As a result of the misfigured HST primary mirror, the FGSs must contend with a spherically aberrated wavefront. As discussed in Chapter 2
, this aberration amplifies the degrading effect of any misalignment of FGS optical elements that produces a shift of the beam at the Koesters prism. The result is a deformed S-Curve with reduced modulation. In order to provide an in-flight means to align FGS1r and thereby guarantee its interferometric performance, a stationary mirror was remounted on a commandable mechanism capable of tip/tilt articulation. This Articulating Mirror Assembly (AMA) is currently available in FGS1r and FGS2r.2 Without the AMA, FGS1r would most likely not have been suitable as a science instrument.
It was anticipated that FGS1r’s performance would evolve during its first year in orbit due to the outgassing of the graphite epoxy composites upon which the instrument’s optical bench is mounted. To monitor the changes, a standard star was observed in Transfer mode, once per month for the first 120 days, and then approximately every 3 months afterward. Consistent with the outgassing hypothesis, the S-Curves on both the X and Y axis were seen to change (degrade) quickly at first but eventually reached an approximate steady state by the first quarter of 1998. Figure B.2
shows the early evolution of the full aperture (F583W) S-Curves, from initial optimization on March 24, to August 10, 1997, and Figure B.3
shows the evolution from September 19, 1997 through February 23, 1998.
On May 8, 1998, the AMA was once again adjusted to restore the interferometer’s performance to yield the S-Curves displayed in Figure B.4
. With near ideal S-Curves on both the X and Y axis, STScI executed an angular resolution test to assess FGS1r’s potential as an astrometric instrument.