5.4 Offsets and Patterns

Offsets are routinely used to reposition the target in the instrument field-of-view. The size of the offset is limited by the requirement that both guide stars remain within the respective fields of view of their FGSs. Offsets within single detectors (the most common type) can be performed to within +/-0.003". Offsets that continue across separate visits (when executed with the same guide stars) will typically have an accuracy of ~ 0.05".

Patterns are used to place the telescope at multiple positions to allow for dithering or mosaic construction. Patterns can define a linear, spiral, or parallelogram series of observation points. Patterns can also be combined to produce a more complex series of observation points. In addition, "convenience patterns" have been predefined to represent typical dither and mosaic strategies; for details see the Phase II Instructions document, available from the Phase II Program Preparation Web page. The possible pattern area is limited by the requirement that the same guide stars be used throughout the pattern. This implies a maximum of about 120 arcsec of linear motion.

For most small or medium-sized imaging programs (e.g., up to a few orbits per target/field combination), conventional dither patterns can be used, which generally consist of offsets designed to provide half-pixel subsampling as well as to move bad pixels and inter-chip gaps to different locations on the sky. Larger programs may benefit by considering more complex dithering strategies, to provide, for example, even finer subsampling of the detector pixels. The data can be combined using the MultiDrizzle software provided as part of PyRAF/STSDAS. More details are provided in the HST Dither Handbook.

In general, undithered observations with the ACS CCD detectors will not be approved without strong justification that such are required for the scientific objectives. Otherwise, hot pixels and other detector artifacts will compromise the program and the archival value of the data. Further details about the options and advantages of ACS patterns can be found in the ACS Instrument Handbook, the Phase II Proposal Instructions, and the ACS Dither Web page.