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Advanced Camera for Surveys Instrument Handbook for Cycle 21 > Chapter 2: Changes After SM4 and Considerations for Cy21 > 2.2 Comparison of ACS/WFC and WFC3/UVIS

The UVIS channel of the Wide Field Camera 3 complements ACS/WFC over wavelengths ~ 3700  to 10,000 . Observers must determine which instrument is more appropriate for their science from the perspectives of field of view, pixel size, throughput, and filter availability. Table 2.2, Figure 2.1 and Figure 2.2 show these characteristics for each instrument. See Figures 5.8 (limiting magnitude for point sources) and Figure 5.9 (limiting magnitude for extended sources). ACS/WFC has a larger pixel scale (0.05 arcsec/pixel) than WFC3/UVIS (0.04 arcsec/pixel), so the field of view of ACS/WFC (202 x 202 arcsec2) is considerably larger than that of WFC3/UVIS (162 x 162 arcsec2). WFC3/UVIS is therefore preferred if angular resolution is more important than field of view. On the other hand, ACS/WFC is more sensitive than WFC3/UVIS at wavelengths longward of ~ 400 nm, so ACS/WFC should be used if greater sensitivity at red wavelengths is important. However, users should also consider ACS’s generally lower charge-transfer efficiency (CTE) compared to WFC3/UVIS, and its increased number of hot pixels caused by its lengthy exposure to HST’s trapped radiation environment. (See Section 4.3.7 for details.)
Table 2.2: Comparison of wavelength coverage, pixel scales, and fields of view of ACS and WFC3/UVIS. HRC is not longer available.
It is also possible to use both ACS/WFC and WFC3/UVIS in parallel. The separation of the two cameras is ~ 360" (see Figure 3.1).

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