The JWST MIRI low-resolution spectrometer (LRS) mode requires target acquisition (TA) when operated in slitless mode for time-series spectroscopy. A dedicated TA procedure has been defined for the mode.

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 slitless spectroscopy, TA is particularly important to be able to account for intra-pixel gain variations in multi-epoch transit observations. TA is mandatory for LRS slitless observations; not specifying the TA parameters in the APT template will result in an error.

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

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TA target

Typically the science target is used for TA. However, the procedure can also be carried out with a nearby bright star, which should be within 60" from the science target. For slitless time-series spectroscopy, most science targets will be bright and compact enough to use for TA; however if the user chooses to perform a dedicated background exposure at the end of the time series, this may require an offset TA sequence.

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TA filters and exposure settings

See also: MIRI Filters and Dispersers, MIRI Target Acquisition

Target acquisition for LRS slitless mode observations can use all filters, read mode, and observation settings described in the MIRI Target Acquisition article. The TA exposure is always carried out in a single integration. A SNR > 20 is recommended for successful TA. 

LRS slitless subarray and target acquisition region of interest (ROI)

See also: MIRI Detector Readout FASTR1, MIRI Optics and Focal Plane

The MIRI LRS uses the SLITLESSPRISM subarray for its slitless time-series observations, to increase the dynamic range of the mode and allow observations of brighter targets. The frame read time in FASTR1 mode is 0.16 s, compared to 2.77 s for a FULL frame read. To avoid switching between FULL and SLITLESSPRISM array configurations on short timescales, which can cause undesirable detector artifacts, the TA region of interest (ROI) defined for LRS slitless TA is part of the SLITLESSPRISM subarray itself, occupying 48 × 48 pixels of the subarray (~5" × 5") and positioned such that overlap with the science spectrum is minimal. The location of the TA ROI is shown in Figure 1.

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Verification image

To provide a reference for the location of the target in the subarray, a verification image is 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. As TA is mandatory for slitless observations, the verification is also taken by default.

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LRS slitless TA sequence

The sequence is as follows:

1. The TA target is placed behind the focal plane mask, at the "TA BLOCK" location. This is to avoid saturation.
2. The TA filter is selected in the filter wheel.
3. The TA target is placed in the center of the LRS Slitless ROI.
4. An image is taken according to the APT specifications for filter, read mode, and number of groups.
5. The onboard centroiding algorithm computes the centroid location of the target, and the offset to the nominal slitless pointing location is computed. This computation includes the offset to the science target, if an offset target is used for TA.
6. The telescope executes a small angle maneuver (SAM) to place the target at required location.
7. A verification image is taken with the requested exposure settings. 
8. The science observation is commenced, which starts with the acquisition of the double prism in the filter wheel.
   
   

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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