The imaging mode for JWST's Mid-Infrared Instrument (MIRI) offers nine broadband filters from 5.6 to 25.5 μm in a 74" × 113" FOV at 0.11 "/pixel plate scale.
For imaging, the MIRI imager offers nine broadband filters covering wavelengths from 5.6 to 25.5 μm over an unobstructed 74" × 113" field of view, and a detector plate scale of 0.11 "/pixel (Bouchet et al. 2015). The MIRI imaging mode also supports the use of detector subarrays for bright targets, as well as a variety of dither patterns that could improve sampling at the shortest wavelengths, remove detector artifacts and cosmic ray hits, and facilitate self-calibration. The Astronomer's Proposal Tool (APT) can be used to design mosaic observations to image larger fields.
Imaging with MIRI is diffraction limited in all filters, with Strehl ratios in excess of 90%, although the detector plate scale of 0.11 "/pixel slightly undersamples the PSF at the F560W band.
MIRI imaging sensitivity is background limited in all the imaging bands (unless one takes short integrations): astronomical background limited at wavelengths <11 μm and telescope background (primary mirror and sunshield) limited at wavelengths >11 μm.
Observers will be able to specify settings for four primary MIRI imaging parameters: (1) filters, (2) dithering pattern, (3) choice of subarray, and (4) detector read out modes and exposure time (via the number of frames and integrations).
Main article: MIRI Filters and Dispersers
All of the MIRI filters available for scientific imaging are broadband (λ/Δλ ~ 5), except for F1130W, which is narrower (λ/Δλ ~ 16) to isolate the 11.3 μm PAH emission feature. They are designed to cover the full wavelength range without significant gaps in wavelength coverage.
Table 1. MIRI filter properties
MIRI operations offers several options for imaging dithers. There are multiple reasons for an observer to use dithers, some of which are unique to MIRI imaging.
Multiple dither patterns are available to support different science strategies (e.g., deep imaging, snapshots, improved PSF sampling) and different target morphologies (e.g., point, compact and extended sources). They're also available for use with predefined detector subarrays.
As with the other near-infrared instruments, MIRI dither specifications can be conceptually separated into large- and small-scale dithers. Large-scale dithers are intended to handle self-calibration and large scale gain variations. Since there is only one imaging MIRI detector, dithers are not required to cover gaps, as is the case for NIRCam. Small-scale dithers are needed to improve image quality when the native plate scale undersamples the PSF. For MIRI, only the F560W PSF is undersampled. The F770W PSF is Nyquist sampled and all other filters lead to oversampled PSFs.
Main article: MIRI Detector Subarrays
MIRI imaging supports a small pre-defined set ofsubarrays for imaging bright sources or bright backgrounds without saturating the detector. The MIRI imaging detector creates subarrays using a different scheme than the near-infrared HAWAII 2RG detectors that are used in other JWST instruments. In particular, frame time gets faster as the subarray gets closer to one edge of the detector. For instance, coronagraphic subarrays are located on the fast side of the array, as are the smallest imaging subarrays, SUB128 1 and SUB64.
Table 2. MIRI subarrays
Imager exposure specifications
MIRI imaging supports two different detector readout patterns: