2.2	FGS Detectors

The FGS photomultiplier tubes (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. Each PMT has a characteristic dark count rate, as well as a “dead time” during which time it is unable to record the arrival of a new photon while it is still processing a previously arrived photon. The FGS1r PMT dead times were measured on orbit using two stars of very similar spectral type (HD 209458 and PO41C) that differ by 4.34 magnitudes.

The FGS1r dark counts for each channel are given in Table 2.1. The FGS1r dead times, in seconds, are given in Table 2.2.

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

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1

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

Table 2.2: FGS1r Dead Times

Ax	2.0759x10-7
Bx	2.3018x10-7
Ay	2.1074x10-7
By	2.2297x10-7