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Optical Field Angle Distortion (OFAD), approaching several arc-seconds, is the largest source of error in obtaining star positions from observations with the Hubble Space Telescope (HST) Fine Guidance Sensors (FGS). The full OFAD calibration of FGS \#3 was done in January 1993, with 600 observations of stars in M35 obtained in 20 orbits. Bootstrap statistical analysis shows that no significant bias exists in the OFAD parameters, and that a positional accuracy of 2-3 milliseconds of arc (MAS) has been achieved over the entire field of view of the FGS. These OFAD parameters have been made available to all HST astrometry users for FGS data reduction.
With evidence that changes could occur both to the telescope and to the FGS which would affect practical applications of the OFAD parameters, a long term stability (LTSTAB) project continues. This series monitors and measures such changes by visiting the same OFAD field at one to two month intervals. Analysis of 11 LTSTAB orbits showed monotonic scale-like change before the November 1993 servicing mission (SM). We see a substantial jump after SM. Extensive studies reveal that this post-SM change is non-linear and cannot be properly modeled by adjusting parameters. With this change uncorrected, positional accuracy has degraded to the 6 MAS level.
We anticipated that the re-collimation and re-focus of the secondary mirror could cause a significant change in the OFAD. To regain the desired accuracy, a new set of observations, called the$\Delta$-OFAD calibration, has been scheduled for late April 1994. Simulations of the $\Delta$-OFAD indicate that a precision of 2-3 MAS can be reattained using the 2-3 MAS precision catalog which resulted from the previous full OFAD. The five orbits required will come from the FGS Instrument Scientist (L. Taff), who was foresightful enough to reserve them. Future monitoring of the FGS calibration by means of a continuing LTSTAB observations is vital to the success of HST FGS astrometric projects.
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