|ACS Instrument Handbook for Cycle 25|
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 unvignetted field of view when used with the HRC, but a vignetted field of view of only 72 × 72 arcseconds 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 5.11 show the location of the filters in each filter wheel. Figure 5.7 shows the integrated system throughputs. Please visit the ACS website or refer to Chapter 10 for a full catalog of the ACS filter throughputs.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.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[/WFC]1 Hα (1%) [/WFC] indicates that polarizer filters, designed for the HRC field of view, induces vignetting when used with the WFC, producing a 72 by 72 arcsecond field of view.
Table 5.2: ACS WFC/HRC filters in Filter Wheel #2.
Hα Ramp—middle segment Hα Ramp—inner segment Hα Ramp—outer segment HRC[/WFC]1 [/WFC] indicates that polarizer filters, designed for the HRC field of view, induces vignetting when used with the WFC, producing a 72 by 72 arcsecond field of view.
Table 5.3: ACS SBC filter complement.
MgF2 (1150 Å longpass) CaF2 (1250 Å longpass) BaF2 (1400 Å longpass) Ly-α (λ = 1200 Å, Δλ = 60 Å) CaF2 Prism (R~100)Figure 5.1: ACS broad-band filters.Figure 5.2: ACS SDSS filters.Figure 5.3: ACS UV and medium-band filters.Figure 5.4: ACS narrow band filters.Figure 5.5: ACS SBC filters.The crosses and open circles are for the ACS narrow and medium band ramps. The open squares are for the 4 WFPC2 ramps. For each of the ACS ramps the peak throughput that was calculated for eleven central wavelength values is plotted. For the WFPC2 ramps, the peak throughput calculated every 100 Å within the field of view of any of the 4 chips, and a 0 filter rotation angle (as mapped in Figures 3.4 and 3.5 of the WFPC2 Instrument Handbook), are plotted.