5.4	Cycle 20 Calibration and Monitoring Program

5.4.1 Active FGS1r Calibration and Monitoring Programs

A short summary is given for each FGS1r calibration and monitoring observation included in the Cycle 20 calibration plan. For specific information, please refer to the STScI FGS Web page (under the topic FGS and Calibration Plans”) and to the HST Schedule and Program Information Web page:

http://www.stsci.edu/hst/scheduling/program_information

5.4.1 Active FGS1r Calibration and Monitoring Programs

•	Prop 13172: Astrometry of Guide Stars and Target Stars in FGS-SI Alignment Calibration

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This proposal gathers the data needed to obtain milli-arcsecond relative astrometry of the guide and science instrument target stars used in the FGS-SI alignment calibration proposal (e.g. 12781 in Cycle 19). To do so, FGS1r is used to observe the guide stars as astrometry targets along with OFAD stars that also lie in the FGS FOV. In addition, the stars observed by WFC3 and STIS in the FGS-SI alignment program also fall within the FGS FOV; these stars will also be observed by FGS1r and subsequently placed in the milli-arcsecond OFAD catalog. These positions will be used to generate a 1 mas catalog that will replace the 40 mas UCAC4 catalog for measuring the FGS-SI alignments.

•	Prop 13173: Mini-OFAD and FGS-FGS Alignments for HST's Three FGSs

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This proposal gathers the data needed to calibrate the geometric distortion (OFAD) and FGS-FGS alignments for all three FGSs. The target field, M35, contains stars from the 1 milli-arcsecond FGS1r OFAD catalog. The relative positions of these stars will be used as the input catalog for the calibration, replacing the much less accurate UCAC positions. The proposal is structured in such a way that targets and guide stars that are not currently part of the OFAD catalog will be brought into the catalog by using FGS1r astrometry visits to observe the position of such stars relative to the OFAD stars. The ultimate goal is to calibrate the distortions to better than 10 mas in FGS2r2 and FGS3 (FGS1r is already calibrated to better than 1 mas), and to obtain FGS-to-FGS alignments accurate to 10 mas. For visits where time permits, several stars are observed with both PUPIL and F583W in order to determine what impact the F583W has on geometric distortion and FGS alignment, in preparation for the use of F583W for guiding, if such becomes necessary.