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The imaging mode for JWST's Mid-Infrared Instrument (MIRI) offers 9 broadband filters from 5.6 to 25.5 μm in a 74" × 113" FOV at 0.11"/pixel plate scale.

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Overview4
Overview4
Introduction

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. 

Info
This mode is not for coronagraphic imaging. 

 


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BasicPerf4
BasicPerf4
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 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).  

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Figure 1. The MIRI imaging FOV

 The imager focal plane, with the imaging FOV highlight on the rightImage Modified

Figure caption
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.

 


 

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MIRIFilters
MIRIFilters
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.

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MIRIImagingFilterTableFig
MIRIImagingFilterTableFig
 

Table 1. MIRI filter properties

Filter
name

λ0
(μm)

Δλ
(μm)

FWHM
(arcsec)

Point source
detection limit
(μJy)

Extended source
detection limit1
(μJy arcsec-2)

Point source
brightness limit2
(mJy)

Comment

 F560W

5.6

1.2

0.220.1820.2213

Broadband Imaging

 F770W

7.7

2.2

0.250.2760.267.4

PAH, broadband imaging

F1000W

10.0

2.0

0.320.5920.5316

Silicate, broadband imaging

F1130W

11.3

0.7

0.361.4651.269

PAH, broadband imaging

F1280W12.82.40.410.9450.8329

Broadband imaging

F1500W15.03.00.481.6180.9337Broadband imaging
F1800W18.03.00.583.8811.966Silicate, broadband imaging
F2100W21.05.00.677.5503.366Broadband imaging
F2550W25.54.00.8226.9599.1192Broadband imaging

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

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.

 

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MIRIImagingFilterFig
MIRIImagingFilterFig
 

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Figure 2. MIRI imaging filter bandpasses

Figure showing MIRI imaging filter bandpassesImage Modified

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Click on the image for a larger view.

 


 

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MIRIDither
MIRIDither
Dithers 

        Main article: MIRI Imaging Dithering
        S
ee also: MIRI Dithering Overview

MIRI operations offers several options for imaging 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 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. 

 


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MIRISubarrays
MIRISubarrays
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 and SUB64. (The bold italic font indicates these are parameters in APT observing templates.)

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subarrays
subarrays

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Figure 3. Subarray locations for the MIRI imager

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

Figure caption

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).

Excerpt

 

Table 2. MIRI subarrays

SubarraySize (pixels)Usable size (arcsec)Frame time (s)

Brightness
limit in F560W
(mJy)

Brightness
limit in F2550W 
(mJy)

FULL1024 × 103279 × 1132.77513192
BRIGHTSKY512 × 51256.3 × 56.30.86542540
SUB256256 × 25628.2 × 28.20.3001202000
SUB128128 × 13614.1 × 14.10.1193004700
SUB6464 × 727 × 70.0854206600

 


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MIRIExp
MIRIExp
Imager exposure specifications

        Main article: MIRI Detector Readout Overview

 MIRI imaging supports two different detector readout patterns: 

  1. Fast mode (default)
  2. Slow mode (only in full array)




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Related Links

JWST User Documentation Home
Mid-Infrared Instrument, MIRI
MIRI Overview
MIRI Coronagraphic Imaging
MIRI Imaging Dithering
MIRI Detector OverviewImaging and Mosaics
MIRI Detector Readout Overview

APT MIRI Imaging Related Articles

MIRI Imaging and MosaicsTemplate APT Guide
MIRI Imaging Template APT Guide
MIRI Parameters
JWST Astronomers Proposal Tool, APTJWST APT website 

ETC Related Articles

JWST Exposure Time Calculator
JWST ETC website

MIRI Sensitivity
MIRI Bright Source Limits

MIRI Detector Related Articles

MIRI Detector Overview
MIRI Detector Readout OverviewMIRI Detector Subarrays
MIRI Detector Readout Fast
MIRI Detector Readout SlowJWST Astronomers Proposal Tool, APTJWST APT website 
JWST Exposure Time Calculator
JWST ETC website

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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

JWST technical documents

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Last updated

Published December 22, 2016


 

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