Pan-STARRS Image Processing Pipeline

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
(No Transcript)
7

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

8

• Photometry Detector Linearization
• stable light source variable exposure time?
• calibrated light source?

9

• 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...

10

• 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

11

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

12

• 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)
13

• 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
14

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

15

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

16
Photometry Atmospheric Stability (short-term
trends)
17

• Photometry Atmospheric Stability conclusions
• photometric conditions exist at lt1 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

18

• 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

19

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

20

• Astrometry Basic Concepts
• RA,DEC lt-gt X,Y
• linear fit
• RA RAo XdRdX YdRdY, etc
• 100 mas _at_ 36 arcsec FOV
• projection fit
• RA,DEC -gt P,Q
• P,Q f(X,Y)

R,D
P,Q
21

• 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
22

• 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
  (N1)th order fit
• fit is insensitive to chip positions, boresite
  center

23

• Astrometry Example from MegaPrime
• chips are probably NOT flat!

24

• Astrometry Example from MegaPrime
• chips are probably NOT flat!

25

• 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