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32.11 Astrometry and Aperture Location


No precise aperture location measures were made in the pre-COSTAR period. The absolute location of the 4.3 aperture was determined to approximately 1" accuracy. Each single aperture was assumed concentric with the 4.3. Post-COSTAR calibrations, which are applicable for the purpose of assessing aperture concentricity, suggest that the circular apertures are indeed concentric but are all displaced by approximately 0.1" from the center of the 4.3 (see FOS ISR 137 for precise aperture offsets).


The absolute locations of the 1.0 apertures were chosen as the fiducial and the relative positions of all the other FOS apertures were measured with respect to them. ACQ/PEAK scan steps with precise pointing telemetry were used to determine the positions of the apertures. The absolute locations of the 1.0 apertures were determined with 1 accuracies of 0.24" and 0.30" for FOS/BL and FOS/RD respectively. All relative aperture locations except for the 4.3, were determined with 0.02" 1 accuracy (see FOS ISR 138). The uncertainty in the relative position of the 4.3 aperture was 0.1" (see FOS ISRs 137 and 139).

The FOS plate scale (see Table 29.2) is accurate to 2% in each coordinate.

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