MIRI LRS Slitless Target Acquisition

Precise wavelength and flux calibrations of MIRI LRS data require accurate knowledge of the target on the detector. For time-series spectroscopy, a dedicated TA helps account for intra-pixel gain variations in multi-epoch transit observations. Users using the slitless mode 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.

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LRS slit and slitless modes each have their own custom TA procedures. TA is recommended for LRS slitless observations.

Please refer to LRS Recommended Strategies for concrete advice. 



TA target

Typically, the science target is used for TA. For slitless time-series spectroscopy, most science targets will be bright and compact enough for TA. A bright, nearby, within the APT visit splitting distance of the science target, point source, with accurate coordinates and proper motion, can also be used as offset TA.



TA filters and exposure settings

See also: MIRI Filters and DispersersMIRI 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 signal-to-noise ratio > 20, is required 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 slitless subarray and target acquisition region of interest (ROI)

See also: MIRI Detector Readout FASTR1MIRI 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 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.



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. While the acquisition of a verification image is optional, it is highly recommended to provide an astrometric reference for precise determination of the location of the undispersed source for post-processing.

Verification images no longer have to use the TA filter, or be part of the TA sequence.



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. An optional 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.

Figure 1. MIRI LRS slitless target acquisition procedure.

Left: Diagram of the MIRI imager module focal plane. The blue box labeled "SLITLESSPRISM_TA" is the 48 × 48 pixels 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 slitless TA procedure. Note: 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

MIRI LRS APT Template

JWST ETC MIRI Target Acquisition


Notable updates
  •  
    Added information about TA verification image

  •  
    Updated info on verification images

  •  
    Updated information on verification images
Originally published