The FGS PMTs (four per FGS) are end-illuminated, 13 stage venetian blind dynode S-20 photon-counting detectors with an effective photocathode area of about 4 mm. The A and B channels for each FGS interferometric axis operate independently. The PMTs are sensitive over a bandpass of 4000-7000A, with an efficiency of ~ 18% at the blue and diminishing linearly to about 2% at the red end.
The FGS1r dark counts for each channel are given in the following table. The values quoted are based on an average of 40Hz PMT counts and associated standard deviations.