Space Telescope Science Institute   5.5.2 Red Leaks  5.6 ACS Point Spread Functions

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. 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.


Table 5.8: Shifts between SBC imaging filters.
Spectral Element
Offset (pixels)
in the x,y directions
F115LP
0,0
F122M
0,0
F125LP
-5,15
F140LP
-7,21
F150LP
-3,11
F165LP
-4,12

 5.5.2 Red Leaks  5.6 ACS Point Spread Functions
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