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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 99.99+% certain to get a star.

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star counts

Star Counts


Sept 2002

Updated Sept 2003

  • 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 99.99+% certain to get a star
two things cause pointing errors
Two things cause pointing errors...



Coarse star trackers



s/c attitude

Coarse sun sensors



Coarse/fine gyros


focal plane guider

Environment: orbit, Sun, Earth, stars....

Cassegrain guider

commands data

sensor noise

What 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 2001
Previous Work:Secroun et al Experimental Astron. v.12#2 2001
  • 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 (13<V<16) at NEP, SEP
    • average star densities from C.W.Allen 3rd Edition
    • observed star densities from GSC1 V<14.5, extrapolated to V=16
  • Result: >95% 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 http archive stsci edu sdss edr main html
SDSS Early Data Release: 462 sqdeg

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= 0.65um “objects”all non-stars, Kodak objects etc rejected

references to star counts
References to star counts

B&S: Bahcall & Soneira, Ap.J.Supp. 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

SDSS: Newberg Richards Richmond & Fan, "Catalog of four color photometry...” 2002 see also... Chen et al, ApJ v.553, pp.184-197, 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.61 1994.

GEMINI: Doug Simms Aug 1995 "Longitudinally Averaged Cumulative Star Counts"


analysis of region using box 0 05degrees this is 180 x180 slightly smaller than secroun s 200 x200
Analysis of region using box=0.05degreesThis is 180”x180”Slightly smaller than Secroun’s 200”x200”
100000 random guider locations in Aldering regionSquare guider box size 0.05, 0.10, 0.15 degHistograms of brightest star within guide box
typical ccd qe curves
Typical CCD QE curves

Front illuminated CCDs:

typical QE ~ 30%

typical BW ~ 400nm

typical QE*BW ~100 to 200nm

  • 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
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
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 <<FOCAL PLANE GUIDER
    • Given by Texp * dT/dt * CTEmoly * Distance = 2nm
    • utterly negligible! about 0.2 millipixel or 0.02 milliarcsecond
  • Creep of coffin center with respect to Gigacam surface <<CASS GUIDER
    • Given by Texp * dT/dt * CTEcfrp * Distance = 10nm
    • negligible: about one millipixel or 0.1 milliarcsecond
  • Compare best gyro errors 40 mas/10seconds = 40000 times worse
additional suggestions 20 sept 2002
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
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.