MIRI Cross-Mode Recommended Strategies

A general set of guidelines regarding exposure time settings, background strategies, and general target acquisition can be applied to all MIRI observing modes.

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Guidelines for aspects of the proposal planning process common to all MIRI observing modes are provided below. Every program has to consider detector operations (i.e., how to choose a combination of groups and integrations per exposure), background strategies to help mitigate the spurious sky + thermal telescope emission, and general target acquisition (TA) aspects.

Specific strategies for all MIRI observing modes can be found in the following JDox pages: MIRI Imaging Recommended StrategiesMIRI MRS Recommended StrategiesMIRI LRS Recommended Strategies, MIRI Coronagraphic Recommended Strategies, and MIRI TSO Recommended Strategies.

Detector readout recommended strategies

See also: Understanding Exposure Times, MIRI Detector Readout

Like other instruments on JWST, MIRI detectors use MULTIACCUM readouts of multiple groups along the integration ramp. Once the final group in an integration is read, the detector circuit is immediately reset, an additional reset is added and if defined, a new integration starts. The on-sky time of each individual exposure (i.e., the time spent in a single dither position) is defined by the number of groups and integrations.

How many groups and integrations should I use?

For MIRI the optimal combination of groups and integrations depends on the target brightness, the background and the desired signal-to-noise ratio. Users should use the ETC to evaluate the exposure time needed to achieve their scientific objectives and follow the guidelines below.

Figure 1. Flow diagram illustrating general recommendations for the MIRI detectors usage

Click on the diagram for a larger view. For time-series observations, please see MIRI TSO Recommended Strategies.

What is the recommended ideal, minimum, and maximum length of an integration?

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

The MIRI Si:As detectors exhibit a set of non-ideal, but known, detector effects (see Ressler et al. 2015, see also MIRI Instrument Features and Caveats). The first few groups in an integration are most strongly affected. An increasing number of groups per integration can mitigate these effects, multiple integrations mitigate the impact of cosmic rays and showers by clearing the accumulated charge and restarting the accumulation of charge.

  • The ideal number of groups will depend on the source brightness and background conditions:
    • Bright sources/high background conditions will generally mean that saturation is reached in a short number of groups (5 to 10). Having one group saturating at the end of the integration will maximize the dynamic range. For very bright cases, imaging users should favor subarrays.

    • Faint source observations will benefit from having longer integrations. In FASTR1 mode, 40 groups (about 100 s in FULL array, shorter when using subarray mode) is suggested as a starting point. Increase or decrease the integration to achieve your desired signal to noise. SLOWR1 is recommended primarily to limit data volume when MIRI. See the MRS Recommended Strategies page for specific MRS guidance.

  • Minimum number of groups: Five groups is typically the minimum number required to obtain a reasonable calibration. For very bright sources, observers are permitted to use between 2 and 5 groups, but at this time there is no guideline for the noise and photometric accuracy we can expect in these very short integrations. When using fewer than 5 groups, and if photometric accuracy is important to your program, you should plan to observe a calibration star with exactly the same exposure parameters. If, instead of accuracy, repeatable precision is your primary need (e.g., transits), then this step may not be necessary.

  • Maximum recommended integration length: Based on in-flight experience, and considering the impact of the background, cosmic rays, cosmic ray showers, and other detector effects, 300 s is the maximum recommended integration length for imaging, coronagraphy and LRS observations. After 300 s about 35% of the MIRI detector pixels are affected by a cosmic ray, either as a direct or secondary impact of a cosmic ray, or a cosmic ray shower (see Figure 2, left). Integrations longer than 300 s may be desired in cases of very low background, such as those measured in the MIRI MRS where the data will be photon noise limited. On-orbit measurements indicate that in a 1,000 s integration, most of the detector pixels are affected by cosmic rays  (see Figure 2, right) and therefore that is the maximum recommended integration length. To understand cosmic ray treatment by the ETC visit the JWST ETC Cosmic Ray Implementation page.

Users are strongly encouraged to follow these general guidelines of integration length optimization. Please reach out to the JWST Help Desk with further questions.

Figure 2. Pixels affected by a direct cosmic ray hit

Cosmic ray impact analysis on a single integration 1000 s (360 groups FASTR1) MIRI detector dark. Left: Number of pixels with one or more cosmic ray hits in a single 1,000 s integration (360 groups FASTR1) MIRI detector dark. Right: MIRI detector pixels flagged as impacted by a cosmic ray "jump". The color bar represents the total number of groups flagged either after the jump or due to multiple hits. The cosmic ray showers flagging and post-jump flagging are currently not used by detault in the JWST calibration pipeline, but are available and can be run independently by observers.

How should I deal with saturation?

In many cases, the maximum integration length will be set by saturation of bright sources within the field, emission lines within a spectrum, or background emission. Because MIRI samples many points "up the ramp," saturating midway in the integration does not mean that data are lost; the pipeline will only fit the initial unsaturated data points. This can be an advantage since you can integrate longer to gain better signal to noise on fainter areas of the field or spectrum. However, saturation, as well as bright sources, could leave latent images in subsequent exposures (although they do not appear to be a strong function of the integrated signal level, and seem to decay in about 30 min). High redundancy in the data (e.g., dithering) is strongly recommended to mitigate the effects of persistence.

When evaluating the impact of saturation as predicted by the ETC, users should keep in mind that the ETC is conservative, and does report saturation at about 80% of the detector full well. Further details can be found in the ETC Saturation Limits article.

Should I use multiple integrations?

The first integration within an exposure differs from the second and subsequent ones. This is because there are multiple detector resets between exposures (the system goes into a continuous reset while the telescope is dithering, filter wheels are moving, etc.), allowing some of the above-mentioned transient features to be cleared out of the first integration of the exposure. For multiple integration data the MIRI detectors use now an additional reset (FASTR1 and SLOWR1), which does allow for the majority of these transient effects to decay. Multiple integrations have the added benefit of allowing for all cosmic rays decay; these also decay during the dithers in between exposures.

To decide whether to use multiple integrations, observers should consider various aspects: brightness of the source, detector performance, dithers, and overheads. These can all be encapsulated in 2 broad cases:

  1. Bright sources/high backgrounds that saturate rapidly: in this case, multiple integrations that allow at least one saturated group will be beneficial to obtain high SNR, and provide better efficiency. Telescope maneuvers to the next dither position are more "costly" than starting a new integration.

  2. Fainter sources that can reach the "ideal exposure length" of 300 s (see above) will benefit from single integrations in all dither positions. It is best to select integration lengths which are as long as your observations allow (i.e., a few long integrations are better than many shorter ones).

Choosing the readout mode: FASTR1 vs. SLOWR1

See also: MIRI Detector Readout Overview

The MIRI detectors can be operated using 2 different readout modes: FASTR1 and SLOWR1. The main differences between these modes are:

  • FASTR1 mode reads out the detector every 2.775 s in full array mode, vs. 23.889 s in SLOWR1 mode. FASTR1 mode offers finer time sampling that allows better characterization of detector effects and more samples for cosmic ray correction. Compared to SLOWR1 mode, in FASTR1 mode there is approximately a factor of 9 less time loss in case of a cosmic ray hit.

  • SLOWR1 mode provides about 9 times less data volume.

FASTR1 is the recommended readout mode for all MIRI observations executed as prime. SLOWR1 mode is useful for parallel observations and in some MRS programs, where it can be essential to limit the data volume.

 Background observations recommendations

See also: JWST Background Model, JWST Background-Limited Observations

Does my program need background observations? 

Please read the following pages for MIRI mode-specific guidance on backgrounds: 

MRS Dedicated Sky Observations 
MIRI Imaging Recommended Strategies  
MIRI LRS Recommended Strategies 
MIRI Coronagraphic Recommended Strategies

Observers should carefully consider the impact the extra emission of the JWST background (modeled by the JWST ETC Backgrounds) will have on their data. Please refer to JWST Background-Limited Observations for guidelines on how to use the Background Limited special requirement in APT.

How often do I need to get a background?

The JWST mission defines visits as individual schedulable units, used to build up the observation timeline. Visits that are not linked by special requirements can be planned at different times of the observing cycle and background variations are expected. Observers should plan on taking background data for every observing period (i.e., non-linked visit) in their programs. 

The JWST Background Variability provides information on the spatial and temporal variability of the background.

Observing programs that need low backgrounds can request visits to be scheduled when the background is predicted to be relatively low. Observing programs that require high accuracy relative calibrations in the results (better than 1%) and use extended targets may consider taking background data before and after the science exposures. Users are encouraged to use the JWST Backgrounds Tool to better understand the impact of the background in their observations, its intensity, and components as a function of time.

Target acquisition

An overview of the MIRI target acquisition process is given elsewhere in the documentation and discussed in the mode-dedicated target acquisition articles (MIRI Imaging Target Acquisition, MIRI MRS Target Acquisition, MIRI LRS Slit Target Acquisition, and MIRI LRS Slit Target Acquisition). These articles also include guidelines on when TA is needed for each MIRI mode.

TA targets

See also: MIRI Target Acquisition

When TA is needed (see JWST Pointing Performance) the science target is typically used for TA. However, the procedure can also be carried out with a nearby bright star, which should be within the APT visit splitting distance of the science target. The splitting distance ranges between 30"–80", depending on the Galactic latitude of the target, and is defined as the total distance between the TA and science exposures which is the combination of:

  1. The distance between the TA target and science target.
  2. The distance between the TA aperture and the science aperture.

If the TA target is not within this visit splitting distance, the observation will not be schedulable by the APT Visit Planner. Users are encouraged to check whether their program can be scheduled when using off-source TA targets. The accuracy of this TA is limited by the precision of the difference between the offset and science targets.

If feasible, using an offset TA target should be considered in the following scenarios:

  • The science target is spatially resolved, resulting in a higher uncertainty on the centroid location (see section below).
  • The science target's spectral energy distribution is not well know in the MIRI TA filters, leading to an uncertain estimation of the exposure time and SNR.
  • The science target requires a long (~100 s or longer) integration to reach an SNR of 20. 

Understanding the TA onboard procedure

The aim of this section is to give details on the onboard TA data process, so users understand the several aspects that might impact its accuracy/outcome. The onboard centroid algorithm for MIRI works as follows:

  • The onboard algorithm uses the first, middle and one-before-last frame to generate two 2-frame difference images. The final image used for TA is then constructed by taking the minimum value of these 2 difference images on a pixel by pixel basis.
  • This raw image is pre-treated (background subtracted and flat fielded).
  • It then finds the brightest 3 × 3 pixel region of the detector region of interest (ROI, see Table 1). The checkbox size (3 × 3) has been defined to encompass the imager PSF.
  • After that, it performs a fine location by calculating the center of mass in the previously located brightest area.

The accuracy of this procedure depends on several aspects:

  • The TA target should be a point source. The algorithm will work in extended/resolved sources, but with less accuracy.
  • The integrated signal-to-noise ratio should be ~20.
  • The TA source should be the brightest source in the ROI. If there is a source brighter (by any factor) than the one selected by the observer, TA will be carried out on that. If there is a source with the same brightness the algorithm will perform TA in the first one encountered following the direction in which the detector is read out.
  • Users should be aware that regions with bright diffuse emission may also result in false identification of the TA target. To reduce the possibility of this happening, the checkbox area is a very small one. 

Table 1. Sizes of detector regions of interest used to perform target acquisition. 

TA region of interest (ROI)Size
MRS (TA performed in imager detector)48 × 48 pixel2
LRS (slit and slitless)48 × 48 pixel2
Coronagraphs48 × 48 pixel2 (16 × 16 pixel2 secondary TA)

Note that the MIRI detector plate scale is 0.11 arcsec/pix.

Additional information is available in these articles: 

MIRI Imaging Target Acquisition 
MRS MIRI MRS Target Acquisition 
MIRI LRS Slit Target Acquisition 
MIRI LRS Slit Target Acquisition

Figure 3. Depiction of a dense MRS TA region (48 × 48 pixels in the MIRI imager)

In this example of a rich field, if the observer selects the central star as a TA target, the centroid algorithm will not be performed there but in the brightest pixel of the field (top right). After the TA is performed on the wrong target, data will not be taken at the coordinates of the science target specified in the proposal. Observers are encouraged to carefully select the TA target so that there are no brighter neighbors.

Target acquisition readout mode: FAST and fast group averaging

MIRI target acquisition can be performed using 6 different readout modes: FAST (FASTR1 is not used for TA),and 5 different flavors of fast group averaging (see MIRI Target Acquisition). Data volume can become a concern when a TA requires a large number of groups—in fast group averaging, each group consists of 4, 8, 16, 34, or 64 co-added FAST mode groups. Fast group averaging is offered for MIRI TA only. The ETC will give warnings when the SNR achieved is not sufficient to successfully finish the TA procedure (see more details in JWST ETC MIRI Target Acquisition).



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

Rieke et al. 2015, PASP, 127, 665R
The Mid-Infrared Instrument for the James Webb Space Telescope, VII: The MIRI Detectors

Lightsey, P.A., 2016, SPIE, 9904, 99040A 
Stray light field dependence for the James Webb Space Telescope

Krick J. E. et al., 2012, ApJ, 754, 53  
A Spitzer/IRAC Measure of the Zodiacal Light

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    Updated with recommendations based on in-flight data
Originally published