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Advanced Camera for Surveys Instrument Handbook for Cycle 20 > Chapter 5: Imaging > 5.1 Imaging Overview

5.1
 
HRC has been unavailable since January 2007. Information regarding HRC is provided for archival purposes.
ACS has been used to obtain images through a variety of optical and ultraviolet filters. 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. (Users need not specify CLEAR in their HST proposals.) For SBC imaging, the single filter wheel is rotated to the desired position. Every third slot (#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.Table 5.1,Table 5.2, and Table 5.3 summarize the filters available for imaging with each channel. Figures 5.1 through 5.6 show the filter transmission curves, and Figure 5.7 shows the integrated system throughputs.
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 arcseconds2 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 .
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, of which only the middle segment was used with the HRC. 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 information is provided for archival purposes only.)
Hα (1%)

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 arcsecond2 field of view.

Table 5.2: ACS WFC/HRC filters in Filter Wheel #2.
(Information regarding the HRC is provided for archival purposes only.).
Filter
name
Hα Ramp—middle segment
Hα Ramp—inner segment
Hα Ramp—outer segment

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 arcsecond2 field of view.

Table 5.3: ACS SBC filter complement.
MgF2 (1150  longpass)
CaF2 (1250  longpass)
BaF2 (1400  longpass)
Fused Silica (1650  longpass)
Ly-α (λ = 1200 , Δλ = 60 )
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.
Figure 5.6: Comparison between the ACS and WFPC2 ramp 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  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.

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