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Modeling and Correcting Time-Dependent ACS PSF for Weak Lensing

This presentation discusses the problems of aliasing and time variation in the Point Spread Function (PSF) of telescopes, specifically for weak lensing measurements. The use of the TinyTim PSF modeling software is proposed as a solution to accurately measure and correct for these effects. Results from the COSMOS dataset and future work are also presented.

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Modeling and Correcting Time-Dependent ACS PSF for Weak Lensing

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  1. Modeling and Correcting the Time-Dependent ACS PSF for Weak Lensing Jason Rhodes, JPL With: Justin Albert (Caltech) Richard Massey (Caltech) HST Calibration Workshop Oct 27, 2005

  2. Motivation :Weak Lensing • Statistical measurement on many galaxies • Lensing induced ellipticities 1-2% • Telescope Point Spread Function (PSF) is the primary systematic concern –it changes shape (e)!

  3. Two Main Problems • Aliasing of the PSF during distortion correction re-pixellization (Multidrizzle) • Time variation of PSF due to thermally induced focus changes

  4. Data Sets • Cosmos 2 Square degree field • Taken cycles 12-13 • 590 orbits of F814 • ACS Parallels • Taken cycles 11-13 • 500 fields of F775W • Stellar Fields (globular clusters) • Taken cycles 11-13 • A few fields in F775W and F814W Not enough stars per field to model PSF Not take often enough to account for time variation

  5. TinyTim • TinyTim Version 6.3 written by John Krist (http://www.stsci.edu/software/tinytim/tinytim.html) • Creates a PSF anywhere on the ACS chip with any filter and spectrum • Can be highly oversampled • Includes diffraction, distortion and charge disffusion • Produces stars as we see in raw (distorted) ACS images

  6. How We Use TT • We use F814W filter, and single wavelength 800nm • We modify the code to measure the effects of multidrizzle • We created a version of multidrizzle that only adds distortion across the field • We modify the code to produce undistorted (post multidrizzle) stars • We manually add charge diffusion as a convolution with a distorted kernel • We avoid the complications of multidrizzle on our model stars

  7. Aliasing From Multidrizzle

  8. “Aliasing” from pixellization Unfortunately, a star’s sub-pixel position affects its observed shape.

  9. “Aliasing” from second Pixellization Then it happens again during DRIZZLE! This at least must be avoidable…

  10. Adjust Pixel scale & DRIZZLE kernel …it is!

  11. Recommended Multidrizzle Parameters • Created TinyTim models with same diffraction everywhere, only distortion is different • Run through multidrizzle w/ various parameters • Large improvement going from 0.05 to 0.03 arcsecond pixels • Gaussian kernel has lower RMS(e) and is more stable

  12. The PSF Problem The PSF pattern is time dependent

  13. Periodic Focus Variation • COSMOS data • ~20 day time scale • Determined by fitting to TinyTim models

  14. Our Solution • Not enough stars→ Make our own! • Use TinyTim PSF modeling software • Create stars without geometric distortion (but with diffraction and other PSF effects • Required modification of TinyTim program • Create dense stellar grids (up to 50x50 across field) • Create at range of focus positions (-10 to +5 μm) • Use stars in each COSMOS field (~10 to 20) to pick the best focus value • Use the template at that focus value to perform correction • Eliminates need for interpolation between stars

  15. TinyTim- The PSF Solution From ISR ACS 2003-6

  16. Focus Values in COSMOS Focus values cluster!

  17. COSMOS Results Before TT E modes B modes After TT

  18. Remaining Work and Conclusions • Still some work to be done • mapping chip height variations with globular clusters • TinyTim stars still slightly too small (diffusion convolution?) • What effect does CTE have on the PSF? • Compare to other methods (e.g. principle component analysis; see Jarvis and Jain astro-ph/0412234 ) • Method greatly reduces PSF systematics • Can easily be extended to other filters • Useful for more than weak lensing

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