1 / 25

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.

tariq
Download Presentation

Pan-STARRS Image Processing Pipeline

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related