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 е blue cutoff. The WFC detectors are two butted 2K by 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 sampled at 11,600 е and undersampled by a factor 3 at the blue end of the WFC sensitivity range (3700 е). 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.
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. See
Section 10.3 for a detailed discussion of the distortion in ACS.
All of the most commonly used WFPC2 filters are included in the ACS filter set. In addition to a medium and a broad V band filter (F550M and F606W), there is a complete Johnson-Cousins
BVI set (F435W, F555W, F814W).
The Sloan Digital Sky Survey (SDSS)
griz 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.
The Hα (F658N), [ΟΙΙΙ] (F502N), and [NII] (F660N) narrow band filters are full-size, and can be used with both the WFC and HRC.
ACS includes a complete set of ramp filters that provide full coverage of the WFC wavelength range at 2% and 9% bandwidth. Each ramp filter consists of three segments. The inner and outer filter segments can be used with the WFC only, while the middle segments could be used by both WFC and HRC. Unlike the WFPC2, where the desired wavelength is achieved by offsetting the telescope, the wavelength of ACS ramps is selected by rotating the filter while the target is positioned in one of the pre-defined apertures. The monochromatic field of view of the ramp filters is approximately 40
by 80
arcseconds. Details of how to use the ramp filters are given in
Section 7.3.
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
72
arcseconds2. More information on the use of the polarizers is given in
Chapter 6.
The CCD channels also have a grism (G800L) for use with both WFC and HRC from 5500 е to 11,000 е, and a prism (PR200L) for use with the HRC from 1600 е to 3500 е. These are described more fully in
Chapter 6.
