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(abridged) We examine the spatial and temporal stability of the HST ACS WideField Camera (WFC) point spread function (PSF) using the two square degreeCOSMOS survey. We show that stochastic aliasing of the PSF necessarily occursduring `drizzling'. This aliasing is maximal if the output pixel scale is equalto the input pixel scale of 0.05''. We show that this source of PSF variationcan be significantly reduced by choosing a Gaussian drizzle kernel and bysetting the output pixel size to 0.03''. We show that the PSF is temporallyunstable, most likely due to thermal fluctuations in the telescope's focus. Wefind that the primary manifestation of this thermal drift in COSMOS images isan overall slow periodic focus change. Using a modified version of TinyTim, wecreate undistorted stars in a 30x30 grid across the ACS WFC CCDs. These PSFmodels are created for telescope focus values in the range -10 microns to +5microns, thus spanning the allowed range of telescope focus values. We then usethe approximately ten well measured stars in each COSMOS field to pick thebest-fit focus value for each field. The TinyTim model stars are then used toperform PSF corrections for weak lensing allowing systematics due toincorrectly modeled PSFs to be greatly reduced. We have made the software forPSF modeling using our modified version of TinyTim available to theastronomical community. We show the effects of Charge Transfer Efficiency (CTE)degradation, which distorts the object in the readout direction, mimicking aweak lensing signal. We derive a parametric correction for the effect of CTE onthe shapes of objects in the COSMOS field as a function of observation date,position within the ACS WFC field, and object flux.Comment: 20 pages, 15 figures. Accepted for the COSMOS ApJ Suppl. special  issu

the astrophysical journal. supplement series/astrophysical journal. supplement series

The Stability of the Point‐Spread Function of the Advanced Camera for Surveys on the <i>Hubble Space Telescope</i> and Implications for Weak Gravitational Lensing

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