MIRI Imaging Recommended Strategies

Recommendations for planning most MIRI imager science observations are provided in this article. 

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The MIRI imager offers 9 broadband filters covering wavelengths from 5.6 to 25.5 μm (Bouchet et al. 2015). Observers should also follow the MIRI Cross-Mode Recommended Strategies and MIRI TSO Recommended Strategies articles.


Detector readout mode

See also: Understanding Exposure Times, MIRI Generic Recommended Strategies (Detectors)

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

The default readout mode for imaging is FASTR1;  FASTR1 mode is mandatory for subarray exposures. FULL array observations may use SLOWR1 mode, primarily to limit data volume (e.g. when MIRI is used in parallel).


Dithering

See also: MIRI Imaging Dithering, MIRI Imaging APT Template

For most science cases, dithering is a highly recommended and necessary practice for the following reasons:

  • Allows good PSF sampling—this is mostly relevant when using the F560W filter; the MIRI imager Nyquist-samples the PSF for wavelengths ≥ 6.25 μm
  • Minimizes problems caused by detector cosmetics and defects, and dithers with large steps between pointings are specifically useful to minimize the effects caused by cosmic ray showers
  • Makes possible accurate background measurements for point sources, and at longer wavelengths permits tracking of potential telescope thermal emission variations
  • Mitigates the impact of bad pixels
  • Allows tracking detector drifts at the timescale of the dwell time per dither position (i.e., total length of time the telescope exposes at a dither position)

The Astronomer's Proposal Tool (APT) offers a set of pre-defined dither patterns for imaging. (Photometric time-series observations are not dithered.) There are 2 main aspects to dithering: (1) choosing an adequate pattern and (2) deciding the dwell time (i.e., how long to stay integrating at a single dither position). 


Choosing a dither pattern  

The user has to select a dither pattern that ensures enough redundancy, hence good quality, in the data. Below is the list of MIRI imaging dithers offered by the APT with usage recommendations.


Table 1. MIRI imaging dithers offered by the Astronomer's Proposal Tool

Dither PatternProsConsMost suitable for

Cycling LARGE (or DEFAULT)

Maximum flexibility in the number of dither points. A minimum of 4 points to provide enough redundancy is recommended. It offers 3 different pattern sizes, the LARGE is very well suited to mitigate cosmic ray showers. Science data with targets smaller or similar size to the pattern are well suited to generate a background from the science exposures.

Slight reduction in the total field of view with full exposure coverage

All cases (point sources, small and large extended sources, mosaics). Considered to be the default pattern.

Cycling MEDIUM

Similar advantages to the cycling large dither pattern.

The separation between the dithers points makes it less effective to mitigate cosmic ray showers.

Point and small extended sources.

Cycling SMALL

Similar advantages to the cycling large dither pattern.

The smaller separation between the dithers points makes it less effective to mitigate cosmic ray showers.

Point sources

4-point extended-source

Prevents persistence issues for small extended sources and can mitigate cosmic ray showers by avoiding the placement of the source onto the same pixels.

Reduces the total field of view with full exposure coverage.

Small extended sources up to 5” in radius

4-point point- source

Provides enough redundancy to correct for bad pixels, cosmic rays, and instrument artifacts. Possible to median-subtract the sky background, especially useful for faint sources on a bright background

Not well suited for minimizing the effect of cosmic ray showers or persistence.

Point sources.

Coordinated ParallelsOptimizes pixel phase sampling for both the prime and parallel instrument modes. If a pattern is not available for a given MIRI filter, using the pattern for a filter at the next longer central wavelength is recommended.May have the same cons as the 2-point pattern if the number of dithers < 4.Used only for coordinated parallel observations with 2 instruments.
2-point point-sourceProvides a well-sampled PSF above 6.35 μm. May also be a more time-efficient alternative to the 4-point pattern if data quality proves to be comparable.

Offers less redundancy required to correct for bad pixels and cosmic ray hits and therefore increases risk that larger fraction of pixels will have a reduced signal-to-noise.*

Cannot median-subtract background.		E.g. a large mosaic program where it is an accepted risk that there will be reduced redundancy for cosmic ray/artifact correction and techniques such as median sky background removal will not be possible.
ReuleauxWill be deprecated in future cycles.


Target acquisition

See also: MIRI Cross-Mode Recommended Strategies (Target Acquisition)

Target acquisition (TA) is currently not being offered for the MIRI imager. Given the expected pointing accuracy of the observatory, most imager science use cases will not need target acquisition. Users interested in imaging TSO science may wish to have TA capabilities for at least the SUB64 subarray, but it is currently not supported.


Filter ordering

At longer wavelengths, the MIRI imager data will be affected by an additional high background component coming from the telescope emission that can potentially imprint latents in the detector. To avoid persistence due to transitions from high to low backgrounds, it is best to sort the imager exposures from short to long wavelengths. The observer has control of the order in which the filters are used: it is the same one that is specified in the MIRI Imaging Template APT at the time of proposal submission.


Background observations

See also: JWST Background Model, JWST Background-Limited Observations, MIRI Cross-Mode Recommended Strategies (Background), APT Targets

The recommendations for obtaining background observations can be found in the general background information for MIRI. In a nutshell:

  • For point sources and sources smaller/similar than the size of the dither pattern, a background can be generated using the science exposures (see Figure 1). The pipeline will not automatically perform this operation.
  • When the emission of the science target covers the entire or a large fraction of the imager FOV, it is advised to obtain background data in every spectral configuration used by the science data. 

When an observation program assigns a background to a science target (in the Astronomer's Proposal Tool), that creates a formal association between them. By doing this, the pipeline will automatically subtract the background exposure from the target exposure. To avoid having undesired residuals from this step, the user should:

  • Choose a background area that is as clean as possible of sources. Given the MIRI sensitivity, it is unlikely to find "empty" regions in the imager FOV (about 74" × 113" in FULL array).
  • Dither the background target. By doing that, the pipeline will stack the dithered images thus removing unexpected sources, and use that combination to remove the background from the science data. Observers should carefully consider how many dither points will be needed to achieve the required SNR in their background, and to remove sources present in the background region with the stacking technique.
  • To avoid increasing the noise in the science data when subtracting the background, use the same exposure time in the science and background. As a minimum, the total exposure time of all background dither positions combined should be the same as that of the total integration time of a single dither position in the science target.

Figure 1. Benefits of background correction in MIRI Imager data, using F770W data from the SMACS J0723.3-7327 ERO program (PID 2736, see Pontoppidan et al. 2022).

Left: Dithered combined image (using the CYCLING LARGE dither pattern). The image shows clear residuals from an imperfect flat field correction (bottom of the FOV) and image striping.
Right: Background-corrected image. In this case the background was created from the data, which is possible because of the small size of the astronomical sources w.r.t. the separation between the dither points.

Imager mosaics

Guidance on MIRI imager mosaics is provided in MIRI Imaging Mosaics. In particular, observers interested in using subarray mosaics should check that page to understand the impact of the current APT mosaic default % overlap.

References

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

Glasse et al., 2015, PASP, 127, 686G
The Mid-Infrared Instrument for the James Webb SpaceTelescope, IX: Predicted Sensitivity

Pontoppidan, C., et al. 2022, ApJL, 936, L14
The JWST Early Release Observations



Notable updates

  • Removed the dwell time section, as this is covered in the cross-mode recommendations page.

  •  
    Made correction to recommendation comments in Table 2
    Removed reference to ground test data, which is superseded by in-flight experience

  •  
    Updated dither recommendations for Cycle 2

  •  
    Changed FAST to FASTR1 and SLOW to SLOWR1

  •  
    Updated Table 1 to reflect new dither pattern recommendations from the MIRI team
Originally published