MIRI Imaging APT Template

Instructions for filling out the Astronomer's Proposal Tool (APT) MIRI imaging template, including full field imaging, subarray imaging, and large imaging mosaic, are provided in this article.

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See also: MIRI ImagingJWST Imaging Roadmap, MIRI Imaging Recommended StrategiesNIRCam Deep Field Imaging with MIRI Imaging ParallelsProposal Planning Video Tutorials

Imaging is one of 4 observing modes available with the Mid-Infrared Instrument (MIRI). For standard imaging, the MIRI imager offers 9 broadband filters centered on wavelengths between 5.6 and 25.5 μm over an unobstructed field of view of up to 74" × 113" and a detector plate scale of 0.11"/pixel. The MIRI standard imaging mode supports the use of detector subarrays for bright targets as well as a variety of dither patterns, which may act to improve sampling at the shortest wavelengths, remove detector artifacts and cosmic ray hits, and facilitate self-calibration. The APT mosaicking tool can be used to design mosaic observations to image larger fields. 

The observer will have control over 4 primary parameters for MIRI imaging:

  1. filter
  2. dithering pattern
  3. subarray 
  4. detector read out mode and exposure time (via the number of groups, integrations, and exposures).  

Allowed values are documented and maintained in MIRI Imaging Template parameters (an article in the JWST Proposal Parameters documentation), but are also described below.

Step-by-step APT instructions are provided below.


Generic parameters

Words in bold are GUI menus/
panels or data software packages; 
bold italics are buttons in GUI
tools or package parameters.

The following parameters are generic to all templates, and are not discussed in this article: observation Number, observation Labelobservations CommentsTarget name, ETC Wkbk. Calc ID (in the Filters dialog box), Mosaic Properties, and Special Requirements.

Coordinated Parallel observations

See also: JWST Parallel Observations

MIRI imaging supports coordinated parallel observations with NIRCam or NIRISS. When the Coordinated Parallel box (visible when the Template parameter is set to MIRI Imaging) is checked, one of these 2 parameter values can be selected:

  1. MIRI-NIRCam Imaging

Note that the default option is None Selected.

MIRI Imaging tab


See also: MIRI Detector Subarrays

A pre-defined set of subarrays are available for selection in the Subarray parameter under the MIRI Imaging tab. They're used for observing targets bright enough to saturate the image in full frame readout. Each subarray is associated with a brightness limit, depending on the filter used, above which a point source will saturate in the shortest 2-group integration. A MIRI imaging observation can only support a single subarray. If the target must be imaged using another subarray, it is necessary to create a separate MIRI imaging observation.

Table 1. MIRI subarrays


Size (pixels)

Usable size (arcsec)

Frame time


1024 × 1032

74" × 113"

2.775 s


512 × 512

56.3" × 56.3"

0.865 s


256 × 256

28.2" × 28.2"

0.300 s


128 × 136

14.1" × 14.1"

0.119 s


64 × 72

7" × 7"

0.085 s



See also: MIRI Imaging Dithering

Dither patterns, that must be created before defining your imaging sequence, are specified in the Dithers dialog box under the MIRI Imaging tab. Each dither is specified by setting several parameters: Dither Type, Starting Point, Number of Points, Starting Set, Number of Sets, Optimized for, Direction, and Pattern Size.

Note that the smallest subarrays are smaller than the largest dithers, so the choice of dither pattern may be constrained by the choice of subarray. The MIRI imaging template in APT will only display dither parameters that are valid for the selected subarray.

Table 2. MIRI dither patterns

Dither typeUse caseNumber of dithers
CYCLINGShallow, flexible imaging, optimized for self-calibration3–311
REULEAUXDeep imaging, optimized for self-calibration12
2-PointFast nod, for snapshots2
4-Point-SetsFlexible dithers for user-optimized imaging of point and extended sourcesMultiple of 4
Patterns for Coordinated ParallelsDithers optimized for both MIRI imaging and the parallel instrument (only available in coordinated parallel mode)2–9

Note that set number 6 of the 4-point-Sets dither type is centered on the FOV of the chosen subarray.

Small scale dithers are supported for imaging with F560W and F770W. To minimize overheads, small dithers are implemented using the telescope fine steering mirror (FSM). After slewing the telescope to each large dither, the small dither pattern will be executed in sequence, leading to a total of Ndithers = Nsmall × Nlarge separate exposures.

The option for no dithers (None) is currently available for the SUB64 subarray if the Times Series Observation Special Requirement is set. See the MIRI Imaging TSOs article for more information.


See also: MIRI Filters and Dispersers

The Filters dialog box, under the MIRI Imaging tab, is used to create the entire imaging sequence for a single observation for each filter. Each MIRI imaging observation can consist of multiple sets of images, each using a unique filter, dither pattern, and exposure configuration. Click on Add, below the dialog box, to add an image set for a filter. Note that image set definitions can be duplicated using the Duplicate button.

An imaging sequence must be completed by selecting an exposure configuration for each imaging filter. Each exposure configuration requires setting the readout pattern and characteristics parameters: Readout Pattern, Groups/Int, and Integrations/Exp. Finally, select a dither pattern from the Dither parameter pull-down list—select a number that corresponds to a dither that was pre-defined in the Dithers dialog box. Exposures will be repeated at each dither position following an offset of the telescope to the new position. The number of exposures at each dither position is set by Exposures/Dith.  

Users should use the Exposure Time Calculator (ETC) to determine the best exposure configuration to optimize the signal to noise.

Readout Pattern

See also: MIRI Detector Readout Overview

MIRI offers 2 Readout Patterns for imaging: 

  1. FASTR1 (default)
  2. SLOWR1

The FASTR1 readout pattern is the default pattern, and is recommended for MIRI imaging observations.

Number of groups and integrations

See also: Understanding JWST Exposure Times

The MIRI exposure configuration is defined by only 2 parameters: 

  1. Groups/Int: The number of groups during an integration, where a group is the time to cycle through and non-destructively read out all the pixels. Groups/Int = 2 requires permission from STScI, which can be obtained with a request through the Help Desk.
  2. Integrations/Exp: The number of integrations during an exposure, where integration is defined as the time between detector resets (destructive reads). 

Other tabs

Mosaic Properties

See also: JWST Mosaic OverviewMIRI Imaging Mosaics

See also:  Specifying Mosaics in APT 

The MIRI imager may be used to obtain data for a region larger than their size by creating a MIRI imaging mosaic pattern under the Mosaic Properties tab.

Special Requirements

A variety of observatory level Special Requirements may be chosen under the Special Requirements tab.


The Comments field (under the Comments tab) should be used for observing notes.

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Originally published