The JWST Mid-Infrared Instrument (MIRI) provides dithering capabilities for the imaging, low-resolution spectroscopy, medium-resolution spectroscopy, and coronagraphic imaging modes for optimal sampling, bad pixel mitigation, and background subtraction.
When using MIRI, dithering is required to achieve accurate photometry and to provide superior sampling (Arendt et al. 2000). Dither patterns for MIRI have been designed to account for the fact that telescope motions larger than ~20" typically result in new guide star acquisitions and larger observing overheads. The fine steering mirror (FSM) can be used for sub-pixel (≤50 mas) offsets, which will be used primarily for coronagraphy. Larger dithers are accomplished by moving JWST.
Dithers and mosaics are distinct in terms of operational ordering. All dither positions must be executed before filter changes may be executed. All filter changes are executed before proceeding to the next tile in a mosaic.
MIRI specific patterns can be implemented in the Astronomer's Proposal Tool (APT) for:
- Imaging dithers
- Coronagraph imaging dithers: no dithering is available
- Low-resolution spectroscopy dithers: ‘Point Source/Staring’ and ‘Extended Source/Mapping'
- Medium-resolution spectroscopy dithers
- Dithering with JWST