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 mode

Parent pages: MIRI Operations → MIRI Mosaics

JWST mosaics allows for moves specific to MIRI imaging. Mosaics for an observation 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 amount of row and/or column shift 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 a row and column shift of 10. The footprints of the 12-point REULEAUX dither pattern are evident in both panels, while the blue shading reflects the level of exposure coverage as a function of position.

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 10% tile overlap, 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. 

Figure 2. 2 × 3 MIRI imaging mosaic with 20% tile overlap, 10 pixel row and column shifts

The same mosaic as in Figure 1, but with 20% overlap between tiles, and a row and column shift of 10. The footprints of the 12-point REULEAUX dither pattern are evident in both figures, while the blue shading reflects the level of exposure time coverage.


References

JWST technical documents


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Last updated

Published January 2, 2017


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Published March 02, 2017