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32.1 Target Location in Aperture

This section discusses factors that affect the degree to which a target is centered in the aperture. Table 32.2 summarizes the nominal initial accuracies for different kinds of acquisitions. Target miscentering affected:

32.1.1 Target Acquisition Limitations


We can not stress too strongly the utility of assessing the accuracy of your target acquisition.

The target acquisition strategy, or lack of one, was of paramount importance in centering the target in the science aperture. Target acquisition methods, their limiting accuracies, and methods of assessing their quality are thoroughly discussed in "Assessing FOS Acquisitions" on page 30-26. In summary, the pointing accuracy limitations of FOS target acquisition methods are:


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.

32.1.2 Guiding and Guide Star Acquisition Limitations

"Assessing FOS Acquisitions" on page 30-26 summarizes the accuracies of HST guiding and provides references to additional information for the detailed assessment of the guiding and the guide star acquisition (and re-acquisition) characteristics associated with FOS observations. For the typical situation, reference to the FOS paper products jitter ball plot for the exposure provides sufficient information to reveal anomalous guiding during the observation.

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%.



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