snap collaboration meeting / paris 10/20071 spectrometer detectors from science requirements to data...
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SNAP Collaboration meeting / Paris 10/2007 1
SPECTROMETER DETECTORSFrom science requirements to data storage
SNAP Collaboration meeting / Paris 10/2007 2
SUMMARY
1. Basic requirements for science2. NIR Baseline3. NIR Dark Current4. NIR and SN : Cosmics, Readout, memory size
and reduction5. NIR and Galaxies : Readout, memory size and
reduction6. CCD : Science, Baseline, requirement and
memory size7. Summary8. Addressed questions
SNAP Collaboration meeting / Paris 10/2007 3
SCIENCES GOALS
Results shown during science talks concerning the SN and WL for the spectrometer have been obtained doing some assumptions on the detectors.
CCD NIR
array size (Mpxl²) 3,5x3,5 2x2
pxl size (µm) 10,5 18
number of detectors 2x½ 2x½
T (K) 140 110
RN (e-) 2 5-7*
DC (e/pxl/sec) 0,0003 0,002
* depend of the integration time. see later…
SNAP Collaboration meeting / Paris 10/2007 4
Spectro NIR Baseline
NIR
array size (Mpxl²) 2x2
pxl size (µm) 18
number of detectors 2x½
T (K) 110
RN (e-) 5-7*
DC (e/pxl/sec) 0,002
Readout modes and memory size have to be thinked for each science
Detector Area (2k x 2k pix)
Minima requirements used for the science simulations
spatial
* depend of the integration time. see later…
SNAP Collaboration meeting / Paris 10/2007 5
Spectro NIR / Dark Current
• From dark current Imager requirement (0.02 e/pxl/sec.) to spectro requirement (0.002 e/pxl/sec.)
• Achievable down to 130K• T requirement 110K
Remarks/questions :• have to be tested on more devices• does the CCD work at this temperature if not it means that two temperature are necessary on the spectro focal plane
2 structures insulation not the same thermal straping for CCD and NIR heater?
Dark current 1.7um cutoff
0.0001
0.001
0.01
0.1
1
10
110 120 130 140 150 160
Temperature (K)
Da
rk c
urr
en
t (e
-/p
ix/s
ec
)
HgCdTe VIRGO 2K-009
InGaAs H1RG-015
HgCdTe H2RG-103
Eg =0.71eV (derived)
Eg =0.73eV (theory)
25% / K
21% / K
R.Smith & M.Bonati,2006-01-07
SNAP Collaboration meeting / Paris 10/2007 6
Spectro NIR and SuperNovae
how to manage such a long time with cosmics rays?
how to reach readout noises at the level of 7e- per exposure?
•Because of the readout noise limitation, science plots have shown clearly that you need long integration time and our baseline for the spectro have to be 3000sec.
SNAP Collaboration meeting / Paris 10/2007 7
Spectro NIR and SN / Cosmics
With18µm pixels calculations show a rate1,3*10-3/s/pxl
1 cosmic/800s/pxl (TBC)
The strategy to reject cosmics is to use an up-the-ramp readout :
sign
al
cosmicYou have to know the previous slope good enough to reject the cosmic
choose the appropriate readout cadence (10-50 TBD)
Some calculations have already shown that the loss of S/N using up-the-ramp and cosmic rejection is affected of only -1.6% for 3000sec exposure (TBC)
(S/N no cosmic–S/N cosmic )/ S/N no cosmic
SNAP Collaboration meeting / Paris 10/2007 8
Spectro NIR and SN / Readout Noise
Caltech, R.Smith measurements
Spatial noise @ 110KEXPOSURE TIME COMPARISON
1
10
100
1 10 100 1000
number of samples
no
ise
(e
-)
3000s 1000s
300s theory
Actual measurements (our baseline) show that being read noise limited larger integration time are necessary but restriction on readout noise have to be under control
With 3000 secondes of exposure time Fowler 100-500 (TBC) are necessary to limit the readout noise
SNAP Collaboration meeting / Paris 10/2007 9
Spectro NIR and SN / Readout strategy
• Need of 3000 secondes exposures• Need of a Up-the-Ramp (cadence 10-50 sec.)• Need of Fowler 100-500
sign
al
ResetN Fowler Samples stored
N Fowler Samples stored
Slow Up-The-Ramp stored+ maximum clocking but not stored
3000sec
This ideal readout mode HAVE TO BE TESTED WITH REAL DATA
SNAP Collaboration meeting / Paris 10/2007 10
Spectro NIR and SN / memory size
• hypothesys : 8h/day divided in 3000sec. exposure read with Fowler 300
• 2 X ½ NIR = 2x2 (Mpxl²) x 2 bytes = 8 Mbytes
8 Mbytes/frame x 3h/day / 3000 sec/expos x 600frames
~ 17 Gbyt/day
to be compared to the 63Gbyt/day NIR imager without data reduction
SNAP Collaboration meeting / Paris 10/2007 11
Spectro NIR and SN / memory size reduction
• 1650 /pxl/3000sec (TBC)• = 1• dynamic range = 1 e/ADCU
7 bit to get 1 extra bit for the sign of the difference
• do not store the frame (on 16bits) but only the differences between frames on 8 bits
if necessary adapt the dynamic rangeN e/ADCU
Detector Area (2k x 2k pix)
spatial
• Reduce the Region Of Interest stored to the SN and its host galaxy
To conclude on that point :1. check the hypothesys of that 17GByt/day2. possibilities to reduce by a factor of 4 (at least)
this volumeUse lossy compression : reduction by a factor up to 5!!!
SNAP Collaboration meeting / Paris 10/2007 12
Spectro NIR and Galaxies
• science inputs :
readout noise=7 e; DC=0.002 e/pxl/sec• Integration time is drived by the imager :
4 x 300sec exposure with 3.5 pixels dithering
Spatial noise @ 110KEXPOSURE TIME COMPARISON
1
10
100
1 10 100 1000
number of samples
no
ise
(e
-)
3000s 1000s
300s theory
300sec. integration time :R.Smith curves show that a Fowler 30 reach the 7 e goal
14 electrons noise in 1200s
1200sec. integration time:R.Smith curves show that a Fowler 100 reach the 7 e goal
7 electrons noise in 1200s
SNAP Collaboration meeting / Paris 10/2007 13
Spectro NIR and WL / data storage
• with 300sec. integration time
220exp/day x 60frame/exp x 8Mbyt/frame ~ 106Gbyts/day
• with 1200sec. integration time
55exp/day x 200frame/exp x 8Mbyt/frame ~ 88Gbyts/day
do we need this?probably NO (coadd.) ~1.8Gbyt
do we need this?probably YES
REDUCTION
Other possible reduction of volume : • depending of the dynamic range of the galaxies one can code on 8bits instead of 16
• check the dynamic range (e/ADCU) versus the number of of galaxies (TBD)•use lossy compression : factor 5 achievable?
SNAP Collaboration meeting / Paris 10/2007 14
Spectro CCD Baseline
what is new for the spectro CCD?• use of the same than the imager :
LBNL 3.5x3.5 Mpxl²; 10.5µm each; same thickness
• Due to the thickness we have the same cosmic limitation on the integration time than for the imager : 300sec whatever the science!• In terms of readout noise requirement we plan to have 2e- and this is achievable changing the readout frequency from 100kpxl/sec (imager) to 50kpxl/sec.
• In terms of Dark Current a requirement of 0.0003 e-/pxl/sec (1 e/pxl/hour) is needed questions : is it feasable at 140K? does it work in space?
Detector Area (2k x 2k pix)
SNAP Collaboration meeting / Paris 10/2007 15
Spectro CCD requirement summary and impact on data storage
CCD
array size (Mpxl²) 3,5x3,5
pxl size (µm) 10,5
number of detectors
2x½
T (K) 140
F (kHz) 50
RN (e-) 2
DC (e/pxl/sec) 0,0003
DATA STORAGE :3.5 x 3.5 (Mpxl²) x 2 (Byt) = 24.5 MByt/expos.
SN :24.5 MByt /exposure x 36 exposures (3h/day)~ 0.9 GByt /day without compression~0.45% of the CCD imager
Galaxies :24.5 MByt/exposure x 220 exposures (100%day)~ 5.4 GByt /day without compression~ 2.4 % of the CCD imager
SNAP Collaboration meeting / Paris 10/2007 16
Spectro CCD summary
imager spectro
CCD
array size (Mpxl²) 3,5x3,5 3,5x3,5
pxl size (µm) 10,5 10,5
number of detectors 36 2x½
T (K) 140 140
F (kHz) 100 50
RN (e-) 6 2
DC (e/pxl/sec) 0,03 0,0003
Readout time (sec) 30 30
SN
integrated time 300 300
exposures per day 220 (100% day) 36 (3h/day)
memory size* (Gbyte) 194 0,9
WL
integrated time 300 300
exposures per day 220 (100% day) 220 (100% day)
memory size* (Gbyte) 194 5,4
*without compression
SNAP Collaboration meeting / Paris 10/2007 17
Spectro NIR summaryimageur spectro
NIR
array size (Mpxl²) 2x2 2x2
pxl size (µm) 18 18
number of detectors 36 2x½
T (K) 140 110
RN (e-) 9 7
DC (e/pxl/sec) 0,02 0,002
Readout time (sec) 1,7 1,7
SN
integrated time 300 3000
exposures per day 220 (100% day) 5 (3h/day)
Read out modefowler 16 coads on boards
Fowler300+Up-the-Ramp50sec
memory size* (Gbyte) 63 17
WL
integrated time 300 300 1200
exposure per day 220 (100% day) 220 (100% day) 60 (100% day)
Read out mode FowlerN coads Fowler30 coadsFowler100+Up-th-Ramp50
memory size* (Gbyte) 63 1,8 88
*without compression
SNAP Collaboration meeting / Paris 10/2007 18
Addressed questions
• stable assumptions inputs from science (TBC)• CCD DC @ 140K TBC• CCD DC OK in space?• cosmic flux and inpact on NIR TBC• Up-the-ramp readout cadence for cosmic rejetion have TBC• Fowler N (100-500) with 3000 sec. TBC with other devices• FowlerN+Up-the-ramp+permanent clocking have to be really tested
– with many devices– @ different T (110-140K)
• NIR DC verify with different devices• CCD works down to which T (is 110K OK?)• dynamic range have to be confirmed :
– for each science– for darks– for stars calibration
• dynamic range versus flux have to be confirmed for galaxies distributions• data storage size and reduction have to be studied• for galaxies should we move from 300sec up to 1200sec ?• does the CCD readout have effect on NIR integration/readout performance?
hard tests. Could be done both in US and FR?
SNAP Collaboration meeting / Paris 10/2007 19
Spectro CCD and NIR summary
CCD NIR
array size (Mpxl²) 3,5x3,5 2x2
pxl size (µm) 10,5 18
number of detectors 2x½ 2x½
T (K) 140 110
F (kHz) 50 TBD
RN (e-) 2 7
DC (e/pxl/sec) 0,0003 0,002
Readout time (sec) 30 1,7
SN
integrated time 300 3000
exposures per day 36 (3h/day) 5 (3h/day)
Read out mode Fowler300+Up-the-Ramp50sec
memory size* (Gbyte) 0,9 17
WL
integrated time 300 300 1200
exposure per day220 (100%
day)220 (100% day)
60 (100% day)
Read out mode
Fowler30 coadsFowler100+Up-th-Ramp50
memory size* (Gbyte) 5,4 1,8 88
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