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We can not stress too strongly the utility of assessing the accuracy of your target acquisition.
) to which an additional uncertainty of 0.15" due to possible uncorrected GIM motion must be added in quadrature. The GIM-related component was effectively removed by the onboard GIM correction (keyword YFGIMPEN=T) implemented April 5, 1993.
algorithm-related pointing uncertainty of 0.08". Post-COSTAR FOS/RD one algorithm-related pointing uncertainty of 0.12".
limiting measurement accuracy was typically 0.1" in each detector coordinate. GIM uncertainty of up to 0.15" should be added in quadrature for observations made prior to April 5, 1993.
Target acquisition was the main cause of target miscenterings in the FOS apertures and miscentering was one of the most important photometric error sources. It usually produced gray light losses, but for larger miscenterings with apertures larger than 0.5, which were relatively common in the pre-COSTAR period, substantial color effects were possible, especially in grating overlap regions or other regions of high "s-curvature." See "Target Miscentering" on page 32-21.
In summary, normal FINE LOCK guiding provided one
guiding accuracies of 0.007" or less. Nominal guide star re-acquisitions allowed the continuation of this pointing accuracy from one orbit to another. All FOS calibration observations were obtained in FINE LOCK with guide stars in two FGSs. 32.1.3 Jitter
Jitter was mostly due to the thermal instability of the solar panels. The greatest excursions occurred when the spacecraft crossed the terminator and lasted for a few minutes. The jitter caused the telescope to mispoint, moving the target in the aperture. This problem was minimized, but not completely eliminated with the introduction of compensating spacecraft commanding in April 1992. Prior to April 12, 1992, rms jitter was 17 mas (orbital day) and 12 mas (night). Starting on April 12, 1992, rms jitter dropped to 7 mas (day and 6 mas (night). For observations obtained after February 5, 1995 jitter excursions can be evaluated by examination of the jitter files or the jitter ball in the FOS paper products. This problem had the largest effect on the small apertures (0.3" and smaller), because the target could move out of the aperture for a short period and even for the larger apertures the associated photometric errors cannot be accurately determined because the exact Y-base of the spectra was unknown. The photometric error introduced by jitter was rarely more than 1%, and always less than 3%.