Star Counts
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Transcript of Star Counts
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Star CountsM.LamptonSept 2002Updated Sept 2003
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MotivationCan SNAP guide itself satisfactorily?Are there enough guide stars?bright enough for low photon shot noisenumerous enough so that a reasonable size guider field is 99.99+% certain to get a star
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Two things cause pointing errors...STATE VECTORs/c attitudeControllercommands dataEnvironment: orbit, Sun, Earth, stars....DynamicsDisturbancesCoarse star trackersCoarse sun sensorsCoarse/fine gyrosfocal plane guiderCassegrain guidersensor noiseWheelsJetsTorquersWhat is the disturbance torque spectrum?What are the various sensor noise spectra?What is the closed-loop response?
- Previous Work:Secroun et al Experimental Astron. v.12#2 2001Calculated the expected sensor position errors vs magnitude and integration timecentroiding: 2x2, 3x3, 4x4 pixel groupsCalculated the Poisson statistics for nominal mean star densities (13
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Big Picture
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SDSS Early Data Release: 462 sqdeghttp://archive.stsci.edu/sdss/edr_main.html
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Aldering Region in GSC 2.2RA=244.0, dec=+55.0DeltaRA=8.75deg, DeltaDec=1.5degon sky: 5.0 deg x 1.5 deg = 7.5sqdegGSC 2.2, DPOSS IIR F band=IIIaF+RG610= 0.65umhttp://www-gsss.stsci.edu/support/data_access.htm15512 objectsall non-stars, Kodak objects etc rejected
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Integral star counts at mid-galactic-latitudesAldering Region at (l,b)=(85,+44)
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1.641.541.91.481.971.91.71.71.413131.591.93
1.981.882.261.932.32.222.11.714141.952.3
2.312.22.432.242.542.52.32.321.731.652.22.59
2.612.482.652.462.792.82.62.72.32.232.162.462.85
2.842.752.932.733.033.12.82.92.42.592.522.773.07
3.142.993.062.953.253.43.13.12.72.882.76183.31
3.43.23.223.143.453.73.33.32.853.12.931919
Allen +40 B
Allen +50 B
B&S -46 V
B&S -51 V
GEMINI +45 R
M&S +40 B
M&S +50 B
EDD +30 V
EDD +60 V
SDSS +60 g*
SDSS -60 g*
Basel +41 G
GSC2.2 +44 R
magnitudes
log stars/sqdeg
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LOG INTEGRAL STAR COUNTS from various references: Galactic latitudes 30 to 60 deg
C.W.AllenC.W.AllenB & S 1984B & S 1984GEMINI 1995M&S 1997M&S 1997EDDINGTONEDDINGTONSDSS+60SDSS-60Basel M13GSC2.2
Glatit40504651454050306060604144
Vmag/Lam0.450.450.550.550.650.450.450.550.550.50.50.50.65
131.641.541.901.481.971.901.701.71.41.591.93
141.981.882.261.932.302.202.002.11.71.952.3
152.312.202.432.242.542.502.302.321.731.652.22.59
162.612.482.652.462.792.802.602.72.32.232.162.462.85
172.842.752.932.733.033.102.802.92.42.592.522.773.07
183.142.993.062.953.253.403.103.12.72.882.763.31
193.403.203.223.143.453.703.303.32.853.12.93
REFERENCES
B&S: Bahcall & Soneira, ApJSupp v.55 67-99 1984
Allen: C.W.Allen "Astrophysical Quantities" Third edition 1973 p.243
Basel: Bahcall et al, Ap.J. v.299 p.616-632, 1985
SLOAN: Newberg Richards Richmond & Fan, "Catalog of four color photometry..." preprint 2002
SLOAN: Chen et al, ApJ v.553, pp.184-197, 2001
EDD: http://star-www.st-and.ac.uk "EDDINGTON Cumulative Star Counts"
M&S: O.Yu.Malkov & O.M.Smirnov, "Testing the Galaxy Model with GSC" ADASS III ASP Conf. v.61 1994
GEMINI: http://www.shef.ac.uk/cgi-bin-cgiwrap/phys/compstars.ps Doug Simms Aug 1995 "Longitudinally Averaged Cumulative Star Counts"
GSC2.2: http://www-gsss.stsci.edu/support/data_access.htm
GSC2.2 Catalog: http://www-gsss.stsci.edu/support/data_access.htm
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Allen +40 B
Allen +50 B
B&S -46 V
B&S -51 V
GEMINI +45 R
M&S +40 B
M&S +50 B
EDD +30 V
EDD +60 V
SDSS +60 g*
SDSS -60 g*
Basel +41 G
GSC2.2 +55 R
magnitudes
log stars/sqdeg
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Allen +40 B
Allen +50 B
B&S -46 V
B&S -51 V
GEMINI +45 R
M&S +40 B
M&S +50 B
EDD +30 V
EDD +60 V
SDSS +60 g*
SDSS -60 g*
Basel +41 G
GSC2.2 +44 R
magnitudes
log stars/sqdeg
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References to star countsB&S: Bahcall & Soneira, Ap.J.Supp. v.55, 67-99 1984Allen: C.W.Allen "Astrophysical Quantities" Third edition 1973 p.243Basel: Bahcall et al, Ap.J. v.299 p.616-632, 1985SDSS: Newberg Richards Richmond & Fan, "Catalog of four color photometry... 2002 see also... Chen et al, ApJ v.553, pp.184-197, 2001EDD: http://star-www.st-and.ac.uk "EDDINGTON Cumulative Star Counts"M&S: O.Yu.Malkov & O.M.Smirnov, "Testing the Galaxy Model with GSC" ADASS III ASP Conf. v.61 1994.GEMINI: http://www.shef.ac.uk/cgi-bin-cgiwrap/phys/compstars.ps Doug Simms Aug 1995 "Longitudinally Averaged Cumulative Star Counts"
GSC2.2: http://www-gsss.stsci.edu/support/data_access.htm
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Analysis of region using box=0.05degreesThis is 180x180Slightly smaller than Secrouns 200x200
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100000 random guider locations in Aldering regionSquare guider box size 0.05, 0.10, 0.15 degHistograms of brightest star within guide box
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What does this mean?at 30 frames/sec...
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GUIDER WORKSHEET EXAMPLES
ASSUMPTIONS
Video guider CCD frame rate30frames/sec
Guider pixel size100milli arcsec
Guider read noise30e RMS
Integral QE * dLambda150nm
Telescope aperture2meters
Telescope efficiency0.7
RESULTS FOR THREE CASES...Good fieldPoor fieldTerrible field
brightest star, R mag131618
photon flux/m2 sec nm631.039.86.3
photoelectrons/frame6934.2437.569.3
RMS jitter, in pixels, one frame0.0070.0730.437
White noise bandwidth, Hz15.00015.00015.000
RMS jitter, in pixels, per root Hz0.0020.0190.113
1-D RMS jitter, 1Hz BW, milli arcsec0.1911.87511.278
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MBD0001CD57.xls
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GUIDER WORKSHEET EXAMPLE
ASSUMPTIONS
Video guider CCD frame rate30frames/sec
Guider pixel size100milli arcsec
Guider read noise30e RMS
Integral QE * dLambda150nm
Telescope aperture2meters
Telescope efficiency0.7
Guider pixels per chip1024x1024pixels
Number of guider chips4
Sky area for guide stars200x200arcseconds
RESULTS FOR TWO CASES...Typical fieldPoor field
brightest star, V mag1316
Percentile among all fields analyzed50%95%
photon flux/m2 sec nm631.039.8
photoelectrons/frame6934.2437.5
RMS jitter, in pixels, one frame0.0070.073
White noise bandwidth, Hz15.00015.000
RMS jitter, in pixels, per root Hz0.0020.019
1-D RMS jitter, 1Hz BW, milli arcsec0.1911.875
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...or at 3 frames/sec...
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GUIDER WORKSHEET EXAMPLES
ASSUMPTIONS
Video guider CCD frame rate3frames/sec
Guider pixel size100milli arcsec
Guider read noise30e RMS
Integral QE * dLambda150nm
Telescope aperture2meters
Telescope efficiency0.7
RESULTS FOR THREE CASES...Good fieldPoor fieldTerrible field
brightest star, R mag131618
photon flux/m2 sec nm631.039.86.3
photoelectrons/frame69342.24375.2693.4
RMS jitter, in pixels, one frame0.0020.0100.047
White noise bandwidth, Hz1.5001.5001.500
RMS jitter, in pixels, per root Hz0.0020.0080.039
1-D RMS jitter, 1Hz BW, milli arcsec0.1590.8333.858
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MBD0001CD57.xls
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GUIDER WORKSHEET EXAMPLE
ASSUMPTIONS
Video guider CCD frame rate30frames/sec
Guider pixel size100milli arcsec
Guider read noise30e RMS
Integral QE * dLambda150nm
Telescope aperture2meters
Telescope efficiency0.7
Guider pixels per chip1024x1024pixels
Number of guider chips4
Sky area for guide stars200x200arcseconds
RESULTS FOR TWO CASES...Typical fieldPoor field
brightest star, V mag1316
Percentile among all fields analyzed50%95%
photon flux/m2 sec nm631.039.8
photoelectrons/frame6934.2437.5
RMS jitter, in pixels, one frame0.0070.073
White noise bandwidth, Hz15.00015.000
RMS jitter, in pixels, per root Hz0.0020.019
1-D RMS jitter, 1Hz BW, milli arcsec0.1911.875
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...or at 10 frames/sec and grasp=360nm...
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GUIDER WORKSHEET EXAMPLES
ASSUMPTIONS
Video guider CCD frame rate10frames/sec
Guider pixel size100milli arcsec
Guider read noise30e RMS
Integral QE * dLambda360nm
Telescope aperture2meters
Telescope efficiency0.7
RESULTS FOR THREE CASES...Good fieldPoor fieldTerrible field
brightest star, R mag131618
photon flux/m2 sec nm631.039.86.3
photoelectrons/frame49926.43150.1499.3
RMS jitter, in pixels, one frame0.0020.0130.064
White noise bandwidth, Hz5.0005.0005.000
RMS jitter, in pixels, per root Hz0.0010.0060.029
1-D RMS jitter, 1Hz BW, milli arcsec0.1040.5832.868
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MBD0001CD57.xls
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GUIDER WORKSHEET EXAMPLE
ASSUMPTIONS
Video guider CCD frame rate30frames/sec
Guider pixel size100milli arcsec
Guider read noise30e RMS
Integral QE * dLambda150nm
Telescope aperture2meters
Telescope efficiency0.7
Guider pixels per chip1024x1024pixels
Number of guider chips4
Sky area for guide stars200x200arcseconds
RESULTS FOR TWO CASES...Typical fieldPoor field
brightest star, V mag1316
Percentile among all fields analyzed50%95%
photon flux/m2 sec nm631.039.8
photoelectrons/frame6934.2437.5
RMS jitter, in pixels, one frame0.0070.073
White noise bandwidth, Hz15.00015.000
RMS jitter, in pixels, per root Hz0.0020.019
1-D RMS jitter, 1Hz BW, milli arcsec0.1911.875
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Typical CCD QE curvesFront illuminated CCDs:typical QE ~ 30%typical BW ~ 400nmtypical QE*BW ~100 to 200nm
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Kodak KAF-3200MEfront illuminated, 2184 x 1472ITO gates not polysiliconLensletsQE * BW = 300 nm
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ConclusionsIf we insist on full video rate 30fps:4 guider chips 1K x 1K is NOT sufficient16 guider chips 1K x 1K is OKIf we can make do with 10fps:4 guider chips 1K x 1K is marginal4 guider chips 1K x 1K with higher QE is OKSample rate requirements depend on disturbance spectrum and behavior of optimized Kalman filterACS dynamic model is needed!SDSS map with u-g-r-i-z would allow better SNR calcNeed to validate the Secroun centroid SNR estimate
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Future WorkGSC 2.2 contains some duplicate objectsoverestimates log(N) curvedoes not affect Monte Carlo calcGSC has poor accuracy -- roughly 0.4 mag RMSbias could invalidate our predictionsWe will probably have *two* guiders: focal plane and cass focusrequire a guide star in FP guider *and* in CF guiderwould convert 99% into 98% success rateno impact if we are 100% coveredWe have non-Aldering fields! Weak Lensing, cal stars....Dont we want to be able to guide *anywhere* on the sky? even NGP?guiding affects PSF -- WL work demands tight guidingUse todays SDSS on NGP region; try mowing some stripesEnlarge SDSS to Aldering region
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Guider CCDs located within GigaCamGuider CCDs located within rear metering structure, on optical axis
- Guider thermal structure creep?Cass guider corrects for s/c pointing, primary and secondary motions, but not small motions of folding flat, tertiary, or GigaCamAssume 1 degC peak-peak over 3 day orbit, coffin and GigaCamdT/dt = 1E-5 degC/sec, or 0.01 degC over a 1000 second exposureCoffin material is CFRP + cyanate ester; CTE=1ppm/degCassume dryout is complete after first month on orbitGigaCam foundation plate is molybdenum: CTE=5.4 ppm/degCCreep within GigaCam baseplate
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Additional Suggestions, 20 Sept 2002Medium format CCDs might be agile: able to quickly dump 99% of a field, and read a selected 1% region *slowly* with excellent SNR. Rockwell HiVISI addressable CMOS chip?Medium format scientific LBL CCDs could have excellent QE*BW products! We should use them, if staff permits. Of course we need a blind storage area to eliminate the need for a shutter. Could run at low pixel rate since only a few rows would have to be read out repeatedly; dump the other rows: 2K x 5rows x 10fps = 100kHz. We would also need a full frame search mode to perform initial localization, probably with a much higher pixel rate. Although we clearly benefit from having a large available chip area, any one given field will need only one CCD running -- dont need 16 full field CCDs running in parallel. We can switch to a different CCD and a different row group when we move to each new field of stars.
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Additional Suggestions, continuedBad columns could seriously spoil the linearity with which a star centroid is recovered, hence radiation damage might cripple a fraction of the guider area. Best to have plenty of extra sky field available on board for tracking so that we can always pick a good guide star located in a functional CCD column. Guider (x,y) centroids control two axes, but how about the third (roll) axis? Dont we need a really good roll guider as well? Would a Ball Aerospace CT-602 serve? Do we need diametrically opposite guide stars in our focal plane?Algorithm for the centroid must be robust against CR hits; perhaps confine centroid calc range to 2x2 or 3x3 pixels and perform sanity trend check of each result.