MIRI LRS Slit Target Acquisition

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, recommended for MIRI low-resolution spectroscopy (LRS) observations with the slit for compact sources.

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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.

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.

TA filters and exposure settings

See also: MIRI Filters and DispersersMIRI 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 > 20 is recommended for successful TA. 

Users should always use the JWST Exposure Time Calculator (ETC) to help choose the best TA filter and exposure settings for their science.

LRS slit region of interest

See also: MIRI Optics and Focal Plane

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

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.

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. The science observation is commenced.

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

Figure 1. MIRI LRS slit target acquisition procedure.

Click on the figure for a larger view.

Left: Diagram of the MIRI imager module focal plane. The blue box labeled "SLIT_TA" is the 48 × 48 pixel TA region of interest (ROI). The reference point is taken to be the midpoint of this ROI. Note the exact locations of these regions are kept in the Science Instrument Apertures File (SIAF). Right: Flow diagram describing the MIRI LRS slit TA procedure. Note: (1) this diagram focuses on the case where the science and TA target and the same; (2) The verification image step is optional and may use different filter and exposure settings than the TA image. 

Additional links

JWST Pointing Performance

MIRI Filters and Dispersers

MIRI Target Acquisition

MIRI Optics and Focal Plane


JWST ETC MIRI Target Acquisition

Notable updates

  • Updated information on offset TA target distance. 
    Updated information on custom TA Verification image exposures

    Updated for Cycle 2

    Updated Verification Image section.

  • Updated information about verification images. 
Originally published