5.5 Ultraviolet Imaging with the SBC

The Solar Blind Channel is the ACS camera optimized for UV imaging. The SBC uses the same optical train as the HRC and is comparable in performance to the FUV MAMA of STIS. The use of the repeller wire increases the quantum efficiency of the detector by ~30%, but adds a halo to the PSF. Bright object limits are discussed in detail in Section 7.2.

5.5.1 Filter Set

Several filters are available for use with the SBC, including a Lyman α narrow band filter (F122M), a long pass quartz filter (F150LP), MgF₂ filter (F115LP), and a CaF₂ filter (F125LP). The SBC also includes two additional long pass filters (F140LP and F165LP) as well as prisms (discussed in Section 6.3). Figure 5.12 shows the filter wheel used by SBC and Table 5.3 lists the filter characteristics in more detail.

5.5.2 Red Leaks

The visible light rejection of the SBC is excellent, but users should be aware that stars of solar type or later will have a significant fraction of the detected flux from outside the nominal wavelength range of the detector. This is discussed in greater detail in Section 4.4.2, and more information is available in Table 5.6. The stellar spectra are from the Pickles catalog (Pickles A.J., 1998, PASP 110, 863).

Table 5.6: Visible-light rejection of the SBC F115LP imaging mode.

5.5.3 SBC Imaging Filter Shifts

The SBC focal surface, like that of the HRC, is tilted significantly with respect to the chief ray. The MAMA detector is a STIS spare, and as such, its window is approximately parallel to the Micro Channel Plate (MCP) surface and the whole detector must tilt to achieve good focus over the field. The window is therefore tilted, and to ameliorate the introduction of “lateral color”, which would result in dispersion-induced degradation of the PSF, the filters are canted in the opposite direction to that of the window. The filter thickness is matched to the mean index of refraction over its bandpass to maintain focus. These result in unavoidable image location offsets between filters. In contrast, the WFC and HRC filters and windows are normal to the chief ray and the detector surfaces are tilted within their housings to match the focal surface tilt. In Table 5.7, we list the shifts for each SBC imaging filter with respect to the F115LP filter. No pointing compensations are made for these offsets.

Table 5.7: Shifts between SBC imaging filters.