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Cerenkov background radiation in imaging detectors

The authors discuss results of an analysis of background dark data obtained with the Digicon detector in the faint object spectrograph on board the Hubble Space Telescope. Time sequenced data are presented which show the background recorded by the detector as it orbits the Earth at an altitude of 600 km. The authors propose that Cerenkov radiation produced by cosmic ray particle interactions with the MgF2 faceplate comprises the major source of this detector noise. Cerenkov light will be emitted whenever a high-energy particle traverses the detector faceplate and can result in large portions of the detector array being illuminated simultaneously. The effects of Cerenkov radiation have been modeled in image tubes by means of a Monte Carlo simulation. This model produces images which qualitatively resemble observed dark data. Moreover, the model closely reproduces the observed average background level and calculates background count statistics which are difficult to determine directly given the rapid sampling rate that would be required. The model can provide optimal settings for the detector burst noise rejection algorithm so that the signal-to-noise ratio of astronomical data can be maximized.