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.

See also: JWST Astronomers Proposal Tool OverviewMIRI LRS APT TemplateAPT Mosaic Planning

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. 

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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. Note that a similar result can be achieved by selecting the MAPPING option for the Dither strategy and specifying the pattern steps in spatial and spectral directions, in arcseconds. The two can also be combined to execute a dither pattern at each mosaic pointing location, if specific spatial sampling of the target is required.

For mosaics, the number of rows sets the number of steps in the dispersion (cross-slit) direction, while the number of columns sets the number of steps in the cross-dispersion direction (the spatial direction, or along the slit).  The mosaic pattern is centered on the coordinates of the specified target.  If the user wishes to observe the position pointed to by the target coordinates in the center of the slit (in the dispersion direction), then they should set an odd number of rows.  To translate the percentage overlap between pointings into angles on the sky, the user should keep in mind that the slit dimensions are approximately 0.52" in the row direction and 4.7" in the column direction.

Users are strongly 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).




Latest updates
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    Added slit dimensions and location of center of mosaic pattern

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