For imaging, the MIRI imager offers 9 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.
Main Article: MIRI Predicted Performance
See also: MIRI Sensitivity, MIRI Bright Source Limits
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 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 4 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).
Figure 1. The MIRI imaging FOV
|Specific sections of the MIRI imager focal plane are used for imaging, coronagraphic imaging, and low-resolution spectroscopy modes. The imaging mode FOV takes up a large section to the right of the imager focal plane.|
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
PAH, broadband imaging
Silicate, broadband imaging
PAH, broadband imaging
|F1800W||18.0||3.0||0.58||3.881||1.9||66||Silicate, broadband imaging|
1 Signal/noise = 10 for 104 s on-source integration time.
2 Saturation based on 13% of flux falling within the brightest pixel for lambda λ ≤ 8 μm and 13% × (8 μm/λ)2 for lambda λ > 8 μm.
Figure 2. MIRI imaging filter bandpasses
Click on the image for a larger view.
Main article: MIRI Imaging Dithering
See also: MIRI Dithering Overview
MIRI operations offers several options for imaging for imaging dithers. There are multiple reasons for an observer to use dithers, some of which are unique to MIRI imaging.
- Dithering allows for the removal of bad pixels and for improving the resolution of undersampled images. For MIRI imaging, only the F560W band produces undersampled images of point sources.
- Dithering by a distance larger than a few times the PSF width on a timescale of a few minutes is necessary to self-calibrate detector gain variations and drifts since detector drifts grow larger with increasing signal.
- At longer wavelengths, when the telescope background dominates the noise, dithering is needed to track temporal variations in the telescope background.
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 of subarrays 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 and SUB64. (The bold italic font indicates these are parameters in APT observing templates.)
Figure 3. Subarray locations for the MIRI imager
Subarray locations for the MIRI imager as viewed from the telescope looking down onto the detector. Imaging templates only provide access to the FULL, BRIGHTSKY, SUB256, SUB128, and SUB64 subarrays. The remaining subarrays are available for coronagraphic imaging (Ressler et al. 2015).
Table 2. MIRI subarrays
|Subarray||Size (pixels)||Usable size (arcsec)||Frame time (s)|
limit in F560W
limit in F2550W
|FULL||1024 × 1032||79 × 113||2.775||13||192|
|BRIGHTSKY||512 × 512||56.3 × 56.3||0.865||42||540|
|SUB256||256 × 256||28.2 × 28.2||0.300||120||2000|
|SUB128||128 × 136||14.1 × 14.1||0.119||300||4700|
|SUB64||64 × 72||7 × 7||0.085||420||6600|
Imager exposure specifications
Main article: MIRI Detector Readout Overview
MIRI imaging supports two different detector readout patterns:
- Fast mode (default)
- Slow mode (only in full array)