5.1 Imaging Overview

ACS has been used to obtain images through a variety of optical and ultraviolet filters. The CCD filter wheels contain filters of two different sizes. Some filters (F435W, F475W, F502N, F550M, F555W, F606W, F625W, F658N, F660N, F775W, F814W, F850LP, and G800L) are full-sized filters that can be used with both WFC and HRC. Others (F220W, F250W, F330W, F344N, F892N, POL0UV, POL60UV, POL120UV, POL0V, POL60V, POL120V, and PR200L) are smaller, giving a full un-vignetted field of view when used with the HRC, but a vignetted field of view of only 72 × 72 arcsec² when used with the WFC. Use of the small UV filters with WFC is not supported due to the unpredictable behavior of the silver coating shortward of 4000 Å.

For WFC and HRC imaging, the desired filter in one filter wheel is rotated into position and a CLEAR aperture in the other filter wheel is automatically selected, see Figures 3.2 and 3.3. (Users need not specify CLEAR in their HST proposals.) For SBC imaging, the single filter wheel is rotated to the desired position. Every third spot (#1, 4, 7, 10) in the SBC filter wheel is opaque, so the wheel must only be rotated to an adjacent slot to block the beam if a bright object limit violation occurs. Tables 5.1, 5.2, and 5.3 summarize the filters available for imaging with each channel. Figures 5.1 through 5.6 show the filter transmission curves. Figures 5.10 and 5.12 show the location of the filters in each filter wheel. Figure 5.7 shows the integrated system throughputs. Please visit the ACS System Throughputs webpage or refer to Chapter 10 for total system throughputs for each ACS filter and dispersing element.

The ACS ramp filters are designed to allow narrow- or medium-band imaging centered at an arbitrary wavelength. Each ramp filter is divided into three segments, each of which can be used with the WFC while only the middle segment was used with the HRC. See Table 5.2 for the list of available ramp filters. More information is available in Section 7.3.1, and schematics of the ramp filters and apertures are given in Figures 7.4 and 7.5.

Filterless WFC or HRC is available, but unsupported, by specifying CLEAR as the filter name. Rough wavelengths and bandwidths for CLEAR imaging are listed in Table 5.1. CLEAR imaging yields significantly degraded WFC PSFs, but only slightly degraded HRC PSFs. See ACS ISR 2003-03 for more details about CLEAR imaging.

Table 5.1: ACS WFC/HRC filters in Filter Wheel #1.
(HRC information is provided for archival purposes only.)

^a [/WFC] indicates that polarizer filters and F892N, designed for the HRC field of view, induce vignetting when used with the WFC, producing a 72 × 72 arcsec² field of view.

Table 5.2: ACS WFC/HRC filters in Filter Wheel #2.
(Information regarding the HRC is provided for archival purposes only.).

^aThe width of the ramp filters is a percentage relative to the central wavelength of the bandpass, Δλ/λ.
^b [/WFC] indicates that polarizer filters, designed for the HRC field of view, induce vignetting when used with the WFC, producing a 72  × 72 arcsec² field of view.

Table 5.3: ACS SBC filter complement.