While perusing one of the sample science programs for the JWST (NIRCam WFSS Deep Galaxy Observations), I came across Step 5, “Decide on dither pattern.”

Instead of guessing — or worse, assuming — what a “dither pattern” is, I thought I’d ask (within the context of program planning and image acquisition):

  1. What is a dither pattern?

  2. Why might one be needed?

  3. How are dither patterns implemented on the JWST?

Extra “points” for sample imagery (real or simulated), that shows the effects of using a dither pattern in this context.


1 Answer 1



In astrophotography, to dither means to shift the pointing of the telescope slightly in random directions between exposures. This allows hot and cold pixels, cosmic ray artifacts, and fixed pattern noise, and even satellite or airplane trails to be removed during the stacking process.

A dither pattern is

a series of exposures of a single target taken at slightly different telescope pointings, with the same set of guide stars.

JWST does this

to improve photometric fidelity by moving sources to a number of locations within the shutter apertures, and to mitigate photometric uncertainties.

JWST's NIRCam implements this by choosing one of a standard set of subpixel pointing offsets between exposures. Some patterns are better at correcting for bad pixels. Others are better at correcting for an undersampled point spread function.

(A more familiar use of this is found in PC graphics cards, which may use "4x SSAA" aka supersampling to improve image quality for related reasons.)

  • 2
    $\begingroup$ MSAA is not supersampling, but multisampling, which supersamples only geometry, but not fragment colors. Full supersampling is, unsurprisingly, called SSAA. $\endgroup$
    – Ruslan
    Mar 21 at 10:30

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