The Wide Field Channel of ACS was designed primarily for high throughput observations at visible wavelengths. The use of protected silver mirror coatings, the small number of reflections, and the use of a red sensitive CCD have provided the high throughput required for this camera at the expense of a 3700 Å blue cutoff. The WFC detectors are two butted 2K by 4K thinned, backside-illuminated, SITe CCDs with a red optimized coating and long- halo fix. The plate scale is 0.050 arcseconds per pixel, which provides a good compromise between adequately sampling the PSF and a wide field of view. The WFC PSF is critically sampled at 11,600 Å and undersampled by a factor 3 at the blue end of the WFC sensitivity range (3700 Å). We expect that it will be possible to achieve a final reconstructed FWHM of 0.100 to 0.140 arcseconds for well-dithered observations. Because the WFC PSF FWHM is largely dependent on the blurring caused by CCD charge diffusion, dithering will not be able to recover the full resolution of the optical system. See Section 7.4 for more discussion of how to use dithered observations to optimally sample the PSF.
The optical design of the camera introduces a two-component geometric distortion. The detectors themselves are at an angle with respect to the optical axis. This produces an 8% stretching of one pixel diagonal compared to the other. As a result, WFC pixels project on the sky as rhombuses rather than squares. These effects are purely geometrical and are routinely corrected in the ACS data reduction pipeline. The second component of geometric distortion is more complex. This distortion causes up to ±9% variation in effective pixel area and needs to be taken into account when doing accurate photometry or astrometry as the effective area of the detector pixels varies nonlinearly with field position.
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