Like the STIS CCD but unlike WFPC2, the HRC CCD is directly sensitive to UV photons and for this reason is much more effective in detecting them. However, whenever a detector has non-negligible sensitivity over more than a factor two in wavelength, it becomes energetically possible for a UV photon to generate more than one electron and be counted more than once. This effect has been seen for STIS and also during the ground testing of the HRC detector. The effect is only important shortward of 3200 Å, and reaches a magnitude of approximately 1.7 e-/photon at 2000 Å. Multiple counting of photons has to be taken into account when estimating the detector QE and the noise level of a UV observation since multiple photons cause a distortion in the Poisson distribution of electrons.
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