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

Calibration of MIRI LRS data requires accurate knowledge of the location of the target on the detector. To this end, users are advised to perform a target acquisition (TA) as part of their observation; this ensures that the target is placed with subpixel accuracy (< 10 mas) at the nominal slit center location (or at the first nod position). 

LRS slit and slitless modes each have their own custom TA procedure. For slit spectroscopy, TA is particularly important to avoid slit losses and wavelength calibration issues that can arise from off-nominal source placement. Cases where TA may not be necessary are:

• when performing an off-source dedicated background observation
• when observing very extended sources.

TA is therefore not mandatory in the LRS APT template; however, for observations of point or compact sources it is highly recommended.

Pointing performance of the telescope is described in JWST Pointing Performance.

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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 60" of the science target. 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 is using an offset TA target, care should be taken with the coordinates (including proper motions) used in the APT for both the TA target and the spectroscopic target.  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).  However, 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.

As integration length increases, cumulative cosmic ray hits increase the risk of inaccurate centroiding. In such cases using a bright nearby point source is likely to give 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. We recommend a SNR of > 20 for successful TA. 

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LRS slit region of interest

See also: MIRI Optics and Focal Plane

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

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TA verification image

To provide a reference for the location of the target in the slit, users may request a TA verification image in the LRS Astronomers Proposal Tool (APT) template. This image is taken at the end of the TA sequence, after the telescope's small angle maneuver places the target at the first pointing position for science, before the prism is selected in the filter wheel. While this image is not used in the calibration pipeline, it can inform the user on the exact placement of the target in the slit (which may affect the quality of the flux and wavelength calibrations). 
 
Observers are strongly encouraged to obtain a TA verification image, especially if they have any concerns about whether the TA procedure has placed the source correctly in the slit.  Knowledge of the position of the source in the slit will permit the observers to manually improve the correction for slit throughput in the pipeline based on the actual target position.  Cases where users should not select this option are (1) if the target used for TA is not the same as the science target and the science target's spectral energy distribution (SED) in the TA filter is unknown, or (2) the science target is too faint in the TA filter band to produce a usable image in the TA exposure time.

The verification image uses the same number of groups and the same filter as for the TA image itself.

If no TA was requested, for example in the case of an extended target, the verification image cannot be requested.

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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 TA 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. 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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