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FGS SMOV4 Requirements
Ed Nelan Art Bradley July 29, 2003


FGS SMOV 4 Requirements
· SM4 will install a refurbished FGS
­ either bay 2 or bay 3 ­ Phase 2 proposals will be provided for either possibility

· FGS considerations generate 11 requirements
­ Very similar to SMOV2 and SMOV3A ­ Approximately 59 HST orbits will be required.


SMOV4 FGS Requirements
FGS Requirement 01 Guide Star Acquisition (BEA test w/FGS1,FGS2) 02 S-curve Optimization for FGS3r (AMA) 03 FGS-FGS Alignment 04 Mini-OFAD FGS3r 05 Guide Star Acq w/FGS3r 06 Pre-SM4 FGS1, FGS2 baseline test 07 Re-commission FGS1, FGS2 (Ops and Science) 08 Near Term Stability Check on FGS3r (2 months) 09 FGS3r PMT Calibration 10 FGS3r PMT Dark Count Calibration 11 FGS3r Obscuration Zone Location (uses ITS) orbits 2 25 12 6 2 4 4 4 -


FGS 01: Guide Star Acquisition Test
· Acquire and track (finelock) guide stars with FGS1, FGS2
­ verifies critical capability ­ characterizes spacecraft jitter

· First pointed observations following release (in BEA). · Use 2 pairs of guide stars which have been used in the past. · Cycle between FGS1, FGS2 as the dominant, primary FGS. · Track guide stars, drop to gyros for several minutes, re-acquire guide stars. · Merle Reinhart, George Chapman will likely produce the phase2 (after launch of SM4).


FGS 02: S-curve Optimization
· FGS3r observations of a standard star in Transfer mode at selected locations in FGS FOV.
­ uses F583W and 2/3 Pupil ­ provides data needed to determine adjustment of articulated mirror assembly (AMA). ­ assess field dependence of interferometric response ­ iterative process, convergence historically achieved in 4 to 5 iterations. ­ 25 orbits over a 7 day period will be required.

· This is a prerequisite for commissioning FGS3r as a guider.


FGS 02: S-curve Optimization

pre-AMA adjustment

post -AMA adjustment


FGS 03: FGS-FGS Alignment
· Propose a full FGS-FGS alignment calibration along with FGS3r commissioning as a guider. · Current accuracy of the FGS-FGS alignment is too poor (~400 mas) to support COS target acquisitions (2" aperture). · Re-calibrating the FGS1r, FGS2r alignments will leave GSC-2 as the dominant source of pointing errors. This is adequate to support COS. · Total additional cost (over just calibrating FGS3r) is 8 HST orbits.




FGS 04: Mini-OFAD Calibration of FGS3r
· Geometrical distortions, or Optical Field Angle Distortions (OFAD) of FGS3r to be calibrated down to ~30 mas. · Calibration will use an astrometric field in M35 (calibrated by HST with FGS3 and FGS1r). · 6 HST orbits are required to achieve the required accuracy. · This calibration is prerequisite to the FGS-FGS alignment calibration.


FGS 05: Guide Star Acquisition Verification for FGS3r
· Verify that FGS3r can acquire and track both bright (V~9) and fa int (V~14) guide stars in finelock. · Final step in commissioning FGS3r for operations. · Prerequisites
­ K-factors derived from observations following final AMA adjustment. ­ PDB updates with the calibrated OFAD distortion coefficients and FGSFGS alignment matrices.

· Use guide star pairs that have been successfully used in the past. · 2 HST orbits required.


FGS 06: Pre-SM4 FGS1r, FGS2r Characterization
· Characterize FGS1r, FGS2r plate scale, geometric distortions, and interferometric response prior to SM4. · Provides baseline data to assess impact of SM4 on guiders.
­ Astrometric field to be observed in Position mode ­ Standard star (Upgren69) to be observed in Transfer mode.

· Total of 4 HST orbits required.


FGS 07: FGS1r, FGS2r Re-commissioning
· Characterize the stability of the S-curves, plate scale, and distortions in FGS1r and FGS2r. · Repeats observations made by FGS 06 after SM4. · Assess the validity of the operational calibrations after SM4. · Enables resumption of astrometry science with FGS1r. · 4 HST orbits required.


FGS 08: Near Term Stability of FGS3r
· After insertion into orbit, FGS3r performance is expected to evolve as the instrument desorbs H2O from its graphite epoxy composites. · Observe a standard star in Transfer mode at 2 and 4 months after commissioning FGS3r.
­ monitors interferometric response, validity of K-factor values used for guide star acquisitions.

· Observe an astrometric cluster in Position mode at 2 and 4 months after commissioning FGS3r.
­ monitors plate scale, geometric distortion, and FGS3r-FGS alignment calibrations. (2 orbits)

· Total of 4 HST orbits required. Longer term monitoring to be carried by FGS/AST yearly calibration program.


FGS 09, 10, 11
·
­ ­ ­

FGS 09: FGS3r PMT calibration
maps stellar V magnitudes to FGS3r photon counts. needed to compute K-factors and fine error signal averaging times. uses data from observations made during OFAD calibration (FGS 04)

·
­

FGS 10: OCS Obscuration Zone Location:
observe the internal test source (ITS) to locate the OCS, a zone of exclusion for acquiring guide stars.

·
­ ­

FGS 11: FGS3r Dark Count Measurement:
needed to compute K-factors uses data acquired during FGS functional aliveness test (in shuttle bay)