Other
Scientific paper
Jul 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001hst..prop.8946n&link_type=abstract
HST Proposal ID #8946
Other
Hst Proposal Id #8946
Scientific paper
This proposal is the first of a pair of proposals that monitors the characteristics of HST's three Fine Guidance Sensors across the boundary of the servicing mission SM3B. This proposal establishes the pre-SM3B baseline measurements. Proposal 8995 gathers the same data but post-SM3B. For each FGS, stars from a selected field in the standard astrometric Galactic cluster M35 are observed in Position mode. This provides a measurement of their relative angular positions, which in turn provides an effective means to detect and monitor any changes in the FGS optical distortions and plate scales. Measurements are made before and after SM3B {prop 8995}. These data are compared to one another by an astrometric plate overlay. The residuals provide an assessment of how much the distortions have changed as a result of the activities associated with SM3B, and if out of tolerance, will indicate the need for a re-calibration of the particular FGS's distortions. The FGS-FGS alignments will also be monitored across the SM boundary. This will be done by using the same guide star pairs in the corrresponding visits of 8946 and 8995. Finally, this proposal and 8995 include Transfer mode observations of the FGS standard star Upgren69. These observations will obtain S-curves at the center of each FGS FOV. These data will provide insight as to whether or not the relative alignment of the FGS optical train with the HST OTA has changed. If changes larger than tolerance are observed, it will be necessary to activate the appropriate contingency proposal that will obtain S-curves are several locations in the FGS FOV. These data will be used to re-compute the optimal values of parameters, or K-factors, that are used in the acquisition and tracking of guide stars {and astrometry targets}. This proposal requires a total of 12 HST orbits.
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