The MultiDrizzle Handbook
4.1 HST Pointing Accuracy and Stability
4.1.1 HST Pointing Accuracy
A knowledge of the capabilities and limitations of HST and its complement of science instruments is of direct importance in deciding whether or not to obtain dithered observations, and what kind of strategies to use if dithering is chosen. A principal issue that must be taken into account when considering strategies for dithered observations is a knowledge of the pointing stability and offsetting accuracy of HST. Regardless of whether integer or sub-pixel dither offsets are being considered, it is important to understand the level to which positioning accuracy can be achieved by the acquisition and tracking system of HST. Specifically, the following issues must first be addressed when considering a sequence of multiple, dithered exposures of the same target with HST, which can be broadly divided into three types of observing program structures:
- Within a single orbit:
- Within a single visit (i.e., multiple contiguous orbits):
- Across multiple visits:
Our statistics on spacecraft behavior are continually improving. There have now been several large campaigns that have made extensive use of HST dithering to optimize the science, for example the Hubble Deep Field North and South (Williams et al. 1996, 2000; Casertano et al. 2000; Gardner et al. 2000), the Hubble Ultra-Deep Field (Beckwith et al, 2006), long-term monitoring campaigns of the globular clusters M22 and 47 Tuc (programs 7615 and 8267 respectively; Sahu et al. 2001; Gilliland et al. 2000). These provide an excellent body of information about the precision and repeatability of HST offsets, as well as the tracking stability of the telescope when no offsets are commanded (e.g., multiple exposures at the same location). Drawing on our experience with these observing programs, we now describe in more detail the HST pointing and stability characteristics for each of the above observing modes, particularly in terms of the positional accuracy of the spacecraft when performing offsets for dithered observational programs.
Gilliland (2005)contains a thorough analysis of the datasets which establish the values given in the table below.
Table 4.1: Typical HST Pointing and Stability Characteristics
Observing Scenario (with fine lock on two guide stars) Type of Program Typical RMS Precision Single pointing Offsets within an orbit
(recommend < 1 arcsec)
Re-acquisition for contiguous orbits in the same visit Repeatability for different visits, same guide stars and same ORIENT Pointing repeatability with different guide stars
4.1.2 HST Tracking Stability at a Single Location
During each orbit, thermal variations in the telescope cause structural variations called "breathing" which leads to changes not only in the optical telescope assembly (OTA) but also in the way in which the Fine Guidance Sensors (FGS) track the guide stars. The breathing in the instrument optics manifests itself as time-dependent changes in the shape and centroid of the PSF across the image, due to the changing focus.
The 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 both guide stars, in which case the drifts would be mainly related to thermal variations and jitter, with effects predominantly in the form of translations. Some small amount of rotation may also occur during the orbit, typically less than a few hundredths of a pixel across the science instrument. The typical r.m.s. tracking accuracy is generally of the order of 2-5 mas or less throughout an orbit, and can always be verified post-facto for a particular observation by examining the jitter files that form part of the archival dataset.
In some observations, however, fine lock is achieved successfully 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 by ~1-5 mas/sec about the guide star. The rate is typically ~1.5 mas/sec, but up to 5 mas/sec can be seen on rare occasions. This will manifest 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 will depend 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 - 20 arcminutes. For these instruments, the maximal scenario of a rotational drift of 5 mas/sec would produce a total translation during one orbit ranging between ~25 - 85 mas. For WFPC2 this maximal shift would be ~50 mas.
Thus, before proceeding with the analysis of dithered data, it is always advisable to examine the jitter data products after the observations to confirm whether two-FGS fine lock was successfully achieved during the observations. If this was the case then the expected translational shifts due to FGS drift should be less than ~3 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 further details on the jitter files and other data products, as well as how to extract the relevant information from these files.
4.1.3 Precision of Commanded Offsets
If the primary reason for dithering is to avoid bad pixels or improve the 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 offsets typically agree with the commanded offsets to an r.m.s. within ~2-5 mas during a single orbit with good lock on both guide stars, ranging up to ~10-15 mas from visit to visit over many days. Occasionally, the actual offsets can differ substantially from the commanded offsets by ~0.1-0.2 arcseconds or more, and with field rotations up to 0.1 degree, as a 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 somewhat larger dither offsets, up to a few arcseconds, are required in order to bridge inter-chip gaps between detectors, as in the multiple cameras of WFPC2 or the two detectors in ACS/WFC. Offsets of this size are unlikely to present any problems with pointing precision, although 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 the guide stars being moved out of the FGS apertures, depending upon the exact configuration of the primary and secondary guide stars. This would necessitate a full acquisition of new guide stars, with substantial associated overhead, and loss of pointing repeatability as a result of the relative positional uncertainties in the guide star catalog (~0.2 - 0.5 arcsec). Such large offsets are more appropriate for mosaicing programs where large areas are being mapped, and would thus involve a fundamentally different observational design than programs involving small dither offsets.
4.1.4 Pointing Repeatability After Guide Star Re-acquisition
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. At the start of the first orbit of a visit, a full guide star acquisition is performed. For each subsequent orbit in the same visit, after the telescope exits from occultation, a re-acquisition of the guide stars is carried out. Since a re-acquisition slews HST in order to force the guide stars to reside in exactly the same location in the pickles as in the previous orbit, the science instrument is typically placed successfully to within ~5 - 20 mas of its location during the previous orbit. This is generally sufficient to perform sub-pixel dithers reliably with most of the HST instruments that have pixel sizes of the order ~0.05 - 0.1 arcseconds. Thus, we generally recommend that the observing schedule be designed to fit all dithered observations of a given target into a contiguous set of orbits within a single visit, whenever possible, to provide improved relative image registration.
4.1.5 Roll Angle Repeatability Over Multiple Visits
Some observing programs are sufficiently large that they necessitate dithered observations of the same target over many orbits. In such cases, breaking up the observations into several separate visits is unavoidable, since single visits are usually constrained by scheduling limitations to contain no more than five orbits. If these multiple visits are scheduled across different dates, then some shifts may be present in the images even when specifying the same pointing, the same guide stars, and same ORIENT as previous visits.
At the start of a new visit, HST first sets up the specified roll for the observation using the gyros and carries out a full acquisition of the dominant guide star, after which it acquires the sub-dominant guide star and tracks it in fine lock. The PCS then preserves this roll angle for the remainder of the visit. In most cases the difference between the desired roll and the actual roll angle will be less than around 0.003 degrees, corresponding to a positional shift of about 73 mas at the sub-dominant guide star (assuming a separation of 1400 arcseconds between the two guide stars). At the 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 with the same 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 degrees and upward can occur (i.e., translational shifts above 100 mas).
The jitter files, which allow the roll angle to be determined based on guide star locations in the FGS, can be used in the case of visits with the same guide stars and same roll, to determine quite accurately the actual roll change that was incurred between visits.
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