MIRI Coronagraphic Imaging Dithering
See also: HCI Coronagraphic Sequences
A roll dither is accomplished by executing two 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 bad pixels.
The roll dither strategy is recommended as the default. 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 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).
LaJoie, C-P, et al. 2016, Proc. SPIE, 9904
Small-grid dithers for the JWST coronagraphs