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.
Several filters are available for use with the SBC, including a Lyman α narrow band filter (F122M), a long pass quartz filter (F150LP), MgF
2 filter (F115LP), and a CaF
2 filter (F125LP). The SBC also includes two additional long pass filters (F140LP and F165LP) as well as prisms (discussed in
Section 6.3). A list of filters is given in
Table 5.3
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 coming 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 5.7.
The SBC focal surface, like that of the HRC, is tilted significantly with respect to the chief ray. Because the MAMA detector is a STIS spare, its window is approximately parallel to the MCP surface and the whole detector must tilt to achieve good focus over the field. Because the window is therefore tilted, “lateral color” is introduced, which would result in dispersion-induced degradation of the PSF, so the filters are canted in the opposite direction to that of the window to ameliorate the color. 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 5.8, we list the shifts for each SBC imaging filter with respect to the F115LP filter. No pointing compensations are made for these offsets.
