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Pan-STARRS Image Processing Pipeline

Pan-STARRS Image Processing Pipeline. Astrometry and Photometry. IFA Pan-STARRS Seminar 735. October 14, 2004. Summary of Topics. Astrometry and Photometry Precision Requirements Summary of the AP Survey Achieving the Photometry Goals Achieving the Astrometry Goals.

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Pan-STARRS Image Processing Pipeline

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  1. Pan-STARRS Image Processing Pipeline Astrometry and Photometry IFA Pan-STARRS Seminar 735 October 14, 2004

  2. Summary of Topics • Astrometry and Photometry Precision Requirements • Summary of the AP Survey • Achieving the Photometry Goals • Achieving the Astrometry Goals

  3. Precision Requirements for Pan-STARRS (PS-4): • 30 milliarcsec relative astrometry • 100 milliarcsec absolute astrometry • 5 millimag relative photometry • 10 millimag absolute photometry (internal system) These goals can only be efficiently met after we have produced a Pan-STARRS Astrometric and Photometric reference catalog

  4. PS-1 AP Survey Parameters: • gizy : bright sweep, 1 x 5 seconds, 2 x 30 seconds • r : bright sweep, 1 x 5 seconds, 6 x 30 seconds (over 6 months) • 1 % photometry ~ 19.5 magnitudes (~ 1 x 109 stars) • saturation: 14 magnitude (5 seconds), 8 magnitude (sweep) • 50% overlap dither pattern • 2% of survey time to calibrations: • 12 standards observations per night per filter (40 min)

  5. Photometry: Science Motivations • galactic stellar populations • galaxy cluster evolution • rare object searches • high-z QSOs • extremely red galaxies • low-mass objects (L & T dwarfs) • YSOs

  6. Photometry: Analysis Overview • linearize detector flux • apply shutter correction • flatten images • photometer objects • apply image zero point, color correction, airmass correction

  7. Photometry: Detector Linearization • stable light source + variable exposure time? • calibrated light source?

  8. Photometry: Shutter Correction • measure shutter fly-over times: dt(x,y) • apply to flat-field or science images • probably small for 30 second exposures (0.1% = 30 ms jitter • may be significant for 5 second exposures...

  9. Photometry: Flat-Fielding Issues : Illumination Source Options • twilight-flat • pros: continuum source, spatially uniform (usually), bright • cons: very blue, limited availability, cirrus issues • dome-flat • pros: continuum source, available anytime, repeatable (?) • cons: spatial structures, low count rates, emission line dangers • night-sky flat • pros: obtained 'automatically' • cons: low count rates, stellar contaminations, spatial structures unknown, emission line source

  10. Photometry: Flat-Fielding Issues : Flat-field Corrections • correct for geometrical distortion & scattered light • stability time scale?

  11. Photometry: Color Corrections • chip-to-chip color terms (possibly linear, small) • internal system vs external system • external transformations are often ambiguous Mginst = -2.5 log (counts / sec) Mgsys = Mginst + Cg + Kg(1-z) + Fg(color) Mgcal = Mgsys + Qg(color)

  12. Photometry: Absolute Photometry • use relative photometry with reference overlaps • measure zero points & atmospheric corrections, apply • combination method (relphot + uniphot) ref 1 ref 1 ref 2 ref 2

  13. Photometry: Zero-point Stability (long-term trends) • system zero-points vary ~0.1 mag on timescales ~ 100 days

  14. Photometry: Atmospheric Stability (short-term trends) • variations in time : Cf(t) • variations in space : Cf(ra,dec) • these variations are NOT strongly correlated

  15. Photometry: Atmospheric Stability (short-term trends)

  16. Photometry: Atmospheric Stability conclusions: • photometric conditions exist at <1% level • sometimes there is haze : Cf(t) • sometimes there is thin cirrus : Cf (x,y) • haze is apparently more common... • make use of external indicators of transparency conditions: • SkyProbe • NIR Camera

  17. Photometry: Bright Stars & Flux Calibrations • OTA guide stars tie 30 sec exposures to 10 msec exposures • OTA 'sweep' can yield survey of stars 8 - 14 mag • Bright stars provide flux calibration (spectrophotometric standards) • SkyProbe A will provide atm transmission function

  18. Astrometry: Science Motivations • proper motions / baseline • high-quality grid for weak-lensing (starting point) • stellar matching in crowded fields

  19. Astrometry: Basic Concepts • RA,DEC <-> X,Y • linear fit • RA = RAo + X*dRdX + Y*dRdY, etc • 100 mas @ 36 arcsec FOV • projection + fit • RA,DEC -> P,Q • P,Q = f(X,Y) R,D P,Q

  20. Astrometry: Mosaic Astrometry • boresite + projection (RAo, DECo, ) • distortion: Nth order polynomial L,M = f(P,Q) • chip coordinates: Xo, Yo,  • watch for stability issues R,D projection P,Q optical distortion L,M L,M chip locations X,Y

  21. Astrometry: Mosaic Astrometry Stability • chip coordinates & distortion are fairly degenerate • direct fitting is a large, multiparameter, non-linear problem • use local gradients instead: • fit L,M assuming no distortion • fit chip parameters from L,M • measure L,M residuals (L, M) • measure local gradients (dL/dP, dL/dQ, dM/dP, dM/dQ) • fit local gradients to Nth order polynomial • resulting terms are coefficients of L,M vs P,Q (N+1)th order fit • fit is insensitive to chip positions, boresite center

  22. Astrometry: Example from MegaPrime • chips are probably NOT flat!

  23. Astrometry: Example from MegaPrime • chips are probably NOT flat!

  24. Astrometry: Calibrating the AP Survey • what is stability of model components? • boresite: changes with every exposure • optical distortion: long-term stability expected • chip warps: short-term stability? temperature dependence? • chip positions: gravity vector dependence? • regularly measure model components & track changes • tie to ICRS with Guide Stars (Tycho) • atmosphere may introduce 50 mas scatter: model in overlaps USNO-B: deep, dense (~20 mag), 150 - 250 mas scatter, large-scale errors UCAC: modest (16 mag), 20 mas scatter + proper motion Tycho: shallow (11.5 mag), 10 mas scatter + proper motion Hipparchos: very shallow (7.3 mag), 1 mas scatter + proper motion

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