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 x
4K thinned, backside-illuminated, SITe CCDs with a red optimized coating and long-λ
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 (Nyquist) sampled at 11,600 Å and undersampled by a factor 3 at the blue end of the WFC sensitivity range (3700 Å). It is 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.
shows the filter wheels used by the WFC.
The Sloan Digital Sky Survey
filter set (F475W, F625W, F775W, F850LP) is designed to provide high throughput for the wavelengths of interest and excellent rejection of out-of-band wavelengths. The filters were designed to provide wide, non-overlapping filter bands that cover the entire range of CCD sensitivity from blue to near-IR wavelengths.
α (F658N), [ΟΙΙΙ] (F502N), and [NII] (F660N) narrow band filters are full-size, and can be used with both the WFC and HRC.
The WFC/HRC filter wheels contain polarizers with pass directions spaced by 60°
, optimized for both the UV (POL0UV, POL60UV, and POL120UV) and the visible (POL0V, POL60V, and POL120V). All the polarizer filters are sized for the HRC field of view. They induce vignetting when used with the WFC, for which the FOV will be about 72 x
. More information on the use of the polarizers is given in Chapter 6