MIRI Target Acquisition

The JWST Mid-Infrared Instrument (MIRI) provides target acquisition capabilities for the low-resolution spectroscopy, medium-resolution spectroscopy, and coronagraphic imaging modes.

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Target acquisition (TA) is a procedure that centroids a source and places it accurately within an aperture, region of interest (ROI), or subarray. The filters available for MIRI TA are F560W, F1000W, F1500W, and FND. TA procedures in the Astronomer's Proposal Tool (APT) are available in these articles:

MIRI target acquisition readout patterns.

See alsoUnderstanding Exposure Times

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

The target acquisition process requires the target acquisition file to be stored on-board. Since on-board memory can only store the equivalent of 10 full frame groups of MIRI data at a time, target acquisition in un-grouped FAST mode is limited to a maximum exposure time that might be shorter than desired by the user.  

The FAST readout mode and the FASTGRPAVG series of readout modes are available for MIRI target acquisition only. The FASSTGRPAVG readout modes allow for longer exposure times. This is done by averaging a given number of MIRI frames together in the ISIM FPAP (Focal Plane Array Processor) as a group. Note that all MIRI target acquisition exposures use one integration.

Readout mode
(as offered in APT)

Number of averaged MIRI frames

Group time
(FULL array)

Group time

Group Time
(MaskLyot subarray)

Group Time
(4QPM subarrays)
FAST12.775 s0.159 s0.324 s0.24 s
FASTGRPAVG411.10 s0.64 s1.30 s0.96 s
FASTGRPAVG8822.20 s1.27 s2.59 s1.92 s
FASTGRPAVG161644.40 s2.54 s5.18 s3.84 s
FASTGRPAVG323288.80 s5.09 s10.37 s7.67 s
FASTGRPAVG6464177.60 s10.18 s20.74 s15.34 s

Latest updates
    Added FAST as readout pattern for TA

    Added links for LRS slit and Slitless TA
Originally published