JWST ETC MIRI Target Acquisition

The Exposure Time Calculator provides a target acquisition (TA) mode for the Mid-Infrared Instrument (MIRI) that allows users to determine the exposure time required to obtain a sufficient signal-to-noise for the TA procedure to achieve the desired centroid accuracy.

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Creating a TA calculation

The procedure for creating a new TA calculation shares many commonalities with regular science exposures. The calculation requires a scene populated with at least one source with appropriate brightness distribution and shape properties. Once the scene and source(s) are defined, the user may specify the instrument and detector setups for the procedure (see Calculations). 

Defining the TA scene and source

For TA calculations, the scenes and sources are defined in the same way as for science calculations. The constructed scene will act as an idealized 2D representation of the target and nearby sources with their defined spectral properties. When populating the scene, users should consider the type of sources supported by each acquisition mode (see mode specific TA calculations).

Creating a calculation

To initialize a MIRI TA calculation, select Target Acquisition from the MIRI instrument drop-down menu. The scene to be used for the TA calculation can be selected in the Scene  tab on the Configuration pane.

Figure 1. Creating a MIRI Target Acquisition Calculation

Create a target acquisition calculation by selecting the Target Acquisition mode from the MIRI instrument drop-down.

What's supported

The ETC allows the user to perform TA calculation for all MIRI modes for which a TA capability is currently supported: 

TA is not currently supported for MIRI Imaging.

Instrument Setup

The Instrument Setup tab has an acquisition mode option which is common to LRS-Slit, MRS, and LRS-Slitless; this is because the TA for these modes are operationally similar except that their subarrays are located in different regions on the detector. Separate options are listed for each of the coronagraphic masks.

Four filters are available for all MIRI TA observations: F560W, F1000W, F1500W and a neutral density filter (FND). The filter can be chosen in the Filter drop-down in the Instrument Setup tab. By default, the F560W filter is automatically selected for all acquisition modes.

Figure 2. MIRI TA Supported Modes and Filters

Top: The ETC supports TA for MRS, LRS-Slit, LRS-Slitless and Coronagraphic Imaging. Bottom: Filters F560W, F1000W, F1500W and the neutral density filter (FND) are available for all acquisition modes. 

Detector Setup

The subarray, readout pattern and exposure configuration are specified in the Detector Setup tab. Each coronagraphic imaging mode uses its own unique detector subarray for both science and TA exposures, i.e., MASK1550, MASK1140, MASK1065  and MASKLYOT. For non-coronagraphic acquisition modes, a single drop-down menu lists the available subarrays, which includes FULL, BRIGHTSKY, SUB256, SUB128, SUB64  and SLITLESSPRISM.  In order to easily distinguish which subarrays are available for each acquisition mode, the applicable modes are clearly indicated alongside the subarray within the drop-down.

Only the FULL and SLITLESSPRISM subarrays are currently supported for target acquisition.

Figure 3. MIRI TA Subarrays

The Detector Setup tab, showing the available subarrays in the drop-down menu.

During TA, the selected subarray will be read out in its entirety; the target will, however, be placed in a dedicated region of interest (ROI), which represents the "search box" in which the centroiding algorithm operates.

Readout pattern

MIRI TA offers two readout modes: FAST and FASTGRPAVG.

MIRI's FAST mode detector readout pattern is the default read mode for target acquisition. The maximum value for NGROUPS, as determined by limitations in the on-board data acquisition system, is <100 for both FAST and FASTGRPAVG. If more time is required than can be provided by the maximum value of NGROUPS and the FAST readout mode, then the FASTGRPAVG should be used. In this mode, each group consists of 4 co-added FAST mode frames. 

Groups, integrations and exposures

For TA calculations in the ETC, the number of groups is selected from a pre-defined dropdown menu, which is dependent on the Subarray selected. For the SUB256, SUB128, SUB64, and SLITLESSPRISM subarrays, the pre-defined list contains even integers between 4 and 98. Due to data storage constraints, the available values for the BRIGHTSKY subarray are a subset of the full list, with a maximum value of 36. For similar reasons, the FULL subarray only allows 4, 6, 8, or 10 to be selected.

The ETC only allows for even group numbers in order to remove the influence of the last-frame effect on the TA centroiding algorithm. The TA algorithm accepts groups in a single integration to perform centroiding; the number of integrations and exposures is therefore fixed at 1.

Figure 4. MIRI Detector Setup tab: Number of Groups

 The number of groups is selected from a pre-defined list. Integrations and exposures remain fixed at 1.


The exposure specification for TA should be chosen to obtain the minimum required SNR of 20 within the extraction aperture defined in the Strategy tab. If the calculation returns a SNR below this threshold, the ETC will issue a "TA may fail" warning. Saturation can also affect the accuracy of the centroiding procedure. Whilst a small number of saturated pixels can still allow accurate centroiding, we recommend avoiding saturation during target acquisition for optimal performance. If any fully or partially saturated pixels are present in the TA exposure, the ETC will issue a warning. The recommendation is to adjust the parameters in the Detector Setup tab (e.g., by decreasing the number of groups) or the Instrument Setup tab (e.g. changing filter selection) to avoid saturation.

Mode-specific TA calculations

Coronagraphic imaging target acquisition

For an in-depth description of target acquisition with this mode, visit MIRI Coronagraphic Imaging Target Acquisition.

Each coronagraphic mask has its own acquisition mode listed under the Instrument Setup tab.

The filters available to users for all coronagraphic imaging modes are F560W, F1000W, F1550W, and the neutral density  filter (FND). The FND provides the strongest flux attenuation and is recommended to avoid saturation and persistence when observing bright targets.

In general, users should consider using the FAST readout mode. However, for TA of a faint star with the Lyot coronagraph (where longer integration times are needed) FASTGRPAVG should be used. This will allow such observations to reach the SNR threshold for TA; using the FAST readout mode could cause data volume issues. In general, FASTGRPAVG is recommended for mitigating data volume issues.

Low Resolution Spectrometer (LRS) target acquisition

For an in-depth description of target acquisition with this mode, visit MIRI LRS Slit Target Acquisition.

To set up a TA calculation for LRS-Slit or LRS-Slitless through the ETC User Interface, select the TA for LRS-Slit, MRS and LRS-Slitless option in the Acq Mode drop down (see Fig. 2). For LRS-Slitless, the user should select the SLITLESSPRISM subarray; LRS-Slit TA uses the FULL array.

The filters available for LRS TA are F560W, F1000W, F1550W, and the neutral density filter (FND)

Target acquisition of faint sources can be accomplished using long exposures (up to approximately 1000s), but may require the FASTGRPAVG readout mode.

Medium Resolution Spectrometer (MRS) target acquisition

For an in-depth description of target acquisition with this mode, visit MIRI MRS Target Acquisition.

To set up a TA calculation for the MRS through the ETC web application, select the TA for LRS-Slit, MRS and LRS-Slitless option in the Acq Mode drop down (see Fig. 2).

The filters available for MRS TA are F560W, F1000W, F1500W, and the neutral density filter (FND).

Observers are prompted to use the FULL subarray for MRS TA procedures. We recommend using the FAST readout mode. 




Latest updates

  • Fixed incorrect links

  • Updated for ETC v1.3

    Update to the recommendation on saturation for MIRI TA