Knife Edge Gap in MIRI Imaging Mosaics
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 (Figure 1).
The default 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.