Space Telescope Science Institute
DrizzlePac 2012 Handbook
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The DrizzlePac Handbook > Appendix B: HST Pointing Accuracy and Stability > B.2 HST Tracking Stability at a Single Location

B.2 HST Tracking Stability at a Single Location
During each orbit, thermal variations in the telescope cause structural variations known as breathing, which leads to changes not only in the optical telescope assembly (OTA), but also in the way that the Fine Guidance Sensors (FGS) track guide stars. The breathing manifests itself as time-dependent changes in the shape and centroid of the PSF across the image due to the changing focus.
Changes related to the FGS, on the other hand, depend largely on whether fine lock has been achieved on one or two guide stars. Most observations are obtained with successful fine lock on two guide stars. In those cases, positional drifts would mainly be related to thermal variations that predominantly manifests as positional translations. A small amount of rotation may also occur during an orbit, typically less than a few hundredths of a pixel across the science instrument. Typical RMS tracking accuracy is generally on the order of two to five mas or less throughout an orbit, and can be verified post-facto by examining the jitter files that are part of the archival dataset for a particular observation.
In some observations, however, fine lock is successfully achieved on only one guide star. In this case, a steady roll angle drift is present as a result of gyro drift. The telescope will rotate about the guide star, typically at a rotation rate of ~1.5 mas/sec but rates of up to five mas/sec have occurred on rare occasions. This manifests itself primarily as a translation of the science instrument, but some slight rotation may also be evident. The actual amount of translation of the science instrument on the sky depends on its location in the focal plane relative to the guide star. For example, STIS and NICMOS are located approximately midway between the optical axis and the FGS apertures, so their distance from a guide star could range from 6 to 20 arcminutes. For these instruments, the maximal scenario of a rotational drift of five mas/sec would produce a total translation during one orbit ranging ~25 to 85 mas. For WFPC2 this maximum shift could be ~50 mas.
Thus, before proceeding with the analysis of dithered data, it is always advisable to examine either the EXPFLAG keyword value or the jitter data products to confirm whether a two-FGS fine lock was successfully achieved during the observation. If a two-FGS fine lock was achieved, the expected translational shifts due to FGS drift should be less than three mas during the orbit, and any apparent rotation should be less than a few hundredths of a pixel across the detector. The HST Data Handbook contains details on how to extract the relevant information from jitter files.

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