The Nancy Grace Roman Space Telescope's Wide Field Instrument (WFI) supports a predefined set of Dither Patterns. Two classes of dither patterns are available: those designed to fill detector gaps (GAP Dither Patterns) and those that provide sub-pixel sampling (SUB Dither Patterns). The available options for both classes are described in this article. 

Dither Pattern Overview

Dither patterns can be used for several purposes, including:

• filling gaps in sky coverage between the 18 WFI detectors,
• mitigating the effects of flat-field uncertainties and scattered light, and
• providing sub-pixel sampling.

Roman WFI dither patterns fall into two categories: GAP dither patterns, which are designed to fill detector gaps, and SUB dither patterns, which provide sub-pixel sampling. This article provides an overview of both types. More detailed information on each category, including guidance on their effective use, is available in the Gap-filling Dithers and Sub-pixel Dithers articles. 

Dither Patterns in the Observation Hierarchy

The Roman Observation Design Hierarchy Figure  illustrates how different layers of an observation are related. For Roman observations, Dither Patterns are executed at the level of a Tile, which corresponds to a single WFI pointing. Tiles are, in turn, executed within user-defined Mosaic Patterns, which together define the overall pointing strategy for a Segment.

WFI dither patterns are selected in the Pass Plans window of the Astronomer's Proposal Tool (APT). GAP Dither Patterns and SUB dither patterns can be used simultaneously; when both are selected, the SUB dither pattern is executed at each pointing of the GAP Dither. 

Dither offsets are specified in the WFI Ideal coordinate system and are defined relative to the base pointing of the selected aperture. The overheads associated with executing dither patterns are described in the Visit Timing & Overhead Estimation article.

Examples of how dither patterns are used in practice can be found in the articles describing the Community Defined Surveys, which illustrate how different dither strategies are applied to achieve specific observation goals. 

Gap-filling Dither Patterns (GAP)

Gap-filling dither patterns (GAP) are large-scale offsets that require telescope slews. These patterns are primarily designed to fill the gaps between the WFI detectors by using  pointings that slightly overlap. In addition, GAP dithers can help mitigate the effects of bad pixels (e.g., hot or dead pixels, or pixels affected by cosmic-ray hits) and reduce both inter-detector and intra-detector systematics.

A GAP dither pattern is characterized by two main features: the geometric arrangement of the dither pointings and the number of pointings, P. Both of these properties are encoded in the name of the dither pattern. Currently, two geometries are supported: LINEGAP, in which pointings are arranged along a line, and BOXGAP, in which pointings are arranged in a grid. 

The gap-filling dither patterns currently available are summarized in the  Table of Gap-Filling Dither Patterns. For additional details on GAP dither patterns and their use, see the Gap-filling dithers article. 

Sub-pixel Dither Patterns (SUB)

Sub-pixel dither patterns (SUB) are small-scale offsets designed to improve sub-pixel sampling and point-spread function (PSF) reconstruction. These patterns shift exposures by fractional pixel amounts without requiring large telescope slews, enabling efficient mitigation of intra-pixel sensitivity variations and improved sampling of the PSF.

The current set of SUB dither patterns includes eight options, SUB2 through SUB9, where the number after "SUB" indicates the total number of dither positions. Optimal sub-pixel sampling typically requires at least four positions; however, two- and three-point patterns can still provide partial improvements, particularly when used in combination with a larger GAP dither pattern.

The sub-pixel dither patterns currently available are listed in the Table of Sub-pixel Dither Patterns. For additional details on SUB dither patterns and their effective use, see the Sub-pixel Dithers article. 

For additional questions not answered in this article, please contact the Roman Help Desk.