Objects that are observed in the coronagraphic mode but that are not placed behind an occulting mask have a PSF that is defined by the Lyot stop. Because the stop effectively reduces the diameter of the telescope and introduces larger obscurations, this PSF is wider than normal, with more power in the wings and diffraction spikes ( Figure 6.14). In addition, the stop and spot substrate reduce the throughput by 52.5%. In F814W, this PSF has a peak pixel containing 4.3% of the total (reduced) flux and a sharpness (including CCD charge diffusion effects) of 0.010 (compare these to 7.7% and 0.026, respectively, for the normal HRC PSF). In F435W the peak is 11% and the sharpness is 0.025 (compared to 17% and 0.051 for the normal F435W PSF). Observers need to take the reduced throughput and sharpness into account when determining detection limits for planned observations. Tiny Tim
can be used to compute off-spot PSFs.
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