MIRI LRS Mosaics

The low-resolution spectroscopy mode for JWST’s Mid-Infrared Instrument (MIRI) offers a mosaicking option for coverage of larger fields of view (FOV) that extend beyond a single pointing.

Mosaics for low-resolution spectroscopy mode

Mosaics for an LRS observation can be implemented in the Astronomer's Proposal Tool (APT) using the JWST APT MIRI low-resolution spectroscopy template. These mosaics provide coverage of larger fields of view (FOV) that extend beyond a single pointing.

Mosaics are only supported for observations with the LRS slit; the LRS slitless mode is dedicated to single-pointing time-series observations. As in a regular observation, the user should select exposure settings for the observation, and pick the FULL1 subarray setting for slit spectroscopy observations. 

Two methods are available for defining the mosaic pattern. By choosing the MAPPING option in the Dither Type drop-down list, the user can specify a number of spatial and spectral offset steps around the slit center position, with the offset provided in arcseconds.

Alternatively, with the number of spatial and spectral steps set to 1, a pointing pattern can be defined in the Mosaic Properties tab in the observations form. Here, the pattern is defined in rows, columns, and percentage overlap between the pointings. Combining the two methods, or selecting the ALONG SLIT NOD Dither Type with a mosaic, allows the user to define a dither pattern at each mosaic pointing location.

Users are encouraged to always check the defined pattern using the APT Aladin visualization functionality, which shows the slit footprints superimposed on the target.

The user should also consider using the MIRI Medium-Resolution Spectrometer (MRS), which provides integral field spectroscopy (IFU) at higher spectral resolution than the LRS (from ~5 to 28.5 µm, albeit without continuous spectral coverage).

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