Hubble Space Telescope Primer for Cycle 11

5.4 Offsets and Patterns

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Offsets (using the same guide stars and performed under the same guide star acquisition) can be performed to an accuracy of about +/-0.02" for the larger slews. 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.