Why Mosaicking and Dithering are Needed
Mosaicking is done with the aim of increasing the area of sky covered by a particular set of exposures, usually with the goal of providing a seamless joining of contiguous frames. The angular offsets used when mosaicking are generally large, up to the size of the field of view. Only programs observing targets larger than the field of view of the detector need to use mosaicked exposures.
Dithering generally involves much smaller telescope offsets, often just a few pixels in size. Most imaging programs are advised to use dithering for several reasons, including:
- removal of hot pixels and other detector blemishes
- improving sampling of the astrometric scene
- improving photometric accuracy by averaging over flat-fielding errors
- bridging over the gap between the chips in the UVIS channel
Dithered and mosiaced exposures can be combined using software such as MultiDrizzle (see the MultiDrizzle Handbook). Note that it is sometimes necessary to use software like MultiDrizzle to combine even CR-SPLIT or repeat exposures, when pointing drift causes slight misalignment of exposures and differences in how PSFs are pixelated, or when gradual changes in focus over the course of an orbit produce changes in the observed PSF. In some programs, especially those observing time-variable phenomena, combining dithered exposures to correct for cosmic rays and transient bad pixels may be scientifically infeasible. In such cases, single-image based methods must be used. These methods use statistical properties of cosmic-ray brightness or sharpness to identify and interpolate across cosmic rays. Single-image cosmic ray rejection schemes are not available through the standard WFC3 calibration pipeline.
WFC3 Dither Patterns
A number of different types of patterns are available to support dithered and mosaicked WFC3 observations. For the most up-to-date information about the pre-defined patterns which have been implemented in APT, please refer to Section 8.4 (Convenience Patterns) of the Phase II Proposal Instructions
Appendix C, section 2 of the WFC3 Instrument Handbook provides summaries of patterns and illustrates the relationship between dithers in detector pixels and dithers in POSTARGs. ISR 2010-09 and ISR 2016-14 present strategies and rationales for selecting and creating dither patterns. Since the IR channel can be more undersampled than the UV channel, ISR 2016-14 provides someadditional dither patterns for WFC3/IR to provide optimal pixel-phase sampling for sets of exposures from N=2 to N=9. Note: Due to document publication schedules, some of the newer patterns and new subpattern capabilities described in the Phase II Proposal Instructions are not documented in the Handbook, or included in ISR 2010-09.