Instrument
Performance
The FOC detectors described in "Detectors" on page 46 may be damaged by illumination levels exceeding 107 photons s-1 pix-1 at the photocathode due to point sources and by an average illumination from a diffuse source over the whole photocathode exceeding 104 photons s-1 pix-1. Because of this danger, the 36 kV HV power supply on the 3 stage image intensifier is set to trip off when the point source illumination exceeds the value given above or if the average illumination from a diffuse source exceeds 200 photons s-1 pix-1. Thus, for safety reasons, no point source delivering more than 106 photons s-1 pix-1 at the photocathode or a diffuse source delivering more than an average rate of 100 photons s-1 pix-1 over the whole photocathode will be allowed to be imaged by the FOC. These values correspond to an 9th magnitude blue star or a diffuse source of surface brightness 10 magnitudes arcsec-2 viewed through the F430W filter with the F/96 relay. Targets brighter than this limit can still be observed by the FOC, provided that 1) enough neutral density filters are selected, so that a linear countrate is observed by the FOC, 2) the filter configuration is checked prior to the observation [Special Requirement CHECK-FILTER = YES].
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