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.
With HSTs 0.23 microns of spherical aberration, a decentering of the wavefront by only 0.25 mm will decrease the modulation of the S-Curve to 75% of its perfectly aligned value. If the telescope were not spherically aberrated (i.e., if the wavefront were planar) misalignments up to five times this size would hardly be noticeable. The impact of HST's spherical aberration and the improved performance of FGS1r are discussed in Chapter 2 of the FGS Instrument Handbook.
FGS Sensitivity Documentation:
FAQs:
Frequently Asked Questions.
Performance Summaries:
Status reports reflecting the current
understanding of instrument characteristics,
performance and calibration.
Handbooks:
The FGS Instrument Handbook is the primary
guide regarding the characteristics and use of
the instrument. The HST Data Handbook is the
primary guide for calibration, reduction and analysis
of FGS data.
Instrument Science Reports:
ISRs are technical reports written by members
of the FGS Group about various aspects of the
instrument and data. They usually contain in-depth
information about specific topics.
Papers and Proceedings:
Selected FGS related published papers and workshop
proceedings.
Examples:
Examples of how to use various FGS-related tools, calibration and analysis techniques.