3.2 Choosing Between Instruments

The primary factors to consider in choosing the preferred instrument are areal coverage, spatial resolution, wavelength coverage, sensitivity, and availability of specific spectral elements. Table 3.1 lists the primary characteristics of the imaging instruments currently available on HST.

For some research programs, the instrument choice may be dictated by the need for a particular spectral element. In this regard, WFC3 offers considerable capability thanks to its broad complement of wide-, medium-, and narrow-band filters both at UV/optical and near-IR wavelengths, as well as one UV grism and two near-IR grisms for slitless spectroscopy.

For studies at optical wavelengths, the trade-offs to consider when deciding between WFC3/UVIS and ACS/WFC include pixel size, field of view and, to some extent, throughput. WFC3 is generally preferable when angular resolution has higher priority than field of view, because of its finer pixel size (since ACS/HRC could not be repaired during SM4, WFC3 offers imaging at the finest pixel scale of any HST instrument at optical wavelengths). On the other hand, ACS/WFC has higher throughput than WFC3/UVIS at wavelengths longward of ~400 nm (see Figure 3.2), and hence may be the best choice when the highest possible sensitivity at such wavelengths is crucial. However, considerations of degraded charge transfer efficiency (CTE) should be kept in mind, since ACS has endured the high-radiation space environment for more than seven years longer than WFC3 has.

At UV wavelengths, WFC3/UVIS is the only imager on HST to offer a large field of view combined with high throughput. However, its spectral coverage does not extend shortward of 200 nm, whereas ACS/SBC and STIS/FUV-MAMA both reach down to 115 nm (STIS/NUV-MAMA reaches 160 nm), and also offer finer spatial sampling (see Section 3.3.3). Thus, WFC3 will be the choice whenever both large field of view and coverage down to 200 nm are required (e.g., multi-wavelength surveys). However, if observations at extreme far-UV wavelengths are necessary, or if the highest available spatial sampling at UV wavelengths is a primary requirement, then ACS/SBC or the STIS UV channels should be considered.

Table 3.1: Comparison of wavelength coverage, pixel scales, and fields of view of HST’s imaging instruments.