The imaging 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 MIRI imaging observations can be implemented in the Astronomer's Proposal Tool (APT) with the JWST APT MIRI imaging template. These mosaics provide coverage of larger fields of view (FOV) that extend beyond a single pointing.

As with single-pointing MIRI imaging observations, the user first selects the desired imaging subarray, dither pattern, readout mode, and filters. The parameters that define the footprint of the MIRI imaging mosaic are the center coordinates, number of rows and columns of the individual pointings (or tiles), the mosaic orientation angle, percentage of overlap region between the tiles, and the percentage of row and/or column shift between tiles that can be used to skew the tiled pattern. For example, Figure 1 shows the footprint of a 2 × 3 mosaic of FULL¹ array tiles, with a 10% tile overlap and no row and/or column shifts. Figure 2 shows the same mosaic, but with 20% overlap between tiles and row and column shifts of 20°. The footprints of the 4-Point-Sets dither pattern are evident in both panels, while the blue shading reflects the level of exposure coverage as a function of position. To ensure that there are no gaps in the exposure coverage across the mosaic, the user should read the documentation on the Knife Edge Gap in MIRI Imaging Mosaics.

Starting with the first tile in the mosaic, the exposures are carried out at each dither step, and the filter changed once the dither pattern has been completed. Once all the user-specified filters have been rotated through for a single tile, the sequence is repeated for the next tile, and so on.

For MIRI imaging mosaics, each tile will usually be a separate visit that requires its own guide star. This depends both on the overlap region between the tiles and the visit splitting distance calculated by the APT. If the entire mosaic cannot be scheduled simultaneously due to missing guide stars, it maybe necessary to split one or multiple tiles into a separate associated observation.

Bold italics style indicates words that are also parameters or buttons in software tools like the APT and ETC. Similarly, a bold style represents menu items and panels.