Instrument Science Report CAL/FOS-069

"FOS Red Detector Flat-field and Sensitivity Degradation"   November 1991

By: George Hartig

Abstract excerpted from the report:

  Significant degredation of the efficiency and fixed pattern noise charac-
  teristics of the FOS Red Digicon in an approximately 200A-wide band centered
  at about 1950A has been detected in observations spanning 10 months.  The
  drop in effciency amounts to about 10% at the center of the band and the 
  strength of apparent absorption features, with typical widths of 5-10 diodes,
  has increased in by up to 15% between Dec 1990 and Sept 1991.  No such degra-
  dation is observed on the Blue detector.  The effect may be due to the devel-
  opment of color centers in the fused silica faceplate of the Red detector due
  to the radiation environment of the HST spacecraft.  A lab test to confirm 
  this hypothesis and a calibration program to monitor the development of the
  effect have been ititiated.


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