distortion in the wfc jay anderson rice university [email protected]
TRANSCRIPT
Background on WFC distortion
• General difficulty calibrating HST– Need high-density field, accurate positions– No satisfactory fields exist– Need self-calibration
• ISR on HRC distortion released a year ago– WFC more complicated:
• Largest HST field
• PSF spatially variable
Overview of this talk
1) PSF issues– Spatial variation
– Time variation
– Fitting stars
– A useful program
2) Distortion solution– Difficulty of calibration
– Form of solution
– Time variability
3) How-to Astrometry with the WFC
PSF Issues (1)
• Need a PSF to measure stars to solve for distortion– Several routines are coming out– My routine: img2xym_WFC.09x10.F
• Similar to the my HRC routine
• Operates on _flt images
• Uses an array of PSFs to deal with spatially dependent charge diffusion
– Between 17% and 24% of a star’s light in central pixel
– Affects photometry at the +/- 4% level
– Affects astrometry at the 0.01 pixel level
Varying flux in
the central pixel
Slices across WFC image
Grid of 9x10 fiducial PSFs
Array of WFC PSFs
Spatial variation
of the WFC PSF
(~10%)
PSF Issues (2): treating the PSF
• The base PSF model• 9x10 array of PSFs• 101x101 pixels• 4x super-sampled• Use bi-cubic interpolation• Covers out to r = 12.5 pix• “Effective” PSF
• Time variability• Typically 5% in the core• Treat as perturbation:
PSF(dx,dy;x,y,NIM) = PSF(dx,dy;x,y) + PSF(dx,dy;NIM)
Variation of the PSF over a month
• Richer’s GO-10424 stare at NGC6397
• Variation is ~ 5%
Zoom of month-long variation
PSF Issues (3): the program
• Operation of program:– Take _flt image– Simple finding criteria– Return (x,y,m) for sources– User collates with other observations
• Measurement quality (internal precision)– Photometry: 0.005 magnitude– Astrometry: 0.01 pix
Internal precision
• 0.01 pixel for each coord
• 0.005 mags
Distortion Solution (1): Why?
• Need for distortion solution– Image rectification
– Stacking to go deep
– Source identification
– Spectra slit/fiber placement
– Lensing analysis
– Astrometry
• Different applications require different accuracies
Distortion Solution (2): Solving for
• Ways to solve for– Best way: calibrated reference frame
• None exists with density/precision useful for HST
– Alternate way: self-calibration• Compare two WFC images of a good-density field
• Hard to know where the distortion error is
• Hard to visualize distortion– 2-d function over a 2-d surface
• Hard to measure distortion outright– But easier to test for errors
Usefulness of different types of data set
Distortion Solution (3): History• Solution history
• Meurer GO-9028• F475W of 47Tuc
• 4th-order polynomial
• Linear-term degeneracy
• Anderson GO-9443• Took orthogonal observation
• Used several filters
• Filter-dependent residuals
• Slightly different quadratic terms
• 68.2666-column pattern, amplitude 0.01 pixel
Distortion Solution (4): Form
• Final form of solution1) Column correction: amplitude 0.01 pixel
2) Polynomial: amplitude 40 pixel
3) Filter-based look-up table: 0.05 pixel
• Software now available for 12 filters• Better for some filters than others
• Used in the drizzle pipeline
• Supplementary program to improve solution for F606W and F814W: GO-10252• Use inner field in Omega Cen: 88,000 stars, even density
• Tables to be improved, PSFs obtained
• Problem: out of focus, just provides a check
• Other checks on solution
The typical residual
table correction:0.05 pixel
Distortion solution (4): Check #1
• Checking the distortion solution– Easier to check than to solve for
– Three tests: short-term, long-term, out-of-focus
• Short-term time variations– GO-10424 (PI Richer)
– 126 orbits taken over 4 weeks
– Each orbit: F814W, F606W, F814W
– Compare each to the average
– Hard to separate distortion variation from PSF variation
– Typical variation is much less than 0.02 pixel
Usefulness of different types of data set
Variation during long
stare
Non-linear
variation– Correlated
with PSF variation
– Only about 0.02 pixel at worst
Distortion Solution (5): Check #2
• Long-term variation– Outer field in 47 Tuc– Observed over 300 times by WFC– Inter-compare exposures, allowing for linear
transformation• Examine astrometric and photometric residuals• Linear variation of linear skew term: 0.1 pixel over
three years• Typical systematic residuals are 0.02 pixel
Usefulness of different types of data set
Initial residual errors
• From early solution– Typically
0.01-0.02 pix
Remaining errors
• Residuals flattened to below 0.01 pixel
Distortion Solution (6): Check #3
• Calibration supplement program GO-10252– 1 orbit for each of F606W, F814W– Aim to improve the fine-scale solution
and provide good empirical PSFs– PSF very much out of focus
• 10% low in central pixel
• Use as comparison test
Independent test in
OMCEN
Distortion Solution (7): Summary
• Short term (weeks)– Linear and quadratic good to 0.02 pix
• Long term (years)– Linear has systematic trends– Quadratic stable to 0.02 pixel
• Out of focus – Errors up to 0.03 pixel at edges
Prescriptions for astrometry (1)
• Accessing the solution– ISR coming very soon, with FORTRAN
programs for finding/measuring/correcting– Included in the drizzle pipeline
• Planning observations– Accuracy: 0.01 pixel per exposure, but…– Beware small systematic errors of ~0.02 pixel
• Planning can minimize/identify these• Ideal dithering depends on goal of project
– Dense field: may be able to solve for PSF– Sparse field: need large dithers to average out spatially
dependent errors
Prescriptions for astrometry (2)
• Reductions– Measure _flt images only (x,y,m)
– Correct for distortion
– Cross-ID stars in different images
– Carefully perform transformations• 6-parameter linear
• Go local if necessary
– Combine similar things first• Identify systematic errors
• Get a handle on random errors
• Lots of Astrometry left to do!