The WFI observation configurations permit a set number of Dither Patterns. There are two types of dither patterns: those that fill detector gaps (GAP Dither Patterns), and those that provide sub-pixel sampling (SUB Dither Patterns). The options for both are described in this article. 

Dither Pattern Overview

WFI dither patterns can be selected in APT (see Pass Plans) and used in tandem to accomplish multiple goals. Dither patterns are used to fill gaps in sky coverage between the 18 detectors, to mitigate flat-field uncertainties and scattered light, and to provide sub-pixel sampling. Dithers are executed in the coordinate system of the selected WFI aperture. 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. GAP Dither Patterns and SUB dither patterns 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 Table Describing GAP Dither Patterns and in the Figure of Gap Filling Dither Patterns. 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 trade-off between expanding the field of view and exposure depth.

Subpixel dither patterns (SUB)

SUB dither patterns are small-scale shifts designed for sub-pixel sampling. The initial set includes only small-grid sub-pixel patterns, which can be executed very efficiently since they do not require telescope slews. Standard (larger) sub-pixel dithers, and dithers optimized for slitless spectroscopy, will be added as needed. Optimal sub-sampling requires at least four dither points, but some benefit is derived from the two and three point dithers (especially in tandem with a GAP dither). The options for sub-pixel dither patterns 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 Figure of Sub-pixel Dither Patterns. 

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