NIRCam Subpixel Dithers

JWST NIRCam subpixel dithers are small pointing offsets between exposures that mitigate an undersampled PSF and bad detector pixels.

NIRCam subpixel dithers are small pointing offsets performed between exposures in imaging, coronagraphy, and wide field slitless spectroscopy observations. They include subpixel offsets designed to mitigate undersampled PSFs. The available subpixel dither patterns serve different purposes for different observing modes:

  • NIRCam standard subpixel dithers, for imaging observations, are small telescope pointing maneuvers (0.2"–2") designed to optimally sample the PSF and improve the spatial resolution of stacked images.
  • NIRCam small grid dithers, for imaging and coronagraphy, are very small pointing shifts (<0.06") performed quickly and precisely with the fine steering mirror:
    • Imaging small grid dithers perform the same optimal pixel subsampling as standard subpixel dithers, but with smaller shifts performed with more efficiency.
    • Coronagraphy small grid dithers are designed to optimize PSF subtraction of a reference star from the science target.
  • NIRCam WFSS subpixel dithers (WFSS) are small telescope pointing maneuvers (~0.6") optimized to improve long wavelength grism observations and short wavelength simultaneously-obtained images.

complete set of primary and secondary subpixel dither patterns (specified in arcseconds) is available as ASCII tables in a .zip file:

This .zip file contains a collection of separate files for all the NIRCam-related dither patterns. Specifically relevant to the subpixel dither patterns described in this article, it includes these subpixel imaging dither patterns (mapping their names to the APT pattern names):

FilenameAPT dither pattern type and pattern name
NircamImagingSmallSubpixel.txtSubpixel Dither Type: SMALL-GRID-DITHER
NircamImagingSubpixel.txtSubpixel Dither Type: STANDARD
NircamWfssSubpixel.txtSubpixel Dither Type: 2-Point, 4-Point, 9-Point


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

Lajoie, C-P et al., 2016, SPIE, 99045K
Small-grid dithers for the JWST coronagraphs

Notable updates

  • Added file (replacing previous file) with updates from in-flight data, and corresponding updates to the text.
Originally published