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.
