Star Counts M.Lampton Sept 2002 Updated Sept 2003
Motivation Can SNAP guide itself satisfactorily? Are there enough guide stars? –bright enough for low photon shot noise –numerous enough so that a reasonable size guider field is % certain to get a star
STATE VECTOR s/c attitude Controller commands data Environment: orbit, Sun, Earth, stars.... Dynamics Disturbances Coarse star trackers Coarse sun sensors Coarse/fine gyros focal plane guider Cassegrain guider sensor noise Wheels Jets Torquers What is the disturbance torque spectrum? What are the various sensor noise spectra? What is the closed-loop response? Two things cause pointing errors...
Previous Work: Secroun et al Experimental Astron. v.12# Calculated the expected sensor position errors vs magnitude and integration time –centroiding: 2x2, 3x3, 4x4 pixel groups Calculated the Poisson statistics for nominal mean star densities (1395% probability of finding a V<16 star provided that the guider field is 200x200 arcsec or larger Concerns: –V is not silicon band; R is more nearly correct –need actual counts, not extrapolations or averages –95% is NOT good enough –Aldering region is not NEP but is closer to NGP, fewer stars!
SDSS Early Data Release: 462 sqdeg
Aldering Region in GSC 2.2 RA=244.0, dec=+55.0 DeltaRA=8.75deg, DeltaDec=1.5deg on sky: 5.0 deg x 1.5 deg = 7.5sqdeg GSC 2.2, DPOSS IIR “F” band=IIIaF+RG610= 0.65um “objects” all non-stars, Kodak objects etc rejected
Integral star counts at mid-galactic-latitudes Aldering Region at (l,b)=(85,+44)
References to star counts B&S: Bahcall & Soneira, Ap.J.Supp. v.55, Allen: C.W.Allen "Astrophysical Quantities" Third edition 1973 p.243 Basel: Bahcall et al, Ap.J. v.299 p , 1985 SDSS: Newberg Richards Richmond & Fan, "Catalog of four color photometry...” 2002 see also... Chen et al, ApJ v.553, pp , 2001 EDD: "EDDINGTON Cumulative Star Counts" M&S: O.Yu.Malkov & O.M.Smirnov, "Testing the Galaxy Model with GSC" ADASS III ASP Conf. v GEMINI: Doug Simms Aug 1995 "Longitudinally Averaged Cumulative Star Counts" GSC2.2:
Analysis of region using box=0.05degrees This is 180”x180” Slightly smaller than Secroun’s 200”x200”
random guider locations in Aldering region Square guider box size 0.05, 0.10, 0.15 deg Histograms of brightest star within guide box
What does this mean? at 30 frames/sec...
...or at 3 frames/sec...
...or at 10 frames/sec and grasp=360nm...
Typical CCD QE curves Front illuminated CCDs: typical QE ~ 30% typical BW ~ 400nm typical QE*BW ~100 to 200nm
Kodak KAF-3200ME front illuminated, 2184 x 1472 ITO gates not polysilicon Lenslets QE * BW = 300 nm
Conclusions If we insist on full video rate 30fps: –4 guider chips 1K x 1K is NOT sufficient –16 guider chips 1K x 1K is OK If we can make do with 10fps: –4 guider chips 1K x 1K is marginal –4 guider chips 1K x 1K with higher QE is OK Sample rate requirements depend on disturbance spectrum and behavior of optimized Kalman filter ACS dynamic model is needed! SDSS map with u-g-r-i-z would allow better SNR calc Need to validate the Secroun centroid SNR estimate
Future Work GSC 2.2 contains some duplicate “objects” –overestimates log(N) curve –does not affect Monte Carlo calc GSC has poor accuracy -- roughly 0.4 mag RMS –bias could invalidate our predictions We will probably have *two* guiders: focal plane and cass focus –require a guide star in FP guider *and* in CF guider –would convert 99% into 98% success rate –no impact if we are 100% covered We have non-Aldering fields! Weak Lensing, cal stars.... –Don’t we want to be able to guide *anywhere* on the sky? even NGP? –guiding affects PSF -- WL work demands tight guiding Use today’s SDSS on NGP region; try mowing some stripes Enlarge SDSS to Aldering region
Guider CCDs located within GigaCam Guider 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 GigaCam Assume 1 degC peak-peak over 3 day orbit, coffin and GigaCam dT/dt = 1E-5 degC/sec, or 0.01 degC over a 1000 second exposure Coffin material is CFRP + cyanate ester; CTE=1ppm/degC –assume dryout is complete after first month on orbit GigaCam foundation plate is molybdenum: CTE=5.4 ppm/degC Creep within GigaCam baseplate <
Additional Suggestions, 20 Sept 2002 Medium 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 -- don’t 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, continued Bad 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? Don’t 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.