Knife Edge Gap in MIRI Imaging Mosaics

The default MIRI imaging mosaic overlap of 10% is used to address the gap produced by the knife-edge structure visible on the left edge of the main imager field of view.

Figure 1. The MIRI imaging field of view (FOV)

Specific sections of the MIRI imager focal plane are used for imaging, coronagraphic imaging, and low-resolution spectroscopy modes. The imaging mode FOV takes up a large section to the right of the imager focal plane.

The structure that protrudes into the main field of view of the MIRI imager, on its left edge, is the "knife-edge" used to evaluate image quality. The default MIRI imaging mosaic overlap of 10% ensures that the gap produced by the knife-edge structure has the same (or greater) exposure coverage as the rest of the imaging FOV for all dither patterns. The 10% overlap also ensures that there is sufficient overlap in the mosaics for the pipeline to match background levels across the mosaicked field of view and is therefore recommended as a minimum.

If optimal background matching across mosaic tiles is not important for the user, a lower percentage of overlap may be selected. A lower percentage may provide shallower coverage of the gap without leaving holes in the mosaic. For example, the 4-point-extended dither pattern covers the entire knife-edge gap with 50% exposure coverage (2 tiles out of 4), even when 0% mosaic overlap is selected. For the dither patterns optimized for point sources, the gap will be covered with at least one tile using 4% mosaic overlap for the 4-point and 6% overlap for the 2-point dither patterns. The exposure coverage as a function of overlap percentage is summarized in Figure 2.

Future versions of APT will include the knife-edge gap in the MIRI imaging footprint in the Aladin display.

Figure 2. Mosaic exposure coverage as a function of overlap percentage

Exposure coverage for a 2-tile mosaic using the 4-point dither patterns optimized for an extended source (top row) and point source (bottom row). The scale indicates the number of frames. A mosaic overlap of 10% is required to cover the knife-edge gap with at least 4 tiles and provide optimal background matching across the field of view. For shallower coverage, but no holes across the gap, a lower mosaic overlap can be used as shown above. 

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