm) −0.00085 ± 0.0072
Na(0.59µm) −0.00049 ± 0.00058
w(0.70µm) −0.00118 ± 0.00034
x(0.86µm) −0.00113 ± 0.00086
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p(0.95µm) −0.00288 ± 0.00094
3.9.3SensitivityPredictionBasedontheHardwareSpecifications
TheDNderivedfromtheONC-TCCDcanbemodeledbyusingthehardwarecomponentspecifications,such
astheCCDquantumefficiency,filterandlenstransmittances.SuchmodelequationsfortheONC-W1and-W2are
describedinEqs.(2)–(4)ofSuzukietal.(2018).Inthissection,wequantifytheONC-Tsensitivitybasedonthe
specificationsofthehardwarecomponents.
WeformulatethattheexpectedsignalIsys,n[DN/s]fromONC-Tisgivenby:
GN¤N,[ =‹
›fifll‡ µ·
ûopt·‚∫
t
‹o(a)„£ì"(a, ∞, ó, ‰)aΦ’0a,OÆÆì,O;ra, (3.22)
whereFopt is the effectiveF-number of the optics (= 9.05),p is the CCDpixel size (=1.33×10-5 [m]), J(λ) is the
irradianceoftheincidencelight[W/m2/μm]ontheobservationtarget,RADF(λ,i,e,α)istheradiancefactorI/F
(dimension-less)oftheobservationtargetataspecificsetofilluminationandviewinganglegeometries(i,e,α;
incidence angle, emission angle, phase angle). The two π symbols are not canceled in this equation to avoid
confusionaboutthephysicalunits,sincethelatterπhasaunitof[sr].
Ontheotherhand,themodelradianceJnforthen-bandcanbeformulateasfollows:
o[ =∫aÂe(d)Ê_Qû(d,L,s,Á)Φ’0a,OÆÆì,O;gd
∫ aΦ’0a,OÆÆì,O;gd. (3.23)
FromEqs.(3.22)and(3.23),thepredictedmodelsensitivitySsys,ncanthenbedescribedas
\N¤N,[ = GN¤N,[/o[ . (3.24)
Table3.11showsthevaluesofthepredictedmodelsensitivityderivedfromEq.(3.24).Toderivethesevalues,we
assumethesolarirradiancespectra(ASTM-E490)forJ(λ),aflatI/FspectraRADF(λ,i,e,α)equalto1andtheCCD
TemperatureTCCD,Tequalto-30[℃].
3.9.4SensitivityCalibrationBasedonStarObservations
AmbiguitiesbetweenthepreflightCCDsensitivity(measuredinthelaboratorybyKamedaetal.(2017))and
theinflightCCDsensitivity,duetotemperaturedifferencesbetweenpreflightandinflightmeasurements,prompted
furtherCCDsensitivitycharacterizationsunderinflighttemperatureconditions.Tomeasuretheabsolutesensitivity
inflight,weobservedeightstars,withV-magnituderangingfrom2.02to4.73,duringthecruisephase.Theobserved
targetsaresummarizedinTable3.10.Thosestarswereselectedbasedonspacecraftattitudeconstraints.The
sensitivitycanbeobtainedfromthelinearrelationshipbetweenthephotonfluxandthedigitalcountobserved
withineachfilter.Forthereferencestarspectra,weusedcatalogsbyHamuyetal.(1992,1994),Alekseevaetal.
(1996),andBurnashev(1996)sincewecouldnotfindasinglecatalogthatincludedallourobservablestars.μAqr,
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φSgr,andτSgrhavenotbeenobservedinwavelengthslongerthan750nm,thus,thesensitivitiesofx-andp-filters
areconstrainedwithmeasurementsfromonlyfivestars.
Although these different catalogs used the same calibration target α Lyr (Vega), they used a different
reference spectrum for α Lyr, which contributes non-negligible systematic errors. Thus, we first derive the
conversion factors to cancel the systematic errors caused by the reference spectrum differences. Using these
conversionfactors,thestarenergyfluxbyAlekseevaetal.(1996)weremodifiedtobeconsistentwiththatofHamuy
etal.(1992,1994)inallfiltersotherthanthep-filter.Notethatweusetheoriginalfluxvaluesforthep-filterfrom
allcatalogssincethep-filterbandpasspartlycontainsthetelluricwaterfeaturebetween930–970nm,causing
particularlyhigherspectralerrorsasdiscussedinHamuyetal.(1992,1994).Burnashev(1996)usedthesame
referencespectrumofαLyrasHamuyetal.(1992,1994)andthereforethestellarspectrafromthiscatalogdidnot
requirere-standardizationtoacommonreferencespectrum.Usingthesespectra,theexpectedfluxJstar,nthrough
eachofthefiltersisobtainedbyusingEq.(3.9).ThedigitalcountsfromthestarobservationsobtainedbyONC-T
arecalculatedasdescribedinSec.3.8.Therangeofsensitivitiesderivedfromthestarobservationscorrelateto
within 5%, except for ul- and p-bands (Fig. 3.33).Note that random scatter between the stars fromdifferent
catalogssuggeststhatthesystematicerrorbetweenthecatalogshasbeenproperlyremoved.Figure3.34displays
therelationshipbetweenthestarfluxesanddigitalcounts.Thesensitivitiesforthefilterswereobtainedusinga
weightedleastsquarelinearregressionfitofthe5or8pointslistedinTable3.11.Theerrorsarederivedbasedon
the 95% confidence level. These deviations are consistent within the errors in the star observations of ~1%
(average)givenbyHamuyetal.(1992,1994)and<3%(90%confidencelevel)fortheb-tox-bandsbyAlekseeva
etal.(1996).Thelargedeviationinthep-filtersensitivitymaybeduetotheloweraccuracyofreferencespectra,
whichhas5–10%errorintheHamuyetal.(1992,1994)and<5%errorintheAlekseeva(1996)spectra.
Forcomparison,thesensitivitybasedonthehardwarespecificationsfromSec.3.9.3isshownalsoinTable
3.10.Eventhoughthereareabsolutedifferencesbetweenthetwosensitivities,ontheorderof~17%,therelative
sensitivitiesbetweenthedifferentfiltersareconsistentexceptforthep-filter(Fig.3.35).Theabsolutesensitivity
differencebetweenthehardwarespecificationandstarcalibrationmeasurementscanbeascribedtosomefluxloss
intheoptics,buttheconsistencyintherelativevaluesindicatethatthetransmittancesforthefilterswerewell
definedpreflight.Thiscomparisonalsosuggestsanotherradiometriccalibrationisneededforthep-filter.Inthe
next section, we update the p-filter sensitivity based on ONC lunar observations. We conclude that the CCD
sensitivityderivedfromthestarobservationsisconsistentwiththepredictionfromhardwarespecificationsexcept
forthep-filter.
Table3.10.Listofobservedstarsduringthecruisephase.
HRNo. Observed
Date
Vmaga) Spectral
Typea)
Filters Referencespectra
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ζPeg 8634 2016-10-19 3.40 B8V ul,b,v,Na,w,x,p Hamuyetal.(1992,1994)
θCrt 4468 2017-05-23 4.70 B9.5Vn ul,b,v,Na,w,x,p Hamuyetal.(1992,1994)
εAqr 7950 2017-10-10 3.77 A1V ul,b,v,Na,w,x,p Hamuyetal.(1992,1994)
μAqr 7990 2017-10-10 4.73 A3m ul,b,v,Na,w Burnashev(1996)
σSgr 7121 2017-10-12 2.02 B2.5V ul,b,v,Na,w,x,p Alekseevaetal.(1996)
ζSgr 7194 2017-10-12 2.60 A2III+A4IV ul,b,v,Na,w,x,p Alekseevaetal.(1996)
φSgr 7039 2017-10-12 3.17 B8III ul,b,v,Na,w Alekseevaetal.(1996)
τSgr 7234 2017-10-12 3.32 K1+IIIb ul,b,v,Na,w Alekseevaetal.(1996)
a) HoffleitandJaschek(1991)
Table3.11.Sensitivityfactorsatthereferencetemperature(TCCD,T=-30℃).
Filtername SensitivityatTCCD,T=-30℃(DN/s)/(W/m2/µm/sr)
starobservations hardwarespecifications
ul(0.40µm) 439.1±2.2 547.3
b(0.48µm) 969.3±7.9 1104.6
v(0.55µm) 1175.0±10.0 1371.9
Na(0.59µm) 546.9±2.0 642.3
w(0.70µm) 1515.0±19.3 1728.4
x(0.86µm) 1499.8±24.6 1736.1
p(0.95µm) 1033.0±71.1* 1023.1
*forthep-filterwewillusethevalue961.2±28.8(DN/s)/(W/m2/µm/sr)fortheradiometriccalibrationpipeline
basedonthelunarobservations(SeeSec.3.9.5).
Figure3.33.Thesensitivitydeviationmeasurementsfromthestellarobservations.Thesensitivitiesarenormalized
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bytheaveragesensitivitywithineachband.
Figure3.34.StarfluxesanddigitalcountsobservedbytheONC-Tfilters.Thedottedlinesindicatelinearregression
linesandtheslopescorrespondtothesensitivitiesofthefilters.
Figure3.35.Sensitivitycomparisonsbetweenthosebasedonthehardwarespecifications(dottedline)andthose
basedonthestellarobservations(solidline).
3.9.5.Jupiter,Saturn,andMoonObservations
Here,weevaluatetheCCDandfiltersensitivitiesbasedontheONC-TobservationsofJupiter,Saturn,andthe
Moon. Comparisons with well-known spectra are useful for evaluating whether the derived sensitivities are
accurate.AlthoughthelunarobservationshavealreadybeenanalyzedbySuzukietal.(2018),thesensitivitieshave
beenupdatedandrequirere-evaluation.Thus,inordertoevaluatetheupdatedsensitivities,werevisittheONC-T
lunar observations. Comparisonwith lunar observations is awell-established technique for evaluating camera
radiometriccalibrations.TheJupiterandSaturnobservationsareanalyzedinasimilarmannerasdescribedinSec.
3.9.2forthecharacterizationoftemperaturedependencies.
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Figure 3.36 shows the comparison between spectra obtained by the ONC-T on 5 December 2015 with
SP/SELENE(Kouyamaetal.,2016)andWAC/LROC(Satoetal.,2014)spectralmodels.Notethatbothmodelsare
calibratedtofittheRoboticLunarObservatory(ROLO)spectralmodel(KiefferandStone,2005).Comparisonsof
theabsoluteradiancecomparisonshowtheradiancebasedontheupdatedONC-TCCDsensitivityexceedsthat
predicted from the lunarmodels.On the other hand, the relative spectral radiance is consistent (within 2.5%)
betweenthemodelsandtheONC-TobservationsTheROLOmodelhasbeenshowntohavea5-10%uncertainty
initsabsolutevaluedetermination,whereastherelativespectraluncertaintyisontheorderoflessthan3%.
TherelativeuncertaintyisbasedontheROLOmodelcomparisonwithlunarbrightnessmeasurementstaken
with Moderate Resolution Imaging Spectroradiometer, which is a well calibrated Earth observing sensor
onboardAquaandTerra(Stone,2008).Thus,theupdatedsensitivityreproducesthelunarradiancespectrumto
withintheambiguityinthemodels.Nevertheless,wecanupdatethesensitivityconversionofthep-filterbasedon
thelunarobservationssincethelunarreferencemodelsaremoreaccuratethanthestellarobservationsatthis
wavelength.Thesensitivityderivedfromthelunarobservationsforthep-filter’snormalizedradiancesareidentical
in both the SP/SELENEmodel and theONC-Tmeasurement.The sensitivity for the p-filter based on the lunar
observations is 961.2±28.8 (DN/s)/(W/m2/µm/sr). This valuewill be used for product generation during the
mission’srendezvousphase.
Figure3.37displaysthenormalizedradiancespectraofJupiterandSaturnatTCCD,T=-29to-27℃comparedto
ground-based observations in 1995byKarkoschka (1998). Karkoschka (1998) observed Jupiter and Saturn at
phaseanglesof6.8˚and5.7˚,respectively,whiletheONC-Tobservedtheseobjectsat3.9˚and4.9˚,respectively.The
JupiterspectrumobtainedbytheONC-Tshowsoverlapwiththeground-basedobservationwhichhas~4%oferror.
Moreover,althoughthesensitivitybasedonthestellarobservationcanreproducetheradiancespectrumtowithin
theambiguityintheobservation,thespectrareproducedbythesensitivitybasedonthelunarobservationsshows
betterconsistencywiththeJupiterground-basedobservation.However,theSaturnspectraacquiredbytheONC-T
shows non-negligible differences between the x-band observations and the ground-based measurements by
Karkoschka (1998). Because the x-band wavelength coincides with a known methane absorption, a higher
reflectancefromtheringscomparedwithSaturn’sdisk(e.g.,Irvine,1971)isexpected.Dependingonthearearatio
oftheringsrelativetothediskintheONC-Timages,thex-bandcouldbebrighterthanthedisk-onlyspectraof
Karkoschka (1998). This explains the variations between the ONC-T x-band and the ground-based Saturn
observations.Moreover,thebluerspectralpropertiesoftheringsmayalsoexplainthedarkerbrightnessseenin
theul-bandcompared to theground-basedmeasurements.Thesespectralcomparisonsreasonably confirm the
ONC-T’sspectralcalibration.
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Figure3.36.ThesegraphscomparetheMoon’sradiancebetweenthatobservedbytheONC-Tandthosepredicted
bylunarspectralmodels.(left)Thedisk-averagedradiance(i<60˚ande<45˚),(center)radiancenormalizedtothe
v-band,and(right)ratioofthenormalizedspectra(showninthecentralgraph)totheONCnormalizedobservation.
GraysymbolsaretheresultsbasedonthesensitivityinSuzukietal.(2018).Thesensitivitiesforallbandsderived
fromthestellarobservationsareshownbythedottedredline.Theupdatedsensitivityforthep-bandderivedfrom
comparisonsbetweenthenormalizedradianceofONC-TandtheSP/SELENEmodelradianceisshownbythesolid
redline.
Figure3.37.NormalizedspectralcomparisonsofJupiterandSaturnobservations.(left)Comparisonofnormalized
radiance between spectra obtained byONC-TandKarakoschka (1998).Dotted lines indicate spectra produced
based on the sensitivity from the stellar observations, and solid color lines indicate those based on theMoon
observations.(right)TheJupiterandSaturnspectraofONC-TdividedbythespectraofKarakoschka(1998)are
shown.TheEarth-basedJupiterspectraandthosefromtheONC-Tareingoodagreement.Thebrightnessinthex-
bandandthedarknessinul-bandbytheONC-TinthespectraofSaturncanbeexplainedbythedifferencesinthe
spectralpropertiesbetweentheringandthedisk(e.g.,Irvine,1971).
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4.EvaluationoftheUpperLimitofaDetectableSodiumAtmosphere
Sodium(Na)isoneofthemostcommonelementalspeciessurroundingsolarsystembodies,duetoits
highabundanceandvolatility.Forexample,sodiumwasthefirstmetalliccomponentdetectedaroundMercury
byground-basedtelescopes(PotterandMorgan,1985).SodiumwasalsodetectedaroundtheMoon(~5kR)
andcomets(1kR~900MRdependingonitsdistancetoSun)throughremoteobservationusingtheresonant
scatteringofsolarlightatthesodiumemissionwavelengthsof589.594nm(D1)and588.997nm(D2)(Rahe
etal.,1976;PotterandMorgan,1988;Leblancetal.,2008).Sodium,ifpresentaroundRyugu,isexpectedtobe
observablebytheONCusingtheNa-bandfilter.Thedensitydistributionanditsseasonalvariationasafunction
ofphaseangleororbitalposition(season)isanimportantvariableinunderstandingtheformationofC-type
asteroidsandthegenerationprocessesofplanetaryatmospheres.Thissectionevaluatestheupperlimitfor
thedetectionofsodiumbytheONC-T,basedonitsobservationsofJupiter’ssodiumnebula.
Methodoutline
ThesodiumemissioncanbedetectedwiththeNa-bandfilteronONC-T.Inordertoeliminatethestray
lighteffectandfluctuationsinbackgroundnoise,animagetakenconcurrentlywiththev-bandfiltershouldbe
subtracted from the Na-band image. This eliminates the stray light component, as its contribution is
independentof theobserving filter.Therefore, thesodiumobservationsequence includesthe imagestaken
alternatelybetweentheNa-andv-bandfilters.Inaddition,thebackgroundnoisecanbeeliminatedbytaking
theratioofbackgroundcount(CNa/v) imagesof“sky-data” taken inboththeNa-bandandv-band,acquired
betweentheNa-(INa(H,V))andv-band(Iv(H,V))imagesofthetarget.Then,thereducedNa-bandimageofthe
targetiscalculatedasfollow:
Target(H,V)=INa(H,V)–Iv(H,V)×CNa/v (4.1)
Datareduction
ItisknownthatJupiterissurroundedbysodiumtorusoriginatingfromitsmoonIo.Thebrightnessofthe
D1andD2linesfromthissodiumtorusareobservedtobe20-80R(e.g.Yonedaetal.,2015).During18to20
May2017,ONCsetitstargetforthewestsideofJupiter’ssodiumtorusandacquired183setsofimages(Na-
and v-bands) continuously. Each imagewas takenwith a 178 s integration time period. The geometrical
configurationoftheobservationsisshowninFig.4.1.Eventhoughwedidnotdetectsignificantcountsfrom
sodiumaroundJupiter,theupperlimitevaluationcanstillbederivedusingthisdataset.
UpperlimitforNadetection
Asafirststep,weidentifyandremoveanyhotpixelsusingthe“darkimage”derivedinSec.3.4.Anypixels
withhighdarkcount� rates(>1-σ)areeliminatedfromthesodium-detectionprocessing.Furthermore,pixels
with higher counts (>3-σ)are alsoeliminated from theprocessing aspossible cosmic ray hits. The image
analysisprocessdescribedintheEq.(4.1)isthenadoptedforthe183imagesets.
TheratioCNa/viscalculatedtobe0.9961usingthemedianfromtheprocessedskyimages(Na-andv-
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band)takenon28April2017.NineROIsarethendefinedasshownintheFig.4.2foreachimage.Inorderto
evaluatesizeeffects,threetypesofROIs(setA,B,andC)aredefined.Thestandarddeviationincountsforeach
pixelintheROIiscalculated.ThisvalueisdefinedastheupperlimitofcountsforsodiumdetectionbytheONC
(1-σ).Integratingoverthemultipleimagesstatisticallyreducestheupperlimit.Asaresult,anupperlimitto
theNabrightnessdetection,asafunctionofthenumberofintegratedimages,canbecalculatedandisshown
inFig.4.3.Ifmorethan100imagesetsareintegrated,thentheupperlimitisforsodiumdetectionisaround
100R. In this calculation the count-to-brightness conversion factor is assumed to be 21R/DN/pix (from
Kamedaetal.,2017).
TheONC-Tobservationsduring18to20May2017didnotdetectasignificantcountfromsodiumaround
Jupiter.This isnot surprising since thebrightnessof thesodiumcloud isknowntobe lessthan80R(e.g.
Yonedaetal.,2015),whichislowerthanthedetectionlimitfortheONC-T.
ThebrightnessaroundRyuguisexpectedtobe10-10skR,althoughtheestimationisstronglydependent
ontheoutgasrateassumptionbasedonthesurfaceandthesodiumg-factor.Whenweassumetheoutgasrate
atRyugutobesameasEarth’smoonorMercury,thebrightnessisextremelylow(10-100R)whichisdifficult
todetectbytheONC-T.Ontheotherhand,ifweassumetheoutgasratetobethebrightnessofseveral10skR
whichevencorrespondstomuchloweroutgasrateoncomets,thesodiumatmosphereisdetectablebythe
ONC-Twithasingleimageset.Itshouldbenotedthatthebrightnessestimationalsodependsontheorbitof
theasteroidbecauseoftheDopplershiftinthesolarlightspectrum,whichisthesourceofthesodiumresonant
scattering(Killenetal.,2009).
Figure 4.1. This image shows the geometry for the Jupiter’s sodium cloud observations
during18to20May2017(Na-bandimage).Jupiteranditsnorthpoleisindicatedbythewhite
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circle and yellow allow, respectively. The known stars are indicated by the white broken
circles.Thecolorgradationiscausedbytheradiatorstraylight.
Figure4.2.AnexampleoftheROIregiondefinitionsforeachset.Thestandarddeviationsfor
countsineachROIwereevaluatedforthe183imagesets.
Figure4.3.TheupperlimitofthebrightnessdetectionforsodiumbytheONCasafunction
ofnumberoftheintegratedimagesets.NocleardependencyontheROIsizeisseen.
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5.InflightCalibrationoftheONC-W1and-W2
Bothpreflightandinflightcalibrationsoftheflat-fields,PSFs,sensitivities,andstraylightcontributions
aresummarizedinthissectionfortheONC-W1andONC-W2.Hayabusa2proximityoperations,suchastouch-
downs,lander-releasing,andgravitymeasurements,willbeconductedprimarilywiththeONC-W1sincethe
longfocallengthoftheONC-Twillbeoutoffocusatpositionsclosertothesurface.Duringtouch-downs,ONC-
W1willtakesequentialimagesuptoaspatialresolutionof~1.5mm/pixel.Thus,thecalibrationsoftheONC-
W1andW2areasimportantasthecalibrationoftheONC-T.Thecalibrationsconductedbothpre-flightand
inflightaresummarizedinTable5.1.
Table5.1.SummaryofthedatausedforthecalibrationsoftheONC-W1and-W2.
Groundmeasurement Inflightmeasurement InflightObservationDate
Bias 0-secexposures 0-secexposures Continuouslyduringthecruisephase
DarkcurrentandHot
pixels
Thermal vacuum test
(variation of CCD, ELE,
andAEtemperatures)
Deep sky observation with
variationofCCDtemperatures
30November2017,2December2017
Sensitivity( ) Integratingspheres StarsforONC-W2andJupiterfor
ONC-W1
9to15February2016,
19Februaryto21March2016,
28June2016,30June2016,
2July2016,17October2017,
30November2017
Flat-Field Flatpanel Star observationwith variation
ofS/Cattitudes
12March2017
Same as images for Sensitivity
calibration
Smear 0-secexposures EarthforONC-W2 4December2015
Geometricdistortion Dotted pattern (Suzuki
etal.,2018)
Stars(Suzukietal.,2018) W1:19February2015
W2:12March2017
AlignmenttotheS/C N/A Stars W1:19February2015
W2:11December2014
SharpPSFs Pinhole Stars, and Mars, Jupiter, and
Saturn
31May2016,9to15February2016,
19Februaryto17March2016,
28June2016,30June2016,
2July2016,30June2017,
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17October2017,30November2017
Straylights N/A Straylightswithvariationofthe
S/Cattitudes.
11December2014,
19February2015,
26February2015,23June2015,
12to17October2015,
9November2015,
14November2015,
22December2015,
9Februaryto17March2016,
21March2016,7June2016,
28Juneto23July2016,
12March2017,13to20June2017,
17to21October2017
5.1BiasCorrection
Weevaluatedthebias levelof theONC-W1and-W2as functionsofCCDandONC-AEtemperatures.We
analyzed0-secondexposureimagestakenduringpreflightthermalvacuumtestsandthecruisephase.Most
ofthedataacquiredduringthecruiseweretakenatthenominalCCDtemperature(-25℃),butattheendof
2017’s, we also acquired 0-sec exposure images at a CCD temperature of +20℃ to test the temperature
dependency.Figure5.1showsthebiaslevel(theaverageforallpixels)oftheONC-W1andW2asfunctionsof
theCCDandONC-AEtemperatures.Thebiaslevelobtainedduringtheinflightobservationsincreasedwith
CCDtemperature,similartotheinflightobservationresultsforONC-T(Fig.3.3)andpreflighttestsforONC-
W1and-W2.ThebiaslevelcanbeempiricallydescribedasafunctionoftheCCDtemperature, OPPQ,(ËtöwËu)
(℃),andAEtemperature, O_R (℃)usingtherelation:
GKLMN = (∆É + ∆tO_R + ∆uO_Ru )OPPQ,(ËtöwËu) + (ÈÉ + ÈtO_R + ÈuO_R
u ). (5.1)
OptimizedvaluesofthesixcoefficientsinthisequationarelistedinTable5.2.Figure5.2showscomparisons
betweenthisempiricalrelationshipandtheactualbiasmeasurements.WecanpredictthebiaslevelfromEq.
(5.1)within10%error.NotethataswasdiscussedinSec.3.2fortheONC-T,thebiaslevelmayalsodependon
thetemperatureoftheELEoftheONC-W1and-W2.WhentheELEtemperaturechangesdrasticallyduring
proximityobservations,theerrormightbegreaterandneedtobecorrectedforbydeductivemethodsbased
ontheONC-T.
Figure5.3showstime-dependentchangeofthebiaslevelatCCDtemperaturesaround-25℃ usingthe
inflightdataacquiredafterlaunch.Wefoundthatthebiaslevelsforbothcamerasstaynearlyconstant,within
1%changeover3years,similarlytotheONC-T’sbiaslevel(Fig.3.5).
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Figure5.1.BiaslevelsfortheONC-W1(left)andtheONC-W2(right)asafunctionoftheCCDtemperature.
Figure5.2.Comparisons between theactual biasmeasurementsand thosepredicted fromEq. (5.1). Left
figureisfortheONC-W1andrightfortheONC-W2.
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Figure5.3.Thetime-dependentchangeinthebiasleveloftheONC-W1andONC-W2.
Table5.2.CoefficientsforEq.(5.1)fortheONC-W1andONC-W2.
ONC-W1 ONC-W2
ƃ 0.680 0.573
∆t −5.74 × 10,ú −2.95 × 10,ú
∆u 1.06 × 10,› 8.92 × 10,≠
ÈÉ 260 288
Èt −1.72 −1.65
Èu 8.50 × 10,ú 6.29 × 10,ú
5.2.DarkCurrentandHot-PixelCorrection
On30Novemberand2December2017,weconducteddeepspaceobservationsusingtheONC-W1and-
W2inordertoevaluatetheirdarkcurrentlevels.TwodifferentCCDtemperatures, −25℃ and +20℃, were
usedforbothcamerasduringtheseobservations.Wetook4-imagesetswithexposuretimesof5.57secatboth
CCD temperatures for ONC-W1, and 44.56 sec at O»»j,Íu = −25℃ and 16.8 sec at O»»j,Íu = +20℃ for
ONC-W2.Afew0-secexposureimageswerealsoobtained,whichwereusedtoanalyzethebiaslevels(seeSec.
5.1).Brightspotsduetocosmicrayswereremovedbytakingamedianofthe4images,andthebiaslevelwas
removed by subtracting the 0-sec exposure images. Signals due to stars were omitted by comparing two
medianimagestakenfromthedifferenttwodays.Unfortunately,wecannotdistinguishstraylightanddark
current from the images takenat O»»j,(ÍtÎHÍu) = −25℃.Thus,weestimatedanupper limit for thedark
currentusingasectionofthemedianimage,wheretheeffectsofthestraylightappeartobesmall(rectangle
area{[200,200];[800,600]}pixelsonONC-W1images,and{[100,100];[900,900]}pixelsonONC-W2images).
At O»»j,(ÍtÎHÍu) = +20℃,weevaluatedthedarkcurrentontheentireimagesbysubtractingthestray-light
componentsestimatedfromtheimagesat O»»j,(ÍtÎHÍu) = −25℃.
Figure5.4(a)showsahistogramofthedarkcurrentfortheONC-W1at O»»j,Ít = −25℃ and +20℃.
TheaveragesandthestandarddeviationsareshowninFig.5.4(b),asfunctionsoftheCCDtemperature.We
definehotpixelsaspixelswherethedarkcurrentislargerthan30DN/s.Figure5.4(c)showsthenumber
ratioofhotpixelstototalpixelsintheanalyzedareaoftheimage.Weconfirmedoneofthepixelsaregreater
than30DN/sat O»»j,Ít = −25℃.SimilardatafortheONC-W2areshowninFig.5.5.Weconfirmedthreeof
the pixels are greater than 30 DN/s at O»»j,Íu = −25℃ . The hot pixel ratio of the ONC-W1 at the CCD
temperatureof +20℃ seemstobelessthanthatoftheONC-W2,andcomparablewiththatoftheONC-T(Fig.
3.7).
WeareplanningtoobservetheasteroidsurfacecloselyusingtheONC-W1,downtoafewmetersaltitude
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duringthetouch-downsequence.WecanunderstandthedetailsofthephysicalpropertiesofRyugu’ssurface,
suchasregolithgrainsizeandmorphology,withanimageresolutionofafewmm/pixelatthetouch-down
sites.Duringtheproximityobservations,theCCDtemperatureispredictedtoriseuptoafewtensofdegrees
Catmost,leadingtohighlargedarknoiselevels,estimatedtobeabout10DN/sat O»»j,Ít = 20℃.Fortunately,
however,theaveragedarkcurrentisnegligiblecomparedwiththeexpectedsignalof~50,000DN/sfromthe
reflectionfromtheasteroidsurface.Nevertheless,thehotpixelsshouldbecarefullyexaminedduringanalyses,
since1%of the imageareahasdark currentvalueshigher than100DN/s,~0.2%of theasteroid surface
brightness(themaximumvalueisashighas680DN/s,~1.3%ofasteroidsurfacebrightness)atthehigher
CCDtemperature(Fig.5.4(c)).Figure5.6showsamapofsuchhotpixels,whichneedstoberecognizedand
accountedfor,especiallyinexaminationsofpixel-scalemorphology.
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Figure 5.4. (a) Histogram of the dark current level, (b) the averaged values as a function of the CCD
temperature,and(c)numberratioofthehotpixelsfortheONC-W1.
Figure 5.5. (a) Histogram of the dark current level, (b) the averaged values as a function of the CCD
temperature,and(c)numberratioofthehotpixelsfortheONC-W2.
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Figure5.6.Hotpixelmapof(a)theONC-W1and(b)ONC-W2attheCCDtemperatureof20℃.Thehotpixels
inthisimagearedefinedasthepixelswherethedarkcurrentislargerthan100DN/s.
5.3.CharacterizationofStrayLightintheONC-W1and-W2
WeakstraylightseeninimagestakenbytheONC-W1and-W2wereobservedduringthecruisephase.
SimilartothetestswiththeONC-T,wehavetakenimagesunderavarietyofspacecraftattitudeswithrespect
totheSuninordertoexaminethepatternsandtheintensityofthestraylight.Wefoundthatthepatternand
intensityofstraylightintheONC-W1and-W2dependsonspacecraftattitudeinasimilarfashionasseenin
theONC-T.
Figure5.7showsthetypicalpatternsofstraylightobservedintheONC-W1.StraylightintheONC-W1
haveaspotlight-likepattern,unlikethegradualintensitychangeintheFOVoftheONC-T.Asthephaseangle
increases(XPNL),theobservedstraylightalsoincreases.Theintensityofthestraylightisevaluatedusingthe
medianofan11x11pixelboxinthespotlight-likezoneinordertoremovetheeffectsduetocosmicraysand
stars.BecauseoptimalexposuretimesfortheONC-W1duringRyuguobservationswillbe<50ms,straylight
darkerthan20DN/scannotbedetected.TheareaaffectedbystraylightisdefinedastheregionintheFOV
withgreaterthan20DN/sintheimage.Figure5.8displaystherelationshipbetweentheintensity,thearea
affectedbystraylight,andthespacecraftattitude.Thespacecraftattitudeisdefinedinthesamemannerasin
theONC-Tdiscussion.Exceptforoneattitude,thestraylightwasweakerthan30DN/s,whichwillresultina
1.5DNfor50-msexposures.AsseenintheONC-T,thespot-likestraylightissmallerandweakeratlarger Ïùn$ .
Moreover,theintensityofthestraylightisusuallyweakerfortheONC-W1thantheONC-Tat Ìùn$ < 50° and
isnegligiblewhenthestraylightfortheONC-Tareavoidedbytwistingthespacecraftaroundthe ÓFP axis.
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Figure5.9showsthetypicalpatternsofstraylightobservedbytheONC-W2.StraylightintheONC-W2
changesgraduallyinFOV.NotethatbrightpartsareseenattheupperandlowerrightcornersoftheONC-W2
FOV.Theyare thepartsof the frameof theONC-W2reflectingthe light source.Weevaluated theaverage
intensityofstraylightfortheONC-W2,excludingtheframeparts(redsquareinFig.5.9).Figure5.10displays
therelationshipoftheintensityofthestraylightandthespacecraftattitude.Optimalexposuretimesforthe
ONC-W2observationsofRyuguisestimatedtobe<20ms,shorterthanfortheONC-W1.Thus,thestraylight
<50DN/swillnotbedetected in suchRyugu images.TheONC-W2 is foundnottohave severestray light
contributionswhen Ìùn$ > 0°, theSun light illuminatesthe+XSCplane,butstraylightcontributionsoccur
whenthe -XSCplane is illuminated.Duringtherendezvous-phaseobservations,Sun lightwillradiateonthe
+XSCpanelatthenominalattitude.Thus, stray light in theONC-W2 isexpectedtobenegligible inmostof
rendezvous-phaseimagesofRyugu.
Figure5.7.Stray lightpatternsat Ïùn$ = 0° withachanging Ìùn$ = 15°, 30°, and48° (fromlefttoright).
Thestraylightisobservedatthelowerleftcornersintheimageswithlarge Ìùn$ .
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Figure5.8.(left)TheintensityclassificationoftheONC-W1straylight.Thestraylightisweakerthan20DN/s
(blueandgreen)andwillnotbedetectedduetotheshortexposuretimesfortheONC-W1,<50ms.(right)The
arearatioclassificationoftheONC-W1straylight.
Figure5.9.StraylightpatternoftheONC-W2at (Ìùn$, Ïùn$) = (0°, 0°).Theredsquareindicatestheregion
wherethestraylightintensitywasassessed.
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Figure5.10.The intensityclassificationof theONC-W2stray light.Thestray light isweakerthan50DN/s
(blueandgreen)andwillnotbedetectedduetoshortexposuretimefortheONC-W2,<20ms.
5.4.Flat-fieldCorrection
The flat-fields of theONC-W1and -W2weremeasured by using the portable self-emissive flatpanel
(CABINCL-5300L),whichwasalsousedtomeasurethe flat-fieldofONC-Tpriorto launch inTanegashima
(launchsite)(seeSuzukietal.,2018).Inthissection,first,weevaluatetheuniformityinreflectanceoftheflat
panelusedforthepreflightmeasurement.Then,theeffectofemissionangle(e)fromtheflatpanelisdiscussed
andtheemissionangleeffect-correctedflatfieldsoftheONC-W1and-W2areshown.
5.4.1Flat-FieldsBasedonthePreflightCalibrationData
Figure 5.11 shows the radiance distribution of the panel taken by the calibration camera (Orion
StarShootG3(Monochrome))whentheemissionangleissettobezerotoshowthereflectanceuniformityof
theflatpanelcalibrationsource.Anedgeofluminousareawithadimensionof350x280mmisclearlyseen
intheleftfigure.TheONC-W1and-W2tookimagesofthispanelfroma30cmdistanceduringthepreflight
calibrationmeasurements.TheapproximateFOVoftheONC-W1and-W2,duringthecalibration,areshown
bytheareaenclosedbywhiterectanglesinthefigure.Sincethereflectanceuniformityofthepanelwithinthis
areaisessentialforaflatfieldevaluationoftheONC,thecolorscaleinbothfiguresshowsvaluesnormalized
by themean radiancewithin this area.Figure 5.11(b) is the same as Fig. 5.11(a) but the color scale is
expanded to enhance the reflectance uniformity of the panel within FOV. The standard deviation of the
radiancewithintheFOVismeasuredbythecalibrationcameratobe1.7%.UnliketheONC-T,anemissionangle
dependenceoftheirradiancefromthepanelneedstobetakenintoaccountsincetheONC-W1and-W2have
wider FOVs. Since the flat plane is a self-emitting luminous plane, only the emission angle should be of
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concerned in this experiment.Figure5.12 shows the configuration for themeasurementsof theemission
angledependenceoftheradiancepropertiesofthepanel.Inthismeasurement,theradiancesfromthepanel
weremeasuredbyacalibrationcameraate=0˚,10˚,20˚,30˚,and35˚.Figure5.13showstheemissionangle
dependenceoftheradiancemeasuredbythecalibrationcamera.Theradianceisdefinedasthemeancounts
withintheareaofFOV(enclosedbywhiterectangleinFig.5.11)normalizedbytheequivalentvalueate=0˚.
Thisshowsthattheradianceofthepanelgraduallydecreasesasemissionangleincreases.Forexample,the
radianceis14%darkerate=35˚whichcorrespondstotheedgeoftheFOVoftheONC-W1.Duetothisrange
inemissionangleacrosstheONC-W1and–W2FOV,asinglerawimageoftheflatpaneltakenate=0˚bythe
ONC-W1or-W2cannotbeusedasaflatfieldimage,unlikethecaseofthenarrowangleFOVONC-T.Thus,a
correctiontoaccountforthiseffectisnecessarytoacquiretheactualflatfielddatafortheONC-W1and-W2
fromasingleflatpanelimage.Thecountdistributionsintheflatpanelimagestakenate=0˚bytheONC-W1
and-W2containboththeactualflatfieldspecificationofthecameraandtheemissionangledependencyof
radiance across the FOV. From the pre-flight measurement, we calculated the emission angle dependent
radiancemodelbyfittingthedatainFig.5.13(redline).AnactualemissionanglefromthecenteroftheFOV
toanypixellocatedinanimagecanbecalculatedbyusingthedistortioncoefficientsforbothcameras(Suzuki
etal.,2018).Therawcountsintheimageoftheflatpanelarethendividedbythenormalizedradiancemodel
ateachangleshowninFig.5.13.Byapplyingthiscorrectiontotherawcountsforallthepixels,theactual
sensitivityfortheflatfieldimagesfortheONC-W1and-W2areobtained.Figure5.14showsthesensitivity
flat fields of the ONC-W1 and -W2 calculated using this methodology. The values in these images are
normalizedbyameanvaluewithin50pixelsfromthecenteroftheimage(thecenterarea).Theprecisionof
these flat fields isdefinedbythespatial inhomogeneityof the flatpanel(~1.7%)sinceasensitivityof the
calibrationcamerawasconfirmedtobemuchmorestablethanthisvalue.Theabsolutesensitivityofthecenter
areameasuredbyusingtheintegratingsphereispresentedforbothwideanglecamerasinSec.5.5.
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Figure5.11.Radiancedistributionoftheself-emissiveflatpaneltakenbythecalibrationcamerawhenthe
emissionangleis0˚.Thetwoimagesaresameexceptforthecolorscaleinordertoenhancetheinhomogeneity
inimage(b).Thewhitesquareinimage(a)indicatestheapproximateFOVoftheONC-W1and-W2inthepre-
flightcalibration.
Figure5.12.Theconfigurationforthemeasurementoftheemissionangledependenceoftheradiance
acrossthewide-anglecameraFOVs.
Figure5.13.Therelationshipofnormalizedradianceandtheemissionangletakenbythecalibrationcamera.
Figure5.14.ThesensitivityflatfieldfortheONC-W1(left)andONC-W2(right).
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5.4.2.Flat-FieldEvaluationBasedonInflightStarandPlanetObservations
ThevalidityofthesensitivityacrosstheFOVoftheONC-W1isexaminedusing35Jupiterimagesobtained
fromFebruarytoMarch2016,approximatelytwoyearsafterHayabusa2’slaunch.Inthisobservationperiod,
Jupiter is locatedatvariouspositionswithintheONC-W1’sFOV.Sameasdescribed inSec.3.6,wemeasure
Jupiter’sbrightnessbyintegratingthedigitalcountsaroundJupiter(toincludebrightnessdistributedoutside
thephysicaldiskbythePSF).Adarkimageforthisobservationperiodiscreatedbyaveragingthe35images
where bright spots, suchas Jupiter, bright stars, and counts from cosmic rays hits are excluded.Flat field
correctionbasedonthepre-flightmeasurementsfortheONC-W1wasnotperformedfortheJupiterimages
duetodifferenceinthebrightnessdependenceontheincidentangleintheopticsbetweenapointlightsource
andasurfacelightsource,whichissignificantatalargeincidentangles.ThoughtheJupiterobservationsmay
notbeusedforvalidatingtheflatfield,theycanbeusedtomonitortheconsistencyintheONC-W1’ssensitivity
acrossitsFOV.
Since Jupiter’sbrightnessvarieswithphaseangle,weperformaphaseangle correctionbasedon the
JupiterbrightnessmeasurementswiththeCassini/ImagingScienceSubsystem(ISS)(Mayorga,etal.,2016).
Here,weusethecoefficientsforthegreen-bandoftheISS,whichhasasimilarwavelengthcoverageasthe
ONC-W1.Thepossibleerrorfromthephaseanglecorrectioniswithin~1%(Mayorga,etal.,2016).Thephase
anglevaluebetweentheSun,JupiterandtheHayabusa2spacecraftdecreasedfrom4°to2°,andthenagain
increased to 4° during this observation period. Finally, we corrected the brightness variation due to the
distance between the Sun and Jupiter and between Jupiter and the Hayabusa2 spacecraft. After these
corrections,themeasuredbrightnessofJupiterbytheONC-W1isstablewithinastandarddeviationof1.8%
(Fig. 5.15(a))withaweak incident angle dependencewith cos0.5i,where i is the incident angle.An ideal
pinholeopticshasacosidependenceforapointlightsource.NotethattheCCDtemperaturewas-25.1±0.3℃
in this observation period, and the sensitivity variation due to CCD temperaturewas not expected to be
significant.
ThesensitivityvalidationoftheONC-W2wasexaminedwithaseriesof60starimagestakenbetween
February2016andNovember2017.Onlystarsbrighterthan0.5V-magnitude(Achernar,Canopus,Procyon,
Rigel,Sirius,andAlphaCentauriAandB)wereusedsincetheyprovidedlargerDNsignalsthanmanyofthe
hotpixelsandcosmicrayhitsduetolongexposuretimeoftheONC-W2observations.Figure5.15(b)shows
thedistribution in the sensitivitydeviationsdeduced from thestarobservations, inwhicheach sensitivity
deviationismeasuredwiththesameapproachdescribedinSec.3.8.Thestandarddeviationofthesensitivity
intheFOVofONC-W2is4.4%,which issomewhat largerthanthoseof theONC-TandONC-W1.The large
standarddeviationcouldbefromthelargernumberofhotpixelsinONC-W2,whichmayoccasionallyoverlap
starlocations.Ifthelargest(15%)andthesecondlargest(9%)deviationsareexcludedfromthecalculations,
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thenthestandarddeviationreducesto3.7%.
Insummary,ithasbeenconfirmedthatboththeONC-W1and-W2donothaveanyclearsensitivitydefects
overtheirFOVs.Thedeviationsinsensitivity,afterflat-fieldcorrection,arewithinafewpercentinlargeparts
oftheirFOVs.Inaddition,wehavenotobservedanyclearsensitivitydegradationwithintheONC-W2detector
duringthe1.5-yearstarobservationperiod.Thevalidityoftheflatfieldcorrectionforasurfacelightsource
willbere-evaluatedwithclose-upobservationofRyugu.
Figure5.15.Sensitivitydeviationsin(a)ONC-W1FOVand(b)ONC-W2FOVasmeasuredfromJupiterand
starobservations,respectively.
5.5RadiometricCalibrations
5.5.1SensitivityCalibrationBasedonthePreflightData
TheradiometricsensitivitiesoftheONC-W1and-W2weremeasuredbyusinganintegratingsphere
priortolaunch.Thedetailsoftheintegratingsphereandtheprocedurestoquantifythesensitivityforthe
ONC-TaredescribedinKamedaetal.(2017).Thus,thedifferencesbetweentheprocedureusedtoquantify
thesensitivityoftheONC-TandthoseusedtoquantifythesensitivityoftheONC-W1and-W2aredescribed
here.ThemajordifferencescomefromthewiderFOVsoftheONC-W1and-W2(~70deg)relativetonarrow
FOVoftheONC-T(~6.5deg).DuetothisFOVdifference,theluminousplaneoftheintegratingspherecould
notfullyfilltheFOVsoftheONC-W1and-W2.Figure5.16displaystheimageoftheintegratingspheretaken
bytheONC-W1,whichshowsabrightcirculararea(theluminousplane)surroundedbyadarkarea.
Althoughitisdifficulttoderivethesensitivityforallpixelsbyusingthissingleimage,theaveraged
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sensitivityofthepixelswithina50-pixelradiusfromthecenteroftheimagecanbemeasured.Bycombining
thesesensitivityvaluesandthenormalizedsensitivityflat-fieldsshowninFig.5.14,thesensitivityofall
pixelsacrosstheFOVcanbecharacterized.SincetheONC-W1and-W2arepanchromaticcameraswitha
broadbandpass,themeanradianceoftheintegratingsphere, "(F,IWÒÒÒÒÒÒÒÒÒ [W/m2/sr/μm]iscalculatedby
"(F,IWÒÒÒÒÒÒÒÒÒ =∫ ûœÚÛ.Ùıˆ<Û.˜ıˆ< (d)f¯*<(d)Sd
∫ f¯*<Û.Ùıˆ<Û.˜ıˆ< (d)Sd
, (cam=W1 or W2) (5.2)
where "(F(a) istheknownspectralradianceprofileoftheintegratingsphere(seeFig.7ofKamedaetal.,
2017)and ΦIW(a) isthesystemefficiencyfunctionoftheONC-W1or-W2(SeeFig.2ofSuzukietal.
2018).Thecurrentoftheintegratingspherewassetto2.75Aforthesemeasurements.Theradiometric
sensitivities, \IWÒÒÒÒÒÒ [DN/s]/[W/m2/sr/μm],forthecenterareaoftheFOVsareobtainedby
\IWÒÒÒÒÒÒ =(
ûœÚ,¯*<ÒÒÒÒÒÒÒÒÒÒ , (5.3)
where G isthesignal[DN/sec]averagedoverpixelswithina50-pixelradiusfromthecenteroftheimage.
The \IWÒÒÒÒÒÒ valuesobtainedfortheONC-W1and-W2are 1.38 × 10ú and 3.84 × 10ú
[DN/s]/[W/m2/sr/μm],respectively.Itisnotedthatthesecoefficientsarereliableonlywhenthespectral
radiationofanobjecthasacomparablecolortemperaturewiththeintegratingsphere.Thus,themean
radianceobtainedbyusingthesecoefficientsisreliableonlywithinanorderofmagnitudeestimation,in
general.TheresultsofthisradiometriccalibrationaresummarizedinTable5.3.
5.5.2SensitivityCalibrationBasedontheInflightJupiterandStarObservations
ThesensitivityoftheONC-W1wasmeasuredbasedontheJupiterobservations,usingthesameimages
described in Sec. 5.4.2. The sensitivity was evaluated by comparing observed Jupiter digital counts and
expectedJupitercountsbasedontheONC-W1’sspecification(Suzukietal.,2018),whichwasdeterminedprior
tolaunch.WeusedtheJupiterimagessinceevenbrightstars(0V-magnitude)provideonlylessthan200DN
intotalsignalinanONC-W1image.Thesestellarobservationsarecomparabletocountsfromhot-pixelsand
typicalcosmicrayhits,duetotheshortexposuretimesoftheONC-W1(upto5.57sec),whileJupiterprovides
morethan2000DNintotalsignal.
Sameaswithstarcalibrations,thedigitalcountsexpectedfromJupitercanbecalculatedbyusingEqs.
(3.18)and(3.19).Unlikestars,however,thedisk-integratedJupiterbrightnessobservedbytheONC-W1has
ageometricaldependenceontheobservationgeometry.Theexpectedirradiance(W/m2/μm)fromJupiter,FJ,
canbeexpressedas
"e(a) = £e(a, ‰) eeÚ(d)
‹‡t_˚
QÚ%¸‚u
, (5.4)
whereαisthephaseangleatthetimeofobservation,AJ(λ,α)isthedisk-equivalentreflectanceofJupiter,ΩJis
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the solid angle of Jupiter as seen from the Hayabusa2 spacecraft, JS(λ) is the modeled solar irradiance
(W/m2/µm)at1AU,andDS-JisthedistancebetweentheSunandJupiterinAU.Thedisk-equivalentalbedois
calculatedfrom
£e(a, ‰) = £e(a, ‰É)˝(d,Á)
˝(d,ÁÛ) , (5.5)
whereΨ(a, ‰) representsthephaseangledependenceofAJ(λ,α),andα0isthereferencephaseangle.Inthis
study,weusedasolarirradiancemodelfromGueymard(2004),theJupiterreflectancedatafromKarkoschka
(1998)takenin1995ataphaseangleof6.8°(i.e.,α0=6.8°),andthephaseangledependencefortheISSgreen-
bandfromMayorgaetal(2016),asdescribedinSec.5.4.SincetheapparentsizeofJupiterwasmuchsmaller
than1-pixelintheONC-W1FOV,thesolidangleofJupiterinsteradianisapproximately
e = •w ˇw !Q¸%"·
, (5.6)
whererJeandrJparetheequatorialandpolarradiiof Jupiter,respectively,andDJ-H is thedistancebetween
JupiterandtheHayabusa2spacecraftatthetimeofobservation.BothDS-JandDJ-Hweremorethan5AUduring
theobservationperiod.
Fromthe16JupiterimagesinwhichJupiterwaslocatednearthecenteroftheFOV,wherevariationsin
the sensitivity are not significant (Sec. 5.4.2), the sensitivity of the ONC-W1 was measured to be
(1.44 ± 0.02) × 10ú [DN/s]/[W/m2/sr/μm],whichishighlyconsistentwiththesensitivitymeasuredpriorto
launch.Theerrorrange isevaluatedfromthestandarddeviationofmeasuredsensitivities from35Jupiter
observations. It should be noted that the sensitivity value from the Jupiter observations has errors from
possibletemporalvariationsinJupiter’sreflectance(forexample,acoupleofpercent(Karalidietal.,2015)),
uncertainties inmeasuring Jupiter’s brightness in theONC-W1 images (2%based on standard deviation),
errorsintheJupiterreflectance(4%inabsolutevaluebyKarkoschka(1998)),anderrorsduetothephase
angledependence(upto1%byMayoruga,etal.(2016)).Inaddition,Jupiterandtheintegratingspherewe
usedinthepre-flightmeasurementshavedifferentspectralprofiles.Thesecouldprovidedifferencesbetween
thesensitivitiesderivedfrompre-flightandinflightmeasurements.
ThesensitivityoftheONC-W2wasmeasuredbasedonstarobservationsconductedfromFebruary2016
toNovember2017.StarsimagesusedforthismeasurementaresamefortheevaluationoftheONC-W2flat-
fielddescribed inSec.5.4. Comparing theobservedDN to theexpectedDNbasedon star irradiances, the
sensitivity of the ONC-W2 in 6 star observations was evaluated as (4.03 ± 0.07) × 10ú
[DN/s]/[W/m2/sr/μm] at the center of FOV, which is consistent with the sensitivity from the preflight
measurement.Inaddition,thisvaluecorrespondstoa47%lowersensitivitythanexpectedfromtheONC-W2
specification,whichwas commensuratewith the results from the Earth observations,where theONC-W2
measuredEarth’sreflectancetobe40%darkerthanthereferencevalue(Suzukietal.,2018).
6 1055 52 364
ItcanbeconcludedthatboththeONC-W1and-W2basicallyretainedtheirpreflightsensitivitiesduring
thecruisephase.Continuoussensitivitymonitoringwithbrighttargetswillprovideacontinuingmeasureof
thestabilityofthesensitivityinflightforthesecameradetectors.
Figure5.16.AnimageoftheintegratingspheretakenbytheONC-W1.
Table5.3.Summaryof the radiometric calibrationof theONC-W1and -W2basedonpreflight integrating
spheremeasurements.
Camera Date Current
settingofthe
integrating
sphere.
\IW
[DN/s]/[W/m2/μm/sr]
ONC-W1 27March2014 2.75A 1.38 × 10ú
ONC-W2 27March2014 2.75A 3.84 × 10ú
5.5.3.PointSpreadFunctions(PSFs)
The PSF of theONC-W1 is evaluated from observations of bright targets, i.e. Mars, Jupiter, and stars
brighterthan1V-magnitude(Betelgeuse,Capella,andProcyon).Marsimagesweretakenon31May2015,and
Jupiter imagesarethesameimagesusedforthe flat-fieldandsensitivityevaluation.Thestar imageswere
takenon12March2017.ThePSFoftheONC-W2wasalsoevaluatedfromstarsbrighterthan1V-magnitude,
takenbetweenFebruary2016toNovember2017.Figure5.17showsasummaryoftheFWHMofeachbright
target for theONC-W1and-W2.TheFWHMvaluesareevaluated froma two-dimensionalGaussian fitting
method,sameasthatusedformeasuringthePSFoftheONC-T(Suzukietal.,2018).Duetotheshortexposure
timeoftheONC-W1(upto5.57sec),onlyalimitednumberofstarimageswereused,while142starsamples
wereavailableformeasuringthePSFoftheONC-W2,whoseexposuretimewas44.56sec.
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AroundthecenteroftheFOV,theFWHMwas~1.8pixels,similartotheONC-TforbothONC-W1and-W2,
andtheyarealmostsameas(slightlybetterthan)theFWHMdeterminedfromthepre-flightmeasurements.
ThelargerFWHMattheimagecornerintheFOVoftheONC-W2couldbecausedbyavignettingeffectfroma
baffle,sameasfortheONC-T(Suzukietal.,2018).WhiletheFWHMoftheONC-W1showsanincreasingtrend
withdistancefromthecenteroftheFOV,theFWHMvaluesinlargepartsofboththeONC-W1and-W2FOVs
are smaller than each FHWM value at the image corner from the pre-flightmeasurements, indicating no
significantdegradationofthePSFafterlaunch.Itshouldbenotedthatsincethestarsare20-50timesdarker
thaneitherJupiterorMarsintheONC-W1,duetotheshortexposuretimeoftheONC-W1stellarobservations
(5.57sec),starobservationsprovideratherlargeerrorsintheFWHMoftheONC-W1.
Figure5.17.ThemeasuredFWHMof(a)theONC-W1and(b)theONC-W2PSFasafunctionofthepixel
distancefromthecenterofFOV.TrianglesanddottedlinesindicatePSFvaluesatthecenterandcornerofthe
FOVmeasuredinpre-flightexperiments.
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6.ONCSystemAlignmentwiththeSpacecraft
UnderstandingthealignmentoftheONCsystemwithrespecttothespacecraftisessentialforcomparing
themeasurementswithinthedifferentFOVsormatchingfootprintswithotherinstruments,suchastheTIR
andNIRS3,orevenwhenwecompareimagestakenfromtheONC-TandONC-W1.Moreover,thealignment
informationisnecessarytocalculatetheaccurateattitudeandpositionofthespacecraftfromimagesduring
operation.Inthissection,wepresenttheresultsofmeasurementsderivingtheviewingdirectionoftheONCs
relativetothespacecraftattitude.AsmentionedinSuzukietal.(2018),theFOVsoftheONC-W1andONC-
Tweredesignedtoaligninthe−ZSCdirectionwithinthespacecraftcoordinatesystem,whilethatofthe
ONC-W2wasslantedbyapproximately30˚fromthe−ZSCdirection.However,actualalignmentsmayhave
small offset from thesedesignedvaluesdue to an ineluctable error in assemblyproceduresprior to
launchor offsets createdby the strongvibrationsduring launchof theHayabusa2 spacecraft. These
offsetsaremeasuredpreciselybyusingstarfiledimagestakenduringthecruisephase.Starfieldimages
usedinthedeterminationandverificationofthedistortioncoefficients(seeSuzukietal.,2018)ofthe
threecamerasarealsovalidatedinthismeasurement.InSecs.3and6ofSuzukietal.(2018),celestial
coordinateswerefittedtothestarfieldimagestoverifytheestimateddistortioncoefficients(SeeFig.5
fortheONC-W1and-W2,andFig.12fortheONC-T).Fromtheseresults,rightascensionanddeclination
anglescorrespondingtothecenteroftheFOVattheobservationtimeforeachcameracanbedetermined.
Ontheotherhand,informationofthespacecraftattitudeattheseobservationperiodsarealsoavailable
fromthetelemetrydata.ThisallowsustopredictpointingdirectionsandthecentersoftheFOVsofthree
camerasduringtheobservationperiodsbyassumingthatthecameraswereperfectlyassembledtothe
spacecraftasdesigned.Thus,theoffsetvaluesbetweenthepredicteddirectionofanopticalaxisofeach
cameraandthatestimatedfromtheactualstarimagecanbedefinedasalignmentinformation.Table6.1
summarizestheobservationdatesandtimesofthestarfieldimages,theobservedcenteroftheFOVin
thecelestialcoordinatesystem,thecenteroftheFOVpredictedbytheattitudeinformation,andtheoffset
betweenthem(i.e.,thealignmentinformation)intermsofanglesandinimagecoordinate(inpixels).The
lastcolumnofTable6.1givesthealignmentinformationintheimagecoordinate(fordefinitionsseeFig.
1ofSuzukietal.,2018)definedas
Δx = xöKN − 511.5[pixels] (6.1)where,the xöKN isobservedlocationofthepredictedcenter(e.g.,thedirectionof−ZSCfortheONC-W1
andONC-T)andthevalue511.5pixelsisthelocationofimagecenter.Wealsocanexpressalignmentby
Eulerangleswithrespecttothespacecraftcoordinatesystem.Thematrixtransformingfromthe
spacecraftcoordinatesystemis
ƒ = %cosΘ) sinΘ) 0− sinΘ) cosΘ) 0
0 0 1*%
cosΘ+ 0 − sinΘ+0 1 0
sinΘ+ 0 cosΘ+*%
1 0 00 cosΘ, −sinΘ,0 sinΘ, cosΘ,
*, (6.3)
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where Θ, ,Θ+ , andΘ, are Euler angles from the spacecraft coordinate system to the instrumentcoordinatesystem.TheEulerangelsforONCarealsolistedinTable6.1.
*Suzukietal.(2018)
Table6.1.Alignmentinformationforthethreecamerasestimatedbyusingstarfieldimages.
7.NIRS3
Hayabusa2 payload includes the near infrared spectrometer NIRS3, whose purpose is to detect and
measuretheabsorptionaround3µm,associatingwiththehydrationofmineralspecies(Iwataetal.,2017).
NIRS3isdesignedtoobservewavelengthsfrom1.8to3.2µm.Itisimportanttoconnectthespectrafromthe
ONC-TandNIRS3instrumentstomeasurethespectralcharacteristicsoverabroadrangeofwavelengths,from
theultraviolet (UV) through thevisible (vis) to thenear infrared (NIR).Thiswill allowus toexamine the
mineralogyofRyuguusingtheintegratedspectrafrombothinstruments.Inthissection,weshowtheONC-T
Camer
a
Observation
time for the
starimages.
YYYY-MM-
DDTHH:mm:
ss
Observed
center of the
FOV in (RA,
DEC)[˚].
Predicted
center of the
FOV in (RA,
DEC).[˚]
Offset
in
angle.
[˚]
Offset of
the center
of FOV in
the image
coordinate
(alignment
informatio
n)
(ΔH, ΔV)
[pixels].
Euler
angles
(
Θ),Θ+ ,Θ,)[Degree]
Distortion
parameter
s*(-t,-u),
°†
= ° + -t°ú
+ -u°≠.
Focal
length*
(mm)
W1 2015-02-
19T09:50:10
(138.880,6.15
0)
(138.789,6.47
5)
0.313 (-3.5,-2.5) (0.0, -
180.2728,
0.1948)
(3.134×10-
7, -
1.716×10-
13)
10.22
W2 2014-12-
11T13:38:12
(136.500,6.02
0)
(136.484,6.05
1)
0.051 (0.5,-0.5) ( -270, -
121,
0)
(2.893×10-
7, -
1.365×10-
13)
10.38
T 2014-12-
11T14:52:53
(77.388,21.96
1)
(77.532,
21.996)
0.151
0
(9.5,-22.5) (0.0, -
179.9415,
0.1386)
(-9.28×10-
9,0)
120.50±0.0
1
6 1055 52 364
andNIRS3spectraoftheMoonobservedalmostsimultaneouslyon5December2015(Kitazatoetal.,2016).
TheconsistencyofthespectraisdiscussedbasedonthecomparisonswiththeSP/SELENEmodel(Yokotaet
al.,2011;Kouyamaetal.,2016)forvis-NIRspectra,sincethereisnooverlapinthewavelengthrangebetween
the two instruments. In this comparison, we used a modified SP/SELENE model which covers longer
wavelengths(~2μm)basedontheupdatedreflectancedatabyYokotaetal.(2012)thanthemodelinYokota
etal.(2011)andKouyamaetal.(2016).ThereflectancespectrumfromtheSPmodelwascorrectedwiththe
ROLOreflectancemodelsincetheSPmodelshowssomewhatdarkerandbrightenertendenciesinshorterand
longerwavelengthranges,respectively(Ohtakeetal.,2013).
First,thealignmentoftheONC-TandNIRS3arecalculatedbasedontheSPICEInstrumentKernel.The
NIRS3alignmenthasbeendeterminedthroughscanobservationsof theEarthcombinedwithslewsofthe
spacecraftattitude.Theerrorinalignmentiswithin+/-0.005˚,correspondingto~5%ofthesizeoffieldof
view.Figure7.1showsthealignmentoftheNIRS3intheFOVoftheONC-T,showinglittleoffsetfromthecenter
oftheFOV.ThefootprintoftheNIRS3is{[476.5,454.3];[492.7,472.1]}pixelsintheONC-Timagecoordinate
system.NIRS3onlyobtainedspectraofpartoftheMoon’ssurfaceduetoitsnarrowfootprint(Fig.7.2).The
comparisonsofthelunarspectrafromdifferentpartsofthesurfaceareshowninFig.7.3.Wealsodisplaythe
modelspectrafromSP/SELENEtoreasonablyextrapolatetheONC-TspectrumtotheNIRS3spectrum.There
seemstobelittleoffsetbetweenthespectraoftheONC-T,NIRS3,andmodelspectra.Notethattheradiometric
calibrationof theNIRS3wasprimarilybasedonpreflight calibration testdata.TheSPmodelspectramay
underestimatetheradianceforlargeincidenceangleregionsasshowninKouyamaetal.(2016),whichisof
importancesincethefootprintsoftheNIRS3arelocatedathigherlatituderegionswheretheincidentangleis
large.Thus,weshifttheSP-modelspectratofittheONC-Tv-bandradianceandtheNIRS3spectratofittheSP-
modelintherangebetween1819to2038nm.WecalculatedtheoffsetfactorbetweentheONC-TandNIRS3
as0.87(Fig.7.4).Usingthisvalue,wecanevaluateslopeoftheintermediatewavelengthfrom0.95to1.8µm.
Thiswavelengthrangewasobservedbyground-basedtelescopes(Moskovitzetal.,2013;LeCorreetal.,2017)
andisreflectiveofthemineralogicalcompositionofthesurface.Moreover,thereisasignificantdifferencein
theslopesinthiswavelengthrange,whichwasobservedbyMoskovitzetal.(2013)andLeCorreetal.(2017).
Thus,wemayobservesomeheterogeneityintheslopeinthiswavelengthrangeoverthesurfaceofRyugu.
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Figure7.1.ThefootprintoftheNIRS3intheFOVoftheONC-T.
Figure7.2.SquaresindicatetheROIsoftheNIRS3observationsinanONC-Timage(red(ROI1):11:35:14on
5December2015,green(ROI2):11:42:27on5December2015,yellow(ROI3):11:45:29on5December2015).
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Figure7.3.Radiance comparisonof the lunar spectrumobtainedby theONC-TandNIRS3.Thegray lines
indicatetheSPmodelspectrabyKouyamaetal.(2016)andthegraysymbolsindicatetheWACmodelspectra
bySatoetal.(2014).NotethatradiometriccalibrationsoftheONC-TandNIRS3wereconductedindependently.
Figure 7.4. Corrected radiance of the Moon observed at 11:35:20 on 5 December 2015. The NIRS3 and
SP/SELENEmodelspectraareshiftedbythecoefficientsinthelegendtofittheONC-Tradiance.
8.ApplicationstoScientificAnalysesonRyugu
Wehaveevaluatedthebias level,EMInoise,dark level,hotpixels, linearity, flat-fields, stray light,and
sensitivityofthedetectorsandfiltersfortheONCsystem.Thissectionaddressestotheevaluationofaccuracy
thekeyscientificanalysestoanswerimportantnatureoftheasteroid.Morespecifically,oneistodetectthe
presenceofhydratedminerals,whichisanindicatorofthepresenceofwater(pastorpresent)ontheasteroid,
and the another is to characterize the real radiance variations on its surface, even when temperature
conditionsarechanging.Weevaluatetheaccuracythatisexpectedbasedonthecalibrationstepswhichare
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describedinprevioussections.
8.1.Signal-to-NoiseRatio
The signal-to-noise ratios (SNRs) of the ONC-T, -W1, and -W2 are important for establishing the
detectabilityofradiancevariationovertheasteroidsurface.Thescientificqualityoftheimagesischangedby
temperatureconditions.Unlikeorbitingmissions,theHayabusa2spacecraftwillexperiencealargerangeof
temperatures,becausethechangeinthedistancebetweenthespacecraftandtheasteroidincludesradiative
heatchangewhenatlowaltitudes.Especially,duringtouchdownstheCCDtemperaturescanbeashighas20
℃.Thereby,understandingtheeffectsofthetemperatureonthedetectorperformanceisnecessaryinorder
toanalyzetheclose-approachimages.Ourcalibrationresultsshowthetemperaturecauseslargeeffectson
darknoise.Here,wediscusstheSNRsdependenceontheCCDtemperature.
Thetotalnoise ∆G canbedefinedasthecombinationofread-outnoise ∆Gwö ,EMInoise ∆GR/( ,shotnoise∆GNfiöA70G[DN] ∙ 3[e, DN⁄ ] 3[e, DN⁄ ]5 whereg=20.95[e-/DN]isthegainfactor,anddarknoise ∆GgMw} :
∆G = 0∆Gwöu + ∆GR/(u + ∆GNfiöAu + ∆GgMw}u [DN], (8.1)
where,onlythedarknoiseisafunctionoftime,buthasanegligiblecontributionatthereferencetemperature
(-30 ℃).BecausethethreecameraswithintheONCusethesamemanufacturedCCDswiththealmostsame
characteristics,theONCcamerasallimagewithaSNR~200atthereferencetemperature.However,oncethe
CCDtemperaturegetshighas20 ℃,thedarknoisecontributionwilldominantinthetotalnoiseandyieldsa
SNR~150fortheONC-Tv-band,whiletheONC-W1andW2willbelessaffectedduetotheshorterexposure
timesduringoperation.Thesevaluesuggestthatthevariationacrosstheasteroidsurfaceoverasingleimage
canbemeasuredwithinerrorsof0.7%attheworstcase.Moreover,theremaybeunavoidableeffectofhot
pixels(>100DN/s),sothat~1%oftheFOVareawillnotbeuseable.
8.2.Detectabilityof0.7-µmAbsorptionBand
One of themost important inquiries for the spectralmapping on Ryugu is the determination of the
presenceofhydratedminerals,whichareindicatorsofaqueousalterationprocessesonprimordialasteroids.
AsteroidsareconsideredtobeapossiblesourceofwatertotheinnerSolarSystem,includingourownEarth
(e.g.,O’Brienetal,2014;Altweggetal.,2014).Moreover,thedegreeofaqueousalterationalsoprovidesinsight
intotheheatinghistoryofasteroids.Here,wediscussthetotalerrorrevealedbythecalibrations,especially
sensitivity,andtheirimpactonthepossibledetectionofhydratedmineralsfromONC-Tmultibandimages.
OurtargetasteroidRyuguiscategorizedasaC-complexasteroid,whicharetheorizedtohaveformedin
the volatile-rich region in theSolar System, in the Bus andBinzel taxonomy (BusandBinzel, 2002). The
presence of hydrated minerals is usually discussed by the most prominent and unambiguous 3-micron
absorption(e.g.,Lebofsky,1980;TakirandEmery,2012).However,the3-µmbandisdifficulttoobserveon
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smallasteroidsfromtheEarth.Thereby,theattributedironcharge-transfer0.7-µmabsorption,whichiseasier
to observe, is used as the reliable proxy for hydrated minerals (e.g., Vilas et al., 1994). Ground-based
observations have established the correlation between the 0.7- and 3-µm absorptions among C-complex
asteroids(e.g.,Howelletal.,2011).ManyobservationsofRyuguhavebeenmadetodatetoobtainmineralogy
informationpriortoHayabusa2’sencounter(Binzeletal.,2001;Vilas2008;Lazzaroetal.,2013;Moskovitzet
al.,2013;Sugitaetal.,2013;Pernaetal.,2017).Mostofthesespectradisplayveryflatspectralshapes,while
only one of them showsevidence of a 0.7-µmabsorption (Vilaset al., 2008). This suggests theremay be
variations in the compositionacrossRyugu’ssurfacethatare characterizedby thepresenceorabsenceof
the0.7-µmabsorptionfeature.Thus,itisimportanttomapoutthe0.7-µmabsorptionbytheONC-Tbasedon
v-,w-,andx-bandobservations.Thedepthof0.7-µmabsorptioncanbemeasuredby
rÉ.6 = 1 −ú.tÊ7
t.õÊ8zt.≠Ê9. (8.2)
Kamedaetal.(2015)reportedthatthe0.7-µmabsorptiondetectiononCM2chondrites,suchasMurchison
andNagoya,bytheflightmodelofONC-T.Fromourupdatedsensitivity,theambiguityinthesensitivitiesfor
the v-, w-, and x-bands are 0.85%, 1.3%, and 1.6%, respectively. The error of 0.7-µm absorption depth
calculatedfromerrorpropagationofEq.(8.2)are1.6%.Thereby,thetypicalabsorptionofserpentine(3-4%)
canbedetectedbyaSNR~2.Thisvaluecanbeimprovedbydecreasingthestatisticalerrorsbasedonmore
starobservations.
8.3.UV-SlopeEvaluation
Asisnotedabove,mostoftheground-basedobservationsofRyugudonotreportanabsorptionaround
0.7-µm. Rivkin (2012) reported that one third of C-complex asteroids in Sloan Digital Sky Survey, which
includeslargenumberofasteroidsdowntosmallobjects,showthe0.7-µmfeature,whereastwothirdsdonot.
Moreover, the0.7-µm feature seen inCMchondrite spectraeasilydisappearswhendehydratedbyheating
(Hiroietal.,1996)andspaceweathering(Matsuokaetal.,2015).Thus,thereispossibilitywewillnotfind0.7-
µmfeatureonRyugu.However,thisdoesnotindicatethattherearenohydratedmineralsonthesurface.Other
than0.7-µmabsorptionfeature,thestrengthofUV-absorptionshortwardof0.55µm,whichiscausedbyFe2+
andFe3+insilicates,alsocorrelatespositivelywiththe3-µmfeature(e.g.,Freierbergetal.,1985).Vilasetal.
(1993)investigatedtherelationshipof0.7-and0.43-µmabsorptionsandsuggestedthatasteroidswith0.7-
µmabsorptionalsodisplaythe0.43-µmabsorption.Moreover,asteroidspectrathatdidnotdisplaythe0.43-
µmfeaturealsodidnotshowthe0.7-µmfeature.Thus, theUVabsorptioncanbealsoaproxyforaqueous
alterationintheONC-Twavelength,suchasaspectralturnovernearthev-band.Thus,wecanusetheUVto
blue spectral slope index instead of the 0.7-µm absorption. TheUV slope can be calculated by ln(Rul/Rv).
AccordingtoHiroietal.(1996),therearelargediscrepanciesofthisvaluefrom-1to0amongC-,G-,B-,andF-
typeasteroidsintheTholentaxonomy(Tholen,1984),andthisvaluecorrelatesnearlylinearlywiththe3-µm
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bandabsorption.Althoughtheyused0.34µmfortheshorterwavelength,ul-bandwavelengthrange,0.4-µm,
canserveasthesameproxy.Basedonoursensitivityambiguity,theerrorforthisvaluewillbe~1%inthe
centeroftheFOV.However,itshouldbenotedthatthespatialerrorcouldbelargebecauseofthedeviationin
ul-bandflat-fieldcausedbytheround-aboutstraylightonthepreflightflat-fieldmeasurement.Forthatreason,
thespatialuncertaintyofmeasuringtheUVslopewillbedominatedby flat-fielduncertaintyof~4%.This
errorestimationstillsuggeststhatwecandistinguishunheatedCMsof ln(Rul/Rv)~-0.6to-0.3 fromhighly
dehydrated(700-1000 ℃)CMsof~0quiteeasily,assumingthatRulandR0.34µmarelinearlycorrelated.
8.4.FirstInflightObservationofRyugu
ThefirstinflightobservationsofRyuguwereconductedon26February2018whentheanglebetween
theSun-Ryugu-Hayabusa2was~1.1-1.6˚.Takingadvantageof the illuminateconditionclosetoopposition,
RyuguwasapparentlybrightandwesucceededinobservationwiththeONC-Tbylongestexposuretime(178
sec).Weconductedtwodifferentobservationsfordifferentpurposes;1)lightcurveobservationand2)Ryugu
multi-band imaging.Onthe lightcurveobservation, thewide-bandwasused inordertostackthesignalas
much as possible. On themulti-band observation, we observed different rotational phases.We took four
sequentialimagesetsforeachbandatonerotationalphase.ThesummaryofobservationsarelistedinTable
8.1.Afterdarkandflatcorrections,invalidimages,whichincludecosmicrayhitsclosetoRyuguduetolong
exposureswereomittedfromtheanalyses.Thebackgroundnoise,suchasdarkcurrentandhotpixels,were
maincauseoferrorsinmeasurements.Thelightcurveisobtainedbytakingthreemovingaveragesovertime
(Fig.8.1).
Onthemulti-bandobservations,theimageswereacquiredevery¼rotationalphasefor7sets.Duetothe
very small signals,we employ two analysismethods to obtain robust results.Onemethod is theaperture
analysis,which isusuallyappliedtostar fluxanalysesasdiscussed inSec.3.Anothermethodistherough
estimationfromthepeaksignalofRyugu.Morespecifically,weobtainedtherelationshipbetweenthepeak
signalandtotalsignalfromstarobservations,inwhichsignalsaremuchlargerthanthebackgroundsignals.
However,thesignalratioofpeaktototalmaychangefrom0.15to0.33withtheRyugudetectedareainone
CCDpixel.Becausethesignalsoftheul-andp-bandsaresmallerthanthedarklevel,<0.05DN/s,thespectral
shapeisdiscussedwithoutthesetwofilters.Figure8.2displaysnormalizedreflectancespectraofRyugufrom
thetwomethodsfromb-tox-bands.Wefoundthatthiscasewithverylowsignalrate,theroughestimategives
smaller errors. Although, the shapes of reflectance spectra are not perfectly reproducible at two similar
rotationalphase,suchasobservationsetof2and6inFig.8.2,theslopeofreflectancespectraforallrotational
phasesvariesto include0(flat)consideringerrors inmeasurements.Suchflatspectraareconsistentwith
ground-basedobservations(Binzeletal.,2001;Vilas2008;Lazzaroetal.,2013;Moskovitzetal.,2013;Sugita
etal.,2013;Pernaetal.,2017).Theremaybedarkening intheb-bandcomparedwiththev-bandformost
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rotationalphases.Furthermore,wecomparethemagnitudeofthev-bandobservedwiththeONC-Tandthe
correspondingV-magnitudeoftheEarth-basedobservationsfromIshiguroetal.(2014).Becausewestackthe
signal fromall 28 v-band images to reduce theerror, the v-bandmagnitude here corresponds to a global
average,ordisk-integratedvalue.Figure8.3showsthecompiledphasecurveofRyugu,displayingconsistency
betweenONC-TandEarth-basedmeasurements,althoughtheerrorbarisstilllarge~10%.Thisresultalso
supportstherobustnessofthecalibrations.
Table8.1.SummaryofRyugufirstinflightobservations.
ObservationTime(UTC) NumberofImages
Lightcurveobservation(wide-band) 26February201803:01–10:28 100
7-bandobservation(1) 26February201810:39–12:01 28
7-bandobservation(2) 26February201812:33–13:56 28
7-bandobservation(3) 26February201814:28–15:50 28
7-bandobservation(4) 26February201816:22–17:45 28
7-bandobservation(5) 26February201818:15–19:39 28
7-bandobservation(6) 26February201820:11–21:34 28
7-bandobservation(7) 26February201822:04–23:28 28
Figure8.1.First inflight lightcurveobservationofRyugutakenon26February2018.Everypointshowsa
threepointsmovingaverageanderrorsareevaluatedbythedeviationbetweenthreepoints.Duetothechange
indistancebetweenthespacecraftandRyugu,thesignalsarenormalizedbythesquareofthedistance.
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Figure8.2.5-bandobservationsofRyuguwiththeONC-Ton26February2018.Thecolorlinesindicatethe
reflectanceobtainedbyapertureanalysesandgraylinesindicateroughestimatesbasedonpeaksignalrate.
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Figure8.3.ThephasecurveofRyugu,compilingtheEarth-baseddatafromIshiguroetal.(2014)(black)and
theinflightmeasurementsfromthisstudy(red).TheinflightONC-TmeasurementoverlapswithEarth-based
measurements.
9.Summary
Inthisstudy,wehavepresentedthecalibrationsofthethreeONCcamerasduringthecruisephaseover
thepastthreeyears;includingcorrectionsforread-outsmearnoise,bias,EMInoise,dark,hotpixels,linearity,
flat-field,straylight,andCCDsensitivity.NotethattheCCDsofONC-T,-W1,and-W2havesamemanufacturer
productnumber.Thus,thebasicbehaviorisexpectedtobesimilarforallthreecameras.
ONC-T
l Read-outsmearcanbeomittedusinginflight0-secexposureimagesubtraction,orbymodelingusingthe
empiricalrelationshipinEq.(3.4),similartothemethoddescribedbyIshiguroetal.(2010).Weshowed
thevalidityofthissmearcorrectionusinganONC-W2imagesincetheverticalchargetransfertimehas
notbeenmeasuredfortheONC-T.WewillmeasuretheverticalchargetransfertimeoftheCCDforthe
ONC-TduringtheapproachphasetoRyugu.
l Biasleveldependsonthetemperatureandcanbedescribedbyanempiricalbiasmodel.Basedonthe
modelthebiaslevelwasshowntobestablewithin1%deviationsoverthepastthreeyears.
l EMInoisecauseswave-likepatternsonimages.Theamplitudeshavebeen<5DNforthecruisephase.
l DarkcurrentandhotpixelsaregreatlychangedbytheCCDtemperature.Atthereferencetemperature,
thedarkcurrentissmallerthan0.09DN/s,whichisconsistentwithpreflightmeasurementsandlessthan
10hotpixels>30DN/sarefound.AttheCCDtemperatureof20℃,thedarkcurrentis20DN/sandhot
pixels>100DN/soccupy~1%oftheFOV.
l ThelinearityoftheCCDwasfoundtobeslightlydegradedfromthe0.6%errorpreflightto1%inflight.
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Theempiricallinearitycorrectionmodelcorrectsthelinearityto<0.1%.
l Flatnessof the flat-fields forv-bandandul-band isevaluatedbasedonstarobservationswithslightly
differentattitudes.Thesensitivityvariationsinthev-bandflat-fieldandtheul-bandflat-fieldareabout
2%and4%,respectively.Theul-bandflat-fielddiscrepancyislargerthanthev-bandbecausetheround-
aboutstraylightmightcontaminateandmakelittlegradationintheFOVduringpreflighttests.
l Scattered light in theopticsarecharacterizedandthePSFmodelsarederivedforallcolor filters.The
effectofabroadPSFintheONC-TissmallerthanthatontheAMICA/Hayabusa.Theeffectcanbecorrected
inawaysimilartothatdescribedbyIshiguro(2014).
l Theghosteffectwasobservedandismostapparentinthep-filter,withasignalasmuchas0.65%the
intensityoftheobjectobserved.
l Suzukietal.(2018)reportedtheradiatorstraylightdependenceonspacecraftattitudewithrespectto
theSun.Theradiatorstraylighthasbeenintensivelyobservedduringthecruisephase.Theattitudeswith
negligiblestraylightwerefoundandreportedinSuzukietal.(2018).Inthisstudywepresentthestray
lightmodel based on PCA. Thismodel reduces the radiator stray light contamination to 25DN/s/pix,
0.25%theexpectedintensityoftheasteroiddisk.
l CCDsensitivitywascarefullyexaminedbasedonstar,Jupiter,Saturn,andMoonobservationsinaddition
to preflightmeasurements.We characterized the sensitivity in twoways; 1) based on the hardware
specificationsoftheONC-T,and2)basedonstarobservations.Moreover,thesensitivityofthep-bandis
moreaccuratelydeterminedbasedonthelunarobservationsthanstellarobservationsduetotheinherent
ambiguityinthestellarreferencespectra.Sincethesensitivitybasedonstellarobservationsfortheul-to
x-bandsandthelunarobservationsforthep-bandreproducedthespectraoftheMoon,Jupiter,andSaturn,
currentcalibrationsusethischaracterizationofthesensitivity,summarizedinTable.3.10.Moreover,we
measured the temperature dependency of the sensitivity using Jupiter observations acquired with
differentCCDandELEtemperatures.Usingtherelationshipderivedfromtheseobservations,weobtained
a method for determining the sensitivity for different temperature circumstances, such as during
touchdowns,androverandlanderreleaseoperations.
ONC-W1and-W2
l EmpiricalbiasmodelsoftheONC-W1and-W2werederivedbasedonthepreflightmeasurements.The
biaslevelduringthecruisephasewasfoundtobestableoverthepastthreeyears.
l DarklevelandthenumberofhotpixelsaresimilartotheONC-T.
l Theflat-fieldsfortheONC-W1and-W2werederivedbasedonthepre-flightmeasurements.However,
becauseasurface lightsource isneededtoassesstheaccuracyof these flat-fields,weareplanningto
evaluatetheflatnessusingwholediskimagesofRyuguduringtheapproachphase.
l CCDsensitivitiesfortheONC-W1and-W2arenewlyderivedbasedonthepreflightintegratingsphere
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measurementsandinflightstarandJupiterobservations.Theinflightsensitivitiesareconsistentwithina
10%difference.Thedifferenceinspectralshapesbetweenthesensitivitiesderivedfromtheintegrating
sphereandtheinflightreferenceobjectsaresuspectedtobethemajorsourceofthedifferences.
l StraylightintheONC-W1and-W2wascharacterized.Thestraylightcontaminationisbasicallyweaker
thanthatseenintheONC-T.Moreover,shorterexposuretimesforONC-W1and-W2yieldsquitesmall
straylighteffects.
Inadditiontotheglobalspectralmapping,weareplanningtoconducthighlightedobservations,suchas
sodium atmosphere observations, hydratedmineral observations, and high-resolution imaging associated
withtouchdowns.
AsodiumatmosphereonRyuguishighlypossible,asRyuguisexpectedhavebeenformedinavolatile-
rich region in the early Solar System. We evaluated the detectability of a sodium atmosphere by testing
observationsofJupiter’ssodiumtorus.Mainlyduetothedarknoise,thesodiumatmosphereonJupiterwas
notdetected,buttheupperlimitfordetectionwasevaluated.WeexpecttodetectNaatmosphereofseveral
10skR,similartocomets(Leblancetal.,2008),byasingleimageset(vandNa)andofseveral100Rbyusing
100ofsets.
Both0.7-µmabsorptionandsteepUVslopearepossible indicatorsofHydratedmineralsonasteroids.
Our calibration results suggested that theerror inmeasurement of 0.7-µm absorption is 1.6% and the in
measurementinUVslopeis<4%usingONC-T.Thesecalibrationaccuraciesimplythatwecandetect0.7-um
absorptionbandwithSNR~2,assumingthetypicalserpentineabsorptionof3-4%andalsowemaydistinguish
unheatedandhighlyheatedCM-likematerialsbasedonUVslope.
Duringthetouchdownsuptoafewmetersofaltitude,theradiativeheatfromRyuguisexpectedtoraise
theCCDtemperatureasmuchas20 ℃.ThishightemperaturemaydegradetheSNRsofthecamerasto150at
most. However, this may still be sufficient to detect the surface radiance variations of 3% caused by
illuminationconditionsoralbedovariation.Thus,quantitativeimagingwithveryhighresolutionuptoafew
mm/pixelispossible.
OurfirstinflightobservationofRyuguon26February2018showsverygoodagreementwiththeEarth-
basedobservationbyIshiguroetal.(2014).Thisconfirmsthatourradiometriccalibrationisrobust.Finally,
drasticdegradationintheONC-Tsystemhasnotbeenobservedduringthe3.5yearcruisephase.However,
someobservations suchas charge transfer timemeasurement, flat-fieldevaluation,and round-about stray
lightinvestigationbasedonRyugudiskobservations,andupdatingsensitivitybymorestarobservationsare
desiredtoimproveandmonitorourcalibration.Andalsotimedependentchangeshouldberecordedatregular
intervalstomonitorthehealthoftheONCsystemforoneandahalfyearsoftherendezvousperiod.
Acknowledgement
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We would like to show our greatest appreciation to Hayabusa2 team members who gave enormous
contributiontoachieveobservations.WewouldalsoliketoexpressourgratitudetoDr.D.Dominguewhogives
insightful and constructive comments.We alsowish to acknowledge the constructive reviews from Dr. C.
Güttlerandananonymousreviewer.ThisresearchissupportedbyJapanSocietyforthePromotionofScience
(JSPS)Core-to-Coreprogram“InternationalNetworkofPlanetarySciences”.
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AppendixA.SpecificationsofONC
DetailedspecificationsofONCthreecamerasaresummarizedhere.Mostoftheinformationarealready
publishedinKamedaetal.(2017)andSuzukietal.(2018).
Table A1. Performance of CCD (E2V CCD47-20 (AIMO)). Note that three cameras of ONC use same CCD
products.
ONC-T ONC-W1 ONC-W2
Format 1056(H)pixels×1024(V)pixels
(16×1024pixelsonbothsidesareOpticalBlackpixels)
CCDpixelsize 13µm
Gainfactor(measuredvalue) 20.95e-/DN 20.86e-/DN 20.11e-/DN
Read-outnoise(measuredvalue) 38.5e- 36.3e- 37.0e-
A/Dconversion 12bit
Full-well(measuredvalue) 91,000e- 84,000e- 96,000e-
Pixelsamplingrate 3MHz
TableA2.Weight,size,andelectricconsumptionofONChardwarecomponents.Thebasicdesignincluding
CCD,electronics,andopticsofW1andW2arethesame.Thus,theirelectricconsumptionisthesame,buttheir
dimensionandweightareslightlydifferentbecauseofdifferenceinradiatorandhooddesign.
Weight[kg] Size[mm] Electricconsumption[W]
ONC-T 2.1 430×155×134 8
ONC-W1 1.355 249×155×95 8
ONC-W2 1.295 222×155×95 8
ONC-AE 1.285 200×220×50 14
DE 2.67 95×221×170 15
TableA3.Focallengthanddistortioncoefficientforwide-,v-,andNa-bandsofONC-T.
Band Focallength[mm]* Distortioncoefficientε1
[1/pix2]*
Note
wide 120.50 −9.28 × 10,:
(RMSerror0.1pixels)
Suzukietal.(2018)
v 120.50 −6.766 × 10,:
(RMSerror0.3pixels)
Newly derived from the star image
hyb2_onc_20151105_065349_tvf_l2a.fit.
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Na 120.49 −1.024 × 10,;
(RMSerror0.3pixels)
Newly derived from the star image
hyb2_onc_20171015_000153_tnf_l2a.fit.
*DetailsforthedistortionmodelaredescribedinSuzukietal.(2018).
Itisnotedthatthefocallengthslistedinthistablearevalidonlywhencombinedwitheachdistortioncoefficient.
AppendixB.ComponentsofONC-TPhotometricSystem
ThequantumefficiencyofCCD,originallymeasured inroomtemperature(20℃), isshowninFig.B1.
Moreover,theCCDtemperaturecanchangethisfunctionshape.Anexampleoffunctionsofquantumefficiency
withtwodifferenttemperaturesbasedonthemanufacturer(E2V)derivedvalues.Thetransmissionsofband-
passfilters(sameasKamedaetal.,2017),theNDfilter,lensesandtheCCDcoverglassaredisplayedinFig.B2.
FigureB1.FunctionsofquantumefficiencyindifferentCCDtemperature.HighCCDtemperatureshowsthe
higherefficiencyinlongerwavelengthrange.
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FigureB2.(Top)Transmittancesoftheband-passfilters.(bottom)TransmittancesoftheNDfilter,lenses,and
theCCDcoverglass.
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AppendixC.CoordinateSystemandDefinitionofAttitudeoftheSpacecraft
FigureC1displaysthespacecraftcoordinatesystem.ThespacecraftattitudewithrespecttotheSuncouldbe
definedbyXPNLandYPNL,whichcorrespondtotheanglesoftheSunto+XSCand+YSCplaneofthespacecraft.
º ∙<FP = sinÌùn$ , º ∙ =FP = sinÏùn$,wheresisaunitvectorfromthespacecrafttotheSun.Theangles ¡ and ¬ arethespacecrafttwistingangle
describedinSuzukietal.(2018).TherelationshipbetweenXPNLandYPNLarederivedas
−sin¡ cos ¬ = sinÌùn$ , sin¡ sin ¬ = sinÏùn$.
FigureC1.Thespacecraftcoordinatesystemandtheimagecoordinatesystem.(afterSuzukietal.(2018))
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