The Stability of the Point-Spread Function of the Advanced Camera for Surveys on the Hubble Space Telescope and Implications for Weak Gravitational Lensing

Creators: Rhodes, Jason D.; Massey, Richard J.; Albert, Justin; Collins, Nicholas; Ellis, Richard S.; Heymans, Catherine; Gardner, Jonathan P.; Kneib, Jean-Paul; Koekemoer, Anton; Leauthaud, Alexie; Mellier, Yannick; Refregier, Alexander; Taylor, James E.; Van Waerbeke, Ludovic

Abstract

We examine the spatial and temporal stability of the Hubble Space Telescope's Advanced Camera for Surveys (ACS) Wide Field Camera (WFC) point-spread function (PSF) using the 2 deg^2 COSMOS survey. This is important for studies of weak gravitational lensing, where the ability to deconvolve the PSF from galaxy shapes is of paramount importance. We show that stochastic aliasing of the PSF necessarily occurs during "drizzling." This aliasing is maximal if the output-pixel scale is equal to the input-pixel scale. This source of PSF variation can be significantly reduced by choosing a Gaussian drizzle kernel with a size of 0.8 input pixels and by reducing the output-pixel scale. We show that the PSF is temporally unstable, resulting in an overall slow periodic focus change in the COSMOS images. Using a modified version of the Tiny Tim PSF modeling software, we create grids of undistorted stars over a range of telescope focus values. We then use the approximately 10 well-measured stars in each COSMOS field to pick the best-fit focus value for each field. The Tiny Tim model stars can then be used to perform PSF corrections for weak lensing. We derive a parametric correction for the effect of charge transfer efficiency (CTE) degradation on the shapes of objects in the COSMOS field as a function of observation date, position within the ACS WFC field, and object flux. Finally, we discuss future plans to improve the CTE correction.

Additional Information

© 2007 American Astronomical Society. Print publication: Issue 1 (2007 September); received 2006 September 21; accepted for publication 2007 January 10. We thank Stefano Casertano and Andy Fruchter for useful discussions on the ACS PSF and how it is affected by drizzle. Our use of the Tiny Tim software would not have been possible without the help of John Krist. The HST COSMOS Treasury program was supported through NASA grant HST-GO-09822. We wish to thank Tony Roman, Denise Taylor, and David Soderblom for their assistance in the planning and scheduling of the extensive COSMOS observations. We gratefully acknowledge the contributions of the entire COSMOS collaboration, consisting of more than 70 scientists, and of the COSMOS PI Nick Scoville in particular. More information on the COSMOS survey is available at http://www.astro.caltech.edu/cosmos. It is a pleasure to acknowledge the excellent services provided by the NASA/IPAC Infrared Science Archive staff (Anastasia Laity, Anastasia Alexov, Bruce Berriman, and John Good) in providing online archive and server capabilities for the COSMOS data sets. The COSMOS Science meeting in 2005 May was supported in part by the NSF through grant OISE-0456439. We thank the anonymous referee for useful comments that improved the paper immensely. We also thank Matthew Lallo and Russell Makidon for providing focus data for the HST during the time the COSMOS observations were made. R. M. was supported in part by grant HST-AR-10964. This research was carried out in part at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA and finded through the internal Research and Technology Development program. Facilities: HST(ACS) Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555; also based on data collected at the Subaru Telescope, which is operated by the National Astronomical Observatory of Japan; the XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA member states and NASA; the European Southern Observatory, Chile; Kitt Peak National Observatory, Cerro Tololo Inter-American Observatory, and the National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation; the National Radio Astronomy Observatory, which is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.; and the Canada-France-Hawaii Telescope, operated by the National Research Council of Canada, the Centre National de la Recherche Scientifique de France, and the University of Hawaii.

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