MIRI Imaging Mosaics

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.

Figure 1. 2 × 3 MIRI imaging mosaic with a tile overlap of 10% and no row or column shifts

Footprint of a 2 × 3 MIRI imaging mosaic using FULL array tiles, with a 10% tile overlap and no row and/or column shifts. The footprints of the 4-Point-Sets dither pattern are evident in the figure, while the blue shading reflects the level of exposure time coverage.
Figure 2. 2 × 3 MIRI imaging mosaic with tile overlap of 20% and pixel row and column shifts of 20%

The same mosaic as in Figure 1 using FULL array tiles, 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 the figure, while the blue shading reflects the level of exposure time coverage.

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Latest updates

  • Replaced figures with those that use the the 4-point-sets dither pattern instead of the Realeaux pattern.
    Added a link to the Knife Edge Gap in Miri Imaging Mosaics page.
    Adressed Karl's comment and added units to the column/shift row percentage values.