FGS Sensitivity

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 HST s 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.

For more specific details on FGS sensitivity, follow the links to the documents listed below.

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.

IHB Chapter 5.2 Transfer Mode Calibrations and Error Sources
Summarizes Transfer Mode sources of error and associated calibrations.
FGS Group 15 Apr 2002 (html)

IHB Chapter 2.2 FGS Detectors
Characteristics of the FGS detectors.
FGS Group 15 Apr 2002 (html)

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.

Correcting FGS Counts for Deadtime in the PMTs
The procedure to remove the effect of deadtime is described in-depth.
M. Lattanzi 16 Apr 2002 (pdf)

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.