2.2 Comparison of WFC and UVIS

The UVIS channel of the Wide Field Camera 3 complements ACS/WFC over wavelengths ~3700 Å to 10,000 Å. 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 2.2Figure 2.1 and Figure 2.2 show these characteristics for each instrument. See also Figures 5.8 (limiting magnitude for point sources) and 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 × 202 arcsec2) is considerably larger than that of WFC3/UVIS (162 × 162 arcsec2). WFC3/UVIS may therefore be 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 ~4000 Å, so ACS/WFC is recommended 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 lengthier exposure to HST's trapped radiation environment. See Section 4.3.8 for details. 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).

Table 2.2: Comparison of wavelength coverage, pixel scales, and fields of view of ACS and WFC3/UVIS. HRC is no longer available. More information can be found at the SIAF website.

Instrument

Wavelength
coverage (nm)

Average Pixel size (arcsec)

Field of View (arcsec2)

WFC3 UVIS200–10000.04162 × 162
ACS WFC370–11000.05202 × 202
ACS HRC200–11000.028 × 0.02529 × 25
ACS SBC115–1700.034 × 0.03035 × 31