NIRCam Dithers and Mosaics

JWST NIRCam dithers and mosaics are pointing shifts performed between multiple exposures to improve sky coverage and image quality.

Dithers and mosaics are used to improve data quality primarily for the NIRCam imaging and wide field slitless spectroscopy observing modes. Detailed background information on principles of dithered observations with JWST are described in Koekemoer & Lindsay (2005), Anderson (2009), Anderson (2011), Anderson (2014), and Coe (2017).

In summary, the multiple offset exposures described below mitigate bad pixels and provide the following additional benefits:

  • Primary dithers are telescope pointing maneuvers (4"–100") to fill gaps in sky coverage between detectors and mitigate flat field uncertainties.
  • Subarray primary dithers are small telescope pointing maneuvers (<0.4") to improve the spatial coverage of the small subarrays. 
  • Standard subpixel dithers are small telescope pointing maneuvers (0.2"–2") to improve the spatial resolution of stacked images.
  • Small grid dithers are very small pointing shifts (<0.06") performed quickly with the fine steering mirror primarily to improve reference PSF subtraction in coronagraphy.
  • Mosaics are, typically, large pointing shifts (arcminutes) to tile over areas larger than the NIRCam field of view (5.1' × 2.2').

None of these pointing maneuvers are allowed in either of the NIRCam time-series observing modes.

Dithers and mosaics are distinct in terms of operational ordering. All dither positions must be executed before any filter changes may be executed, and all filter changes are executed before proceeding to the next tile in a mosaic. The following ordered list summarizes this sequence of (mostly optional) operations, also shown graphically in Figure 1:

Figure 1. Operational ordering for dithers, filter changes, and mosaics

Operational ordering for dithers, filter changes, and mosaics

This example observing sequence includes 2 subpixel dither steps at each of 2 primary dither steps. (Two are shown for illustration; more dither steps are preferred in practice.) This pattern repeats after changing filters, and these multi-filter observations repeat at each pointing in the mosaic. Most of these operations (e.g., changing filters) are optional.
Additional articles about NIRCam dithers and mosaics:


Anderson, J. 2009, JWST-STScI-001738
Dither Patterns for NIRCam Imaging

Anderson, J. 2011, JWST-STScI-002199
NIRCam Dithering Strategies I: A Least Squares Approach to Image Combination

Anderson, J., 2014, JWST-STScI-002473
NIRCam Dithering Strategies II: Primaries, Secondaries, and Sampling

Coe, D. 2017, JWST-STScI-005798
More Efficient NIRCam Dither Patterns

Koekemoer, A. M. & Lindsay, K. 2005, JWST-STScI-000647
An Investigation of Optimal Dither Strategies for JWST


Notable updates

  • Clarified dither sizes in parentheses
Originally published