The WFI observation configurations permits a set number of Dither Patterns. There are two types of dither patterns: (1) those that fill detector gaps, Appendix: WFI Dithering, and (2) those that provide sub-pixel sampling, Appendix: WFI Dithering. The options for both are described below. 

Dither Pattern Overview

WFI dither patterns can be selected in APT (see Pass Plans), and used in concert, to accomplish multiple goals: fill gaps in sky coverage between the 18 detectors, mitigate flat field uncertainties and scattered light, and provide sub-pixel sampling. Dithers are executed in the coordinate system of the WFI aperture selected. Small-grid dithers can be executed very efficiently with fine steering, whereas larger dithers require telescope slews and additional overhead time. The maximum distance allowed between pointings in a single visit is a function of Galactic latitude for Fixed Targets. Dither Patterns are executed for each Tile within the user-defined Mosaic Patterns to define the overall pointing strategy within a Segment. Appendix: WFI Dithering and Appendix: WFI Dithering can be used at the same time and the SUB dither will be executed for each pointing in the GAP Dither. 

Gap-filling dither patterns (GAP)

GAP dither patterns are large-scale shifts that require telescope slews. These patterns are designed to fill-in the gaps between the WFI detectors, with overlaps that are small compared to the field-of-view. The strategy behind each GAP pattern is provided in the Appendix: WFI Ditheringand in the Appendix: WFI Dithering. Most GAP patterns perform the minimal shift needed to span the gaps between the detectors (with small overlaps), but the WIDE versions perform the maximum shift that covers the gaps, as a tradeoff between expanding the field of view and exposure depth.

Subpixel dither patterns (SUB)

SUB dither patterns are small-scale shifts designed for subpixel sampling. The initial set includes only small-grid subpixel patterns which can be executed very efficiently, as they do not require telescope slews. Standard (larger) subpixel dithers, and dithers optimized for slitless spectroscopy, will be added as needed. Optimal subsampling requires at least 4 dither points, but some benefit is derived from the 2 and 3 point dithers (especially in concert with a GAP dither). The options are: NONE (default), SUB2, SUB3, SUB4, SUB5, SUB6, SUB7, SUB8, SUB9 where the number after "SUB" indicates the number of sub-pixel dithers. 

The positions for the eight SUB dither patterns are shown in the Appendix: WFI Dithering. 

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