MIRI Imaging Recommended Strategies

Recommendations for planning most MIRI imager science observations, based on pre-launch knowledge of the instrument. 

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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 follow the mode-independent general guidelines described for MIRI. Additional information is available in the MIRI Cross-Mode Recommended Strategies and MIRI TSO Recommended Strategies articles.



Detector readout mode

See also: Understanding Exposure TimesMIRI Generic Recommended Strategies (Detectors)

Words in bold italics are buttons 
or parameters in GUI tools. Bold 
style represents GUI menus/
panels & data software packages.

The default readout mode for imaging is FASTFAST mode is mandatory for subarray exposures. FULL array observations may use SLOW mode, primarily to limit data volume 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 detector cosmetics and defects
  • 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 may make use of a no-dither option. 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

4-point point- source

Provides a well-sampled PSF in all MIRI filters and 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

None

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

4-point extended-source

Prevents persistence issues for small extended sources 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

Cycling

Offers maximum flexibility in the number of dither points.

May have the same cons as the 2-point pattern if the number of dithers < 4.More specialized cases that require a larger number of dithers in many locations.
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 two 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.

* The data quality resulting from the 2-point dither pattern will be assessed during commissioning.



Dwell time limit

Dwell time is defined as the length of time spent at a single dither position. Since multiple exposures are discouraged at a single dither position, the dwell time should also define the exposure length. For the long wavelength filters, the dwell time is limited to the amount of time it takes to reach a ceiling in the signal-to-noise ratio (SNR) due to high background levels. This is known as the dwell time limit. The following table gives recommendations on the length of time observers should spend at a single dither position (i.e., exposure length). These estimations are based on ground measurements of flight-like detectors. 


Table 2. Recommendations on the length of time at a single dither position (exposure length)

MIRI filterBackground typeLimitation in dwell time?Recommendation
F560W to F1800WLowNo

If possible, a minimum of at least 40 groups in FULL/FAST mode (111 s)  will minimize the effects of drifts at low backgrounds.  A maximum of 360 groups in FULL/FAST is recommended (although not required).

F560W to F1800WMediumNoA minimum of 5 groups and a maximum of 1,000 s integration is recommended  (but not mandatory)
F2100W and F2550WAlways highYes

Maximum dwell time of 8 minutes; longer exposures will reach a ceiling in the SNR.


Guidelines on the exposure length can be also found in the MIRI Cross-Mode Recommended Strategies article. Users should also note that the observatory imposes a limit of 10,000 s on the length of an individual exposure to allow for moves of the high gain antenna (HGA). This is only waived for TSO observations.



Target acquisition

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

Target acquisition (TA) is currently not being offered for the MIRI imager. Given that 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 ObservationsMIRI Generic Recommended Strategies (Background), APT Targets

The recommendations for obtaining background observations for MIRI imaging can be found in the general background information for MIRI.

It is important to note that when an observation program assigns a background to a science target, 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.


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




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