The FGS is a white light, broad bandpass interferometer, and its S-Curve is essentially a single fringe interferogram. The spectral incoherence of white light causes the higher order fringes to be strongly damped. Because the S-Curve is a normalized function, its amplitude is not sensitive to the target's magnitude provided the background and dark contributions to the input beam are relatively small. However, as fainter targets are observed (i.e., V greater than 14.5), the S-Curve's amplitude will be reduced because background and dark contributions are not coherent with light from the target. The effect of dark+background is easily calibrated and therefore does not compromise the instrument's scientific performance in either Position or Transfer Mode.
The characteristics of observed S-Curves depend on several factors: the quality and fabrication of the internal optics, the relative sensitivity of the PMTs, the alignment of the internal optics, the filter in use, the color of the target, and the effect of the spherically aberrated HST primary mirror. Some of the effects can be removed during processing and calibration, while others limit the performance of the instrument.
Given the presence of spherical aberration from the HST's misfigured primary mirror, the wavefront presented to the Koesters prism is not flat but has curvature. This greatly amplifies the effects of misalignments in the FGS optical train. A decentered spherically aberrated beam introduces a phase error between the re-combining transmitted and reflected beams, resulting in degraded S-Curve characteristics. The interferometric response (in filter F583W) of the 3 original FGSs are shown in Chapter 2 of the FGS Instrument Handbook. Decenter emerges as morphological deformations and reduced modulation of the fringes. Of the original three FGSs, FGS3 was the only instrument with sufficient fringe visibility to perform as an astrometric science instrument.
For complete details on the interferometric response of FGS, please see the FGS Instrument Handbook.
For other details on the FGS interferometric response, see Section 2.1.2 of the FGS Instrument Handbook.
FGS Interferometric Response Documentation:
Frequently Asked Questions.
Status reports reflecting the current understanding of instrument characteristics, performance and calibration.
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 of how to use various FGS-related tools, calibration and analysis techniques.