Fine Guidance Sensor Instrument Handbook for Cycle 17

2.2 FGS Detectors

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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 Table 2.1.

Table 2.1: FGS1r Dark Counts

PMT	counts/second1	stdev
AX	174.6	±2
BX	84.4	±2
AY	164.0	±2
BY	252.0	±2

1Values based on an average of 40Hz PMT counts and associated standard deviations.