supernova / acceleration probe system engineering mike roberto and mike amato november 16, 2001

SuperNova / Acceleration Probe

System Engineering

Mike Roberto and Mike AmatoNovember 16, 2001

November 16, 2001 SNAP System Engineering P2Roberto/Amato

ISAL Team

Mike Amato System EngineeringJeff Bolognese Structural AnalysisArt Bradley Star Field /Fine GuidanceJennifer Bracken ISAL Team LeadJudy Brannen Mechanical DesignMick Correia Mechanical DesignPaul Earle ElectricalDennis Evans OpticsRodger Farley Mechanical SystemsLandis Markley Guidance, Navigation, and ControlWes Ousley ThermalMike Roberto ISAL SystemsCarl Stahl DetectorsJohn Wood ISAL Science LiaisonEric Young Electro-optical Systems


Summary

1. Requirements2. Trades and Issues3. Instrument Diagram4. Orbit Parameters5. Observation Strategy Summary6. Optics7. Detectors8. Mass, Data Rate9. Conclusions


Requirements

Requirements Summary2000 type 1A supernovae from 0.3 to 1.7 during three year missionShortest Wavelength 3.50E-07 mLongest Wavelength 1.70E-06 mTelescope entrance aperture diameter 2.0 mTelescope focal length 21.4 mTelescope focus, decenter, and tilt adjustmentField of View 1.0 degreeFocal plane temperature 140 KGuide CCDs on focal plane for ACSShutter for focal planeone spectrograph (plus redundant spectrograph)pointing stability 0.03 arc sectelemetry rate 5.00E+07 bits/sec


Trades and Issues

1. Baselining Hubble type structure rather than lower structureHubble like and upper structure tripod. Metering structureis stiffer, less obscuration, and more thermally isolated. However,it has higher mass and more complicated baffle integration.

2. Converged on ‘two vane’ shutter design. Advantage is thesame illumination time for each pixel, but feedback controlis needed. Alternate shutter type did not need feedback controlbut required very rapid operation to minimize time differencesin pixel illumination.

3. Baselining five degrees of freedom secondary mirror adjustmentBased on flight design, because secondary mirror is the most sensitiveto alignment errors. Tertiary mirror adjustment mechanism does notseem to be needed.

4. May not need thermal control of metering structure, but this could easilybe added (approximately 150 W). Plan to use low coefficient of thermal expansion graphite epoxy materials.


Trades and Issues (continued)

5. Do not see unusual stray light problems, but careful baffling is necessary.

6. Baselining 16 bit analog to digital converters. Could also use 12 bit A/D converters with multiple gains.

7. Baselining fixed filters on focal plane which requires stepping across focal plane. Advantage is the reduction in the number of mechanisms (filter wheelnot needed.

8. Future Trade – consider placements of spectrographs so behind focal plane to reduce stray light.


Instrument Diagram


Orbit Parameters

Modified Chandra orbit for complete observation of~ northern or southern ecliptic in one orbit and equalizingspectrograph and focal plane observation times.

Radius of perigee 3.0 Earth radiiRadius of apogee 24.5 Earth radiiPeriod 3.0 daysHeight for data collection =/> 60 x 10^3 kmTime spent near perigee 12 hoursTime for focal plane observations 29 hoursTime using spectrograph 31 hours


Observation Strategy Summary

Art Bradley and Landis Markley provided the inputs.

• Inside electron belts, slew 180 degrees, download data, orbit maintenance,shutter closed

2. Outside electron belts, use ACS guide star CCDs on focal plane for finepointing control when shutter is open

3. Observation time = 200 seconds

4. Focal plane shutter is then closed for 20 seconds while focal plane is read out,drift is fixed, and pointing is changed by ¼ of CCD position

5. Sky observations on focal plane are repeated for 480 steps in one direction, covering about 5 degrees in 29 hours


Optics

OpticsThree mirror reflective telescopeTelescope aperture diameter 2.0 mEffective focal length 21.4 mShortest Wavelength 3.50E-07 mLongest Wavelength 1.70E-06 mTelescope field of view 1.0 degree

Diffraction limit spot diameter for shortest wavelength = 2.44*lambda /d *EFL

9.14E-06 m

Diffraction limit spot diameter for longest wavelength = 2.44*lambda /d *EFL

4.44E-05 m

CCD pixel size 1.05E-05 mAngular dimension of CCD pixel 4.91E-07 radiansAngular dimension of CCD pixel 1.01E-01 arc sec

spectrograph with prism to separate wavelengths.


DetectorsDetectorDetector Information Discussion with Carl StahleVisible/NIR Detectors Silicon CCDShortest Wavelength or Lambda 3.50E-07 mLongest Wavelength 1.00E-06 mNumber of array pixels (each dimension) 1600Pixel size 1.0500E-05 mNumber of science CCD arrays 144Number of ACS CCD arrays 4Operating temperature 140 kTime to reset well 1.000E-07 secDetector well capacity 1.00E+05 electronsDetector Read Noise 3 electronsDark current 3 electrons/minPixel Rate for Each ROIC Output 1.000E+06 Hz analog pixelsquantum efficiency 0.8note: QE will change with lambda and temperatureconsider radiation tolerancesIR DetectorsPhotovoltaic HgCdTeShortest Wavelength or Lambda 1.00E-06 mLongest Wavelength 1.70E-06 mNumber of array pixels (each dimension) 2000Pixel size 1.8000E-05 mNumber of science CCD arrays 44Operating temperature 140 kTime to reset well 1.000E-07 secDetector well capacity 1.00E+05 electronsDetector Read Noise 10 electrons (3 goal; NGST)Dark current 3 electrons/minMax Pixel Rate for Each ROIC Output* 1.000E+06 Hz analog pixelsquantum efficiency 0.8* NGST @ 100 KHz

Spectrograph DetectorPhotovoltaic HgCdTenumber of detector arrays 1Shortest Wavelength or Lambda 3.50E-07 mLongest Wavelength 1.70E-06 mNumber of array pixels (each dimension) 1000Pixel size 1.8000E-05 mOperating temperature 140 kNotes: substrate thinned for spectrograph detector


Mass

Instrument Mass Estimate KG

Primary mirror 172

Main optics bench 119

Metering main structure 98Exterior baffle tube 70

Aft metering structure-aka 'Coffin' 62Telescope door 61

Dewar/radiation shell 60

Secondary spider 48Spacecraft Interface structure 50

Interior baffle tube 35

Other hardware (see Excel spreadsheet) 32Blankets 26Tertiary mirror 25Focal plane subassembly and mount 23Focal plane radiator 20Secondary mirror mechanism 18Fold mirror 16Other hardware 15Main electronics box 20Focal plane electronics boxes (2) 8Shutter mechanism 10Secondary mirror 9Spectrographs 8

Total Instrument Mass Estimate 1005


Data Rate

1. Focal plane average data rate during an observation 40 M bits/sec(144 CCD arrays with 1600x1600 pixels, 44 HgCdTe arrays with 2000x2000

pixels,16 bits per pixel, 220 seconds to complete observation plus read out focal plane)

2. Average data rate during spectrograph observation 73 K bits/sec(one HgCdTe array with 1000 x 1000 pixels, 16 bits per pixel)

3. Data collected per orbit (assuming about half focal plane observations and half spectrograph observations, assume data compression by factor of 2)

Focal plane telemetry per orbit 2.1 T bitsSpectrograph telemetry per orbit 4.0 G bitsHousekeeping telemetry per orbit 47 M bits

4. If focal plane data taken all the time, and data compressed by factor or two,total data collected during orbit about 4.2 T bits.


Backup Slides


Orbit Parameters

Orbit Parameters used in the studyEarth Radius 6.380E+06 mmu = Earth Gravitational Constant = GM 3.986E+14 m̂ 3 s -̂2rp = radius of perigee = distance from Earth center to closest approach to Earth 3.0 Earth Radii

ra = radius of apogee = distance from Earth center to farthest point on the orbit

24.5 Earth Radii

rp expressed in meters 1.914E+07 mra expressed in meters 1.565E+08 ma = semi-major axis of ellipse = one half the length of the major axis = (rp+ra)/2

8.782E+07 m

e = eccentricity = ra / a -1 0.782r = position vector of satellite relative to the center of the Earthnu = the polar angle of the ellipse, also called true anomaly, measured in the direction of motion from the direction of perigee to the position vector

129.0 degrees

nu expressed in radians 2.251E+00 radiansOrbit Period = 2 * pi *(a 3̂/mu)̂ 0.5 259003.7 secOrbit Period in days 3.00 daysn = mean motion, or average angular velocity, determined from the semi-major axis of the orbit = (mu/a 3̂)̂ 0.5

2.426E-05 radians/sec

E = eccentric anomaly equals intermediate variable to account for satellite's constantly changing velocity = acos((e+cos(nu))/(1+e*cos(nu)))

1.2653E+00 radians

M = mean anomaly = fraction of an orbit period which has elapsed since perigee, expressed as an angle = E - e*sin(E)

0.519 radians

M expressed as a fraction of the orbit time0.083

Fraction of orbit time for spacecraft at or above the the altitude associated with polar angle of the ellipse

0.165

t = time for satellite to go from perigee to specified angle from perigee = M/n

2.14E+04 sec

[r] = magnitude of the position vector = a*(1-e 2̂)/(1+e*cos(nu)

6.716E+07 m

Altitude of satellite at polar angle of ellipse = r - Earth radius

6.078E+07 m

Orbit parameters reference: Space Mission Analysis and Design, Larson and Wertz (editors),Second Edition, Chapter 6, 1992


Data RateData Rate

Number of science CCD arrays on focal plane 144Number of HgCdTe arrays on focal plane 44Number of pixels per CCD array 2.56E+06Number of pixels per HgCdTe array 4.00E+06Total number of focal plane pixels 5.45E+08Number of bits per pixel 1.60E+01total number of bits for focal plane 8.71E+09 bitsTime for one focal plane data set 2.00E+02 secTime to readout focal plane 2.00E+01 secAverage focal plane data rate during an observation 3.96E+07 bits/secNumber of focal plane observations per orbit 480Number of focal plane bits per orbit 4.183E+12

Time for focal plane observations 2.93E+01 hoursTime for one orbit 7.19E+01 hoursTime with instrument not collecting science data inside electron belts

1.19E+01 hours

Time available for spectrograph observations 3.07E+01 hoursNumber of HgCdTe arrays for spectrograph 1Number of pixels per spectrograph HgCdTe array 1.00E+06Number of possible 220 second spectrograph observations

503

Number of bits per spectrograph pixel 16Maximum number of spectrograph bits per orbit 8.05E+09

Housekeeping (H/K) telemetry points 300Number of bits per H/K telemetry point 8Approximate additional data packet information 500H/K plus additional data packet bits per frame 2900Average time between telemetry points 8 secNumber of 8 second periods per orbit 3.24E+04Number of H/K telemetry points per orbit 9.39E+07 bitsTotal bits per orbit 4.19E+12 bits

Average Data Rate During Focal Plane Observation

3.96E+07 bits/sec

Total instrument bits collected during an orbit

4.19E+12 bits

supernova / acceleration probe system engineering mike roberto and mike amato november 16, 2001

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