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DrizzlePac 2012 Handbook
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The DrizzlePac Handbook > Appendix B: HST Pointing Accuracy and Stability > B.3 Precision of Commanded Offsets

B.3 Precision of Commanded Offsets
If the primary reason for dithering is to avoid bad pixels or improve PSF sampling, then dither offsets less than about one arcsecond are recommended. Examination of HST behavior in previous dither campaigns reveals that for offsets of this size, the actual measured dither offset will agree with the commanded offset to an RMS within about two to five mas during a single orbit with good lock on both guide stars. The RMS of this offset typically increases to a range of up to ~10 to 15 mas when comparing one visit to another over several days. Occasionally, the actual offsets can differ substantially from the commanded offsets by ~0.1 to 0.2 arcseconds or more, with field rotations of up to 0.1. This is generally the result of FGS false lock on a secondary null, or other FGS interferometric peculiarities. This behavior was observed in two out of nine pointings during the HDF-N campaign.
In some cases, larger dither offsets of up to a few arcseconds are required to bridge inter-chip gaps between detectors, as in WFPC2’s four CCDs and the two detectors in ACS/WFC. Offsets of this size are unlikely to present any problems with pointing precision but observers should be aware that such offsets may introduce more non-uniform subsampling across the field as a result of the geometric distortion inherent in the instruments.
Offsets larger than several tens of arcseconds may result in guide stars moving out of the FGS apertures, depending on the exact configuration of the primary and secondary guide stars. This would necessitate a full target acquisition using new guide stars, with substantial associated overhead, as well as a loss of pointing repeatability due to the relative positional uncertainties in the guide star catalog (~0.2 to 0.5 arcseconds). Such large offsets are more appropriate for mosaicing programs where large areas are being mapped, and would therefore involve a fundamentally different proposal design than those programs involving small dither offsets.
B.3.1
For many HST programs, dithered observations of a target are obtained during a number of separate orbits, often contiguous, which are in turn grouped into one or more visits.
The first orbit in a visit begins with a full guide star acquisition. For each subsequent orbit in the same visit, HST will reacquire the same guide stars upon exit from occultation. In post-occultation guide star reacquisitions, the instrument pointing is typically within ~5 to 20 mas of its location in the previous orbit.
The precision of HST’s guide star reacquisition is based on its ability to force the post-slew position of the guide stars to reside in the exact same location in the guide star acquisition field-of-view (i.e., pickles), as in the previous orbit. This is generally sufficient to reliably perform subpixel dithers for most HST instruments that have pixel sizes of the order ~0.05 to 0.1 arcseconds. Therefore, it’s recommended that, whenever possible, the observing proposal should be designed to fit all dithered observations of a given target into a contiguous set of orbits within a single visit to provide improved relative image registration.
B.3.2
Some observing programs are sufficiently large enough to necessitate dithered observations of the same target over many orbits. In such cases, it is necessary to break the observations into several visits because the length of a single visit is constrained by available scheduling windows depending on the target’s position in the sky. For all targets outside the CVZ, single visits are usually constrained by scheduling limitations to contain no more than five orbits. If multiple visits of the same target were scheduled across different dates, images in one visit may have small offsets relative to images from other visits, even if the same pointing, same guide stars, and same ORIENT were specified for the visits.
At the start of a new visit, HST sets up the specified roll for the observation using the gyros, and carries out a full acquisition of the dominant guide star. This is followed by the acquisition of the sub-dominant guide star, which enables the telescope to track in fine lock. The pointing control system (PCS) then preserves this roll angle for the remainder of the visit.
In most cases, the difference between the desired roll angle, and the actual roll angle, will be less than ~0.003. This corresponds to a positional shift of approximately 73 mas at the sub-dominant guide star, assuming a separation of 1,400 arcseconds between the two guide stars. For WFPC2, this shift is 38 mas, i.e., just less than the size of a WFPC2/PC pixel. Therefore, multiple visits at the same specified roll, target, and guide stars will, under nominal circumstances, show repeatability to this level. It is not uncommon for scheduling constraints to affect the time between updates to the Fixed Head Star Trackers (FHSTs) and FGS acquisitions, in which case roll angle deviations of 0.01 and greater can occur (i.e., translational shifts above 100 mas). For visits with the same guide stars and requested roll angle, the actual roll changes incurred between visits can be accurately determined from the locations of guide stars in the FGS as recorded in the datasets’ jitter files.

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