MIRI MRS Target Acquisition

The MIRI medium-resolution spectrometer (MRS) may be used with or without target acquisition (TA) for both point and extended sources.

Observers using the MIRI medium-resolution spectrometer (MRS) integral field unit (IFU) may wish to use a target acquisition (TA) procedure to refine the blind JWST pointing accuracy prior to science observations, especially at the shortest wavelengths. This TA improves the pointing precision of the MIRI MRS from ~0.3'' without TA to 90 mas (1-σ radial) with TA, which is approximately the half width of a slice at the shortest wavelength. TA may be performed with the FND, F560W, F1000W, and F1500W filters. Note that the TA centroiding procedure loses accuracy if the pixels are saturated so a brightness limit (Table 1) must also be considered for the target.  Without TA, the uncertainty of the pointing is dominated by the guide star catalog position errors and pointing errors due to roll control.

Table 1. Saturation limits for MRS target acquisition for sources of a given blackbody temperature


300 K hard saturation

1,000 K hard saturation



The MRS target acquisition sequence uses a region of interest (ROI) on the MIRI imager located as close to the MRS as possible to minimize offset distances after target acquisition. As illustrated in Figure 1, a 48 × 48 pixels ROI (approximately equal to 5x5'') is defined at the upper right corner of the MIRI imager.

Figure 1. MIRI MRS target acquisition procedure

MIRI MRS TA procedure. Left: Depiction of the MIRI imager module focal plane, with the 5 × 5'' ROI used to perform TA shown in blue. Next to it, the MRS channel 4 FOV is shown in black. The exact locations of these regions are kept in the Science Instrument Apertures File (SIAF). Right: A flow diagram describing the MRS TA steps.
Science targets that are spatially extended should not be used for target acquisition. Instead, bright point sources can be used for target acquisition if they are within 16" of the science target (the TA source position must not require a greater than 40" spacecraft move to put the science target in the IFU; see Target Acquisition Considerations). This requires accurate coordinates for both the science target and the TA source.

Additionally, some observations with the MRS either do not require or cannot perform target acquisition. Examples of such observations are spectral mapping of extended objects, measurements of faint diffuse targets, or dedicated background observations.

Latest updates
    Revised TA box size to 48 × 48 arcsec.

    Updated discussion to include no-TA option available in APT 25.4.2
    Streamlined page by linking to extraneous information on other pages that was previously duplicated

    Clarified units (milliarcseconds)
    Removed paragraph on offset sources over 50"

    Introduction: Specifying that F770W is an available filter for TA.
    Figure 1: Specification of the axes and MRS reference frame.
    Specification of the MRS ROI, including its size and mid-point.
Originally published