Star Counts

Star CountsM.LamptonSept 2002Updated Sept 2003

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

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

Big Picture

SDSS Early Data Release: 462 sqdeghttp://archive.stsci.edu/sdss/edr_main.html

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

Integral star counts at mid-galactic-latitudesAldering Region at (l,b)=(85,+44)

Chart4

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

Sheet1

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

Sheet1

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

Sheet2

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

Sheet3

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

Analysis of region using box=0.05degreesThis is 180x180Slightly smaller than Secrouns 200x200

100000 random guider locations in Aldering regionSquare guider box size 0.05, 0.10, 0.15 degHistograms of brightest star within guide box

What does this mean?at 30 frames/sec...

Sheet1

Sheet2

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

Sheet3

MBD0001CD57.xls

Sheet1

GUIDER WORKSHEET EXAMPLE

ASSUMPTIONS







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

Sheet2

Sheet3

...or at 3 frames/sec...

Sheet1

Sheet2


ASSUMPTIONS















Sheet3

MBD0001CD57.xls

Sheet1


ASSUMPTIONS



















Sheet2

Sheet3

...or at 10 frames/sec and grasp=360nm...

Sheet1

Sheet2


ASSUMPTIONS















Sheet3

MBD0001CD57.xls

Sheet1


ASSUMPTIONS



















Sheet2

Sheet3

Typical CCD QE curvesFront illuminated CCDs:typical QE ~ 30%typical BW ~ 400nmtypical QE*BW ~100 to 200nm

Kodak KAF-3200MEfront illuminated, 2184 x 1472ITO gates not polysiliconLensletsQE * BW = 300 nm

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

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

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

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.

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.

Star Counts

Documents

nominal mean star densities

guide stars

basel m13gsc2

bahcall et

secroun et

chen et

pointing errors

various references