Astronomy and Astrophysics – Astronomy
Scientific paper
Jul 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000hst..prop.8830n&link_type=abstract
HST Proposal ID #8830
Astronomy and Astrophysics
Astronomy
Hst Proposal Id #8830
Scientific paper
It is well known for our experience with FGS3, and later with FGS1r, than an FGS experiences long term evolution, presumably due to disorption of water from the instrument's graphite epoxy composits. This manifests principly as a change in the plate scale and secondarily as a change in the distortions. These effects are well modeled by adjustments to the rhoA and kA parameters which are used to transform the star selector servo angles into FGS {x, y} detector space coordinates. By observing the relative positions of stars in a standard cluster at a fixed telescope pointing and orientation, the evolution of rhoA and kA can be monitored and calibrated to preserve the astrometric integrity of the FGS. This is usually not an issue for a non-astrometric {science} FGS, such as FGS2r, since the calibrations needed to support operations for guiding HST are tolerant to the largest changes in rhoA and kA under normal situations. However, FGS2r was just recently instlled into HST during SM3a and is expected to undergo significant desorption during its first two years in orbit {as did FGS1r}. This proposal monitors the instrument's plate scale and distortions to assure its continued reliably as a guiding FGS.
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