MIRI 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 faciliatate self-calibration. The Astronomer's Proposal Tool (APT) can be used to design mosaic observations to image larger fields. 

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Do not use the MIRI imaging mode for coronagraphic imaging.



Basic performance

Main article: MIRI Predicted Performance
See also: MIRI SensitivityMIRI 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 in the F560W band.

MIRI imaging sensitivity is background limited in all the imaging bands (unless one takes short integrations): astronomical background limited at wavelengths <15 μm and telescope background (primary mirror and sunshield) limited at wavelengths >15 μm.

Observers will be able to specify settings for four primary MIRI imaging parameters: (1) filters, (2) dither 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

 The imager focal plane, with the imaging FOV highlight on the right


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.


Imaging filters

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

Filter
name

λ0
(μm)

Δλ
(μm)

FWHM
(arcsec)

Point source
detection limit
(μJy)

Extended source
detection limit
(μJy arcsec-2)

Comment

 F560W 1

5.6

1.2

0.22

0.182

0.22

Broadband Imaging

 F770W

7.7

2.2

0.25

0.276

0.26

PAH, broadband imaging

F1000W

10.0

2.0

0.32

0.592

0.53

Silicate, broadband imaging

F1130W

11.3

0.7

0.36

1.465

1.2

PAH, broadband imaging

F1280W

12.8

2.4

0.41

0.945

0.83

Broadband imaging

F1500W

15.0

3.0

0.48

1.618

0.93

Broadband imaging

F1800W

18.0

3.0

0.58

3.881

1.9

Silicate, broadband imaging

F2100W

21.0

5.0

0.67

7.550

3.3

Broadband imaging

F2550W

25.5

4.0

0.82

26.959

9.1

Broadband imaging

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†  Signal/noise = 10 for 104 s on-source integration time


Figure 2. MIRI imaging filter bandpasses

Figure showing MIRI imaging filter bandpasses



Dithering performance 

Main article: MIRI Imaging Dithering        
See also: MIRI Dithering

MIRI operations offers several options for imaging dithersThere 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 of 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. 



Subarrays 

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 1 and SUB64.

Figure 3. Subarray locations for the MIRI imager

Subarray locations for the MIRI Imager as viewed from the telescope looking down onto the detector.

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)

FULL

1024 × 1032

74 × 113

2.775

BRIGHTSKY

512 × 512

56.3 × 56.3

0.865

SUB256

256 × 256

28.2 × 28.2

0.300

SUB128

128 × 136

14.1 × 14.1

0.119

SUB64

64 × 72

7 × 7

0.085

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Imager exposure specifications 

Main article: MIRI Detector Readout Overview   
See also: Understanding Exposure Times  

 MIRI imaging supports two different detector readout patterns: 

  1. FAST mode (default)

  2. SLOW mode (only in full array)



References

Bouchet, P. et al. 2015, PASP, 127, 612
The Mid-Infrared Instrument for the James Webb Space Telescope, III: MIRIM, The MIRI Imager
Updated version

Ressler, M.E. et al. 2015, PASP, 127, 675
The Mid-Infrared Instrument for the James Webb Space Telescope, VIII: The MIRI Focal Plane System
Updated version

Rieke, G. et al. 2015, PASP, 127, 584
The Mid-Infrared Instrument for the James Webb Space Telescope, I: Introduction
Updated version




Published

 

Latest updates

  • Removed column “Point source brightness limit (mJy)" from Table 1