HST Call for Proposals and HST Primer for Cycle 24

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5.4	Offsets, Patterns, and Dithering

Offsets are routinely used to reposition a 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 dithered or mosaiced observations. 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 at the Phase II Program Preparation webpage. 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 arcseconds of linear motion.

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

In general, undithered observations with the ACS CCDs, and WFC3’s CCD and IR detectors, will not be approved without strong justification for why it is 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 webpage. Information on dithering for WFC3 observations is found in Appendix C of the WFC3 Instrument Handbook, the Phase II Proposal Instructions, and WFC3 ISR 2010-09.

STIS CCD observations may normally benefit from dithering to eliminate hot pixels and improve PSF sampling, although for spectroscopic observations, this may complicate data reduction. See the STIS Instrument Handbook and the Phase II Proposal Instructions for further details.