MIRI Coronagraphic Imaging Dithering

Since coronagraphy requires the target to be placed at the center of the coronagraph, dithering for MIRI coronagraphy is restricted to roll dithers and small grid dithers

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Commissioning (Boccaletti et al. 2022) and Early Release Science (ERS-01386; Carter et al. 2022) results indicate that the use of roll and small grid dithers (SGDs) both significantly improve contrast. The advantages and disadvantages of each method are still being evaluated, including the tradeoff between telescope efficiency versus contrast.

bservers should carefully consider the contrast performance necessary to meet their science goals, and adjust their PSF subtraction strategy accordingly.  Contact the JWST Help Desk for guidance.

Roll dithers

See also: HCI Coronagraphic Sequences

A roll dither is accomplished by executing 2 sets of observations, each offset in spacecraft roll angle. Such roll-dithers can be used to increase the contrast, since instrumental/telescope artifacts remain fixed to the telescope reference frame, while astrophysical objects move. This allows more advanced processing such as ADI, KLIP, etc. The roll dither also mitigates against residual cosmic rays and other detector artifacts.

The allowed offset from nominal varies from ±3.5° (7° total) to ±7° (14° total), as a function of solar elongation (longitude of the sun) at the time of the observation. Forcing the roll offset toward the upper end of the range becomes very restrictive to scheduling because the windows of time where larger roll offsets can be accommodated get very small (see JWST Position Angles, Ranges, and Offsets for more information).

Small grid dithers

See also: HCI Small Grid Dithers

Small grid dithers (SGDs) are executed by the fine steering mirror (FSM) to move the target a few milliarcseconds (currently 10 mas per step) in a 5 × 5 or 9 × 9 grid pattern. These SGDs provide phase diversity for optimizing the reference point source subtraction and can increase the contrast by a factor of 10 or more (LaJoie, C-P, et al. 2016).


Carter, A. et al. 2023, ApJL, 951, L20
The JWST Early Release Science Program for Direct Observations of Exoplanetary Systems I: High Contrast Imaging of the Exoplanet HIP 65426 b from 2-16 µm

Boccaletti, A. et al. 2022, A&A, 667, A165
JWST/MIRI coronagraphic performances as measured on-sky

Lafrenière, D., Marois, C., Doyon, R., Nadeau, D., Artigau, E., 
2007, ApJ, 660, 770
A New Algorithm for Point-spread Function Subtraction in High-Contrast Imaging: A demonstration with Angular Differential Imaging

LaJoie, C-P, et al. 2016, SPIE 9904
Small-grid dithers for the JWST coronagraphs

Soummer, R., Pueyo, L. Larkin, J., 2012, ApJ, 755, L28

Detection and Characterization of 
Exoplanets and Disks Using Projections on Karhunen-Loeve Eigenimages

Notable updates
    Added information about the use of these dither patterns, and a published reference describing ERS results
Originally published