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DrizzlePac 2012 Handbook
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The DrizzlePac Handbook > Appendix B: HST Pointing Accuracy and Stability > B.1 HST pointing stability

B.1 HST pointing stability
An understanding of HST’s pointing stability and offsetting accuracy is essential for planning dither observing strategies, regardless of planning for integer or subpixel offsets. Multiple, dithered exposures of the same target with HST could have three types of observing scenarios.
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The precision with which HST can be offset to different dither locations during an orbit (i.e., a comparison between the commanded and actual telescope offsets)
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Astrodrizzle-combined images from the pipeline are observations taken within a visit (the first and second scenario described above), as specified in the exposure logsheet either using the CR-SPLIT, or special requirements, or dither patterns. Such exposure groupings are known as associations. In the pipeline, exposures in an association are aligned based on the WCS of each image, and drizzle-combined to create a combined distortion-free image with suffix drz.fits. In most of these cases, the image alignment based on WCS, performed by AstroDrizzle in the pipeline, produces satisfactory results. On some occasions, the alignment may not be optimal due to pointing anomalies. These associations will have to be re-processed by the observer who will need to change the default AstroDrizzle processing for an image association to produce better results.
In post-pipeline image processing, AstroDrizzle and other associated tasks can be used for more complex image combination, such as combining observations taken across several visit or taken at different orientations, as described above.
Statistics on HST pointing behavior have been continually improving thanks to extensive use of dithering to optimize scientific output in several large observing campaigns. These include 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), and 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 observations have provided an excellent body of information regarding precision and repeatability of HST offsets, as well as tracking stability of the telescope when no offsets are commanded (e.g., multiple exposures at the same location). Drawing on experience with these observing programs, more details about HST pointing and stability characteristics for each of the observing scenarios listed above can be described, particularly in terms of positional accuracy of the spacecraft when performing offsets for dithered observational programs. Gilliland (2005) contains a thorough analysis of the datasets, which established the values given in Table B.1.
Table B.1: Typical HST pointing and stability characteristics
Not recommended unless unavoidable, e.g., due to scheduling constraints
Unavoidable in many crowded fields such as those along the galactic plane
For ACS: 0.052 arcsec/orbit in WFC

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