Precise wavelength and flux calibrations of MIRI LRS data require accurate knowledge of the target on the detector. Users are advised to perform a target acquisition (TA) as part of their observation to ensure their target is placed with subpixel accuracy (<10 mas) at the nominal slit location. The pointing performance of the telescope is described in JWST Pointing Performance.

Target acquisition is, therefore, strongly recommended for MIRI low-resolution spectroscopy (LRS) observations with the slit for compact sources.

LRS slit and slitless modes each have their own custom TA procedure. For slit spectroscopy, TA is important to avoid slit losses and wavelength calibration issues that can arise from off-nominal source placement. 

Please refer to LRS Recommended Strategies for concrete advice.

TA may not be necessary when performing an off-source dedicated background observation.

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Self TA vs. offset TA

Typically, the science target is used for TA (i.e., a self-TA). However, the procedure can also be carried out with an offset TA target, such as a nearby bright star, which should be within the visit splitting distance of the science target. Finding this distance is described in the MIRI Cross-Mode Recommended Strategies TA section. Use of an offset TA target may be advisable in the following cases:

• the science target is spatially resolved, giving a higher uncertainty on the centroid location;
• the science target's spectral energy distribution is not well known in the TA filters; or
• the science target requires a long (~100s of seconds) integration to reach SNR of 20. 

If the observer uses an offset TA target, care should be taken with the coordinates (including proper motions) used in the APT for both the TA and the spectroscopic targets.  Self TA will correct for small errors in position, provided that the spectroscopic target is the brightest object in the TA region of interest (ROI, see below).  For an offset TA, any error in the position of either the offset TA target or the spectroscopic target will result in misplacement of the spectroscopic target in the slit.

For faint sources requiring longer integrations, please note that cumulative cosmic ray hits associated with longer integrations may increase the risk of inaccurate centroiding. In such cases, using a bright, nearby point source will likely yield a better result.

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TA filters and exposure settings

See also: MIRI Filters and Dispersers, MIRI Target Acquisition

Target acquisition for LRS slit mode observations can use all filters, read mode, and observation settings described in the main article on MIRI target acquisitions. The TA exposure is always carried out in a single integration. A signal to noise ratio (SNR) of > 50 is recommended for successful TA. 

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LRS slit region of interest

See also: MIRI Optics and Focal Plane

The LRS slit target acquisition uses a region of interest (ROI) measuring 48 × 48 pixels (~5 × 5")  located in the imager field, close to the location of the slit (see Figure 1). Note that the ROI is not a detector subarray; TA exposures are performed with FULL array mode.

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Verification Image

To provide a reference for the location of the target in the subarray, a verification image should be taken at the end of the TA sequence, before the filter wheel is moved to the prism (P750L) setting. As of APT version 2023.1.1, TA verification images are no longer exclusively taken with the same exposure settings as the TA image itself. The user can specify any of the available imager filters, and change the number of groups and integrations to obtain an optimally useful pre-image to the spectroscopic observation. The ability to edit the exposure configuration between the TA exposure and TA verification exposure is particularly useful in cases where an offset target was used for TA.

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LRS TA sequence

The sequence is as follows:

1. The TA target is placed in the center of the LRS slit ROI (SLIT_TA).
2. An image is taken according to the APT specifications for filter, read mode, and number of groups.
3. The onboard centroiding algorithm computes the centroid location of the target, and the offset to the first pointing location is computed (the slit center, or the first nod position). This computation includes the offset to the science target, if an offset target is used for TA.
4. The telescope executes a small angle maneuver (SAM) to place the target at required location in the slit.
5. An optional verification image is taken if it was requested in the APT template.
6. The double prism in the filter wheel is moved into place 
7. An optional verification image is taken with the requested exposure settings.
8. The science observation is commenced.
   
   

Exposures taken during the TA sequence are downloaded to the archive and made available to the observer.

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Additional links

JWST Pointing Performance

MIRI Filters and Dispersers

MIRI Target Acquisition

MIRI Optics and Focal Plane

MIRI LRS APT Template

JWST ETC MIRI Target Acquisition

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