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FGS Instrument Handbook for Cycle 24 |
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FGS interferometry relates the wavefront tilt to the normalized difference of intensity between the two beams emerging from the Koesters prism (see Figure 2.3). As the tilt varies over small angles (as when the IFOV scans the target), this normalized intensity difference defines the interferogram, or “S-Curve”, given by the relation,where Ax and Bx are the photon counts from PMTXA and PMTXB respectively, accumulated over 25 milliseconds intervals when the IFOV is at location x. The Y-axis S-Curve is defined in an analogous manner. Figure 1.1 shows an S-Curve resulting from several co-added scans of a point source.2.4.1 The Ideal S-Curve2.4.2 Actual S-CurvesReferring back to Figure 2.3, if the tilt axis is of the incident beam is not at point ‘b,’ the beam is said to be decentered with respect to the Koesters prism. 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 Figure 2.4. 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.Figure 2.4: Full Aperture S-Curves of the Original FGSsThe degrading effects due to the misalignment of an FGS with the spherically aberrated OTA can be reduced by masking out the outer perimeter of the HST primary mirror. This eliminates the photons with the largest phase error. The 2/3 PUPIL stop accomplishes this and restores the S-Curves to a level which allows the FGS to track guide stars anywhere in the FOV. Unfortunately, it also blocks 50% of the target’s photons, so nearly a magnitude of the HST Guide Star Catalog is lost. Figure 2.5 shows the improvement of the S-Curve signature with the 2/3 PUPIL in place relative to the full aperture for the three FGSs. The PUPIL has been used for HST guiding since launch.The S-Curve measurements in the original three FGSs indicated large decenters of the Koesters prisms in FGS1 and FGS2 and field dependency in FGS3. FGS1r also shows field dependence, as can be seen for three positions across the FGS1r FOV in Figure 2.6 (however, note that its x,y fringes are near ideal at the FOV center).