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 FOVs of their FGSs. Offsets within single detectors (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, available from the Phase II Program Preparation Web page. If guide stars are used, the possible pattern area is limited by the requirement that the same guide stars be used throughout the pattern. This implies about 120 arcsec of maximum linear motion.

For most small or medium-sized imaging programs (e.g., up to a few orbits per target/field combination), the 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.