MIRI MRS Mosaics
JWST mosaics can be constructed for the MIRI medium-resolution spectrometer (MRS) and implemented in the Astronomer's Proposal Tool (APT) with the JWST APT MIRI MRS template. These mosaics provide coverage of larger fields of view (FOV) that extend beyond a single pointing, and can be constructed using any of the 4 channels as the primary field of view.
Primary channel selection and mosaics
See also: MIRI MRS Field
Choosing Channel 1 (or ALL1) in APT places the target at the pointing origin of the channel 1A (i.e., Channel 1, SHORT(A)) field of view, and the boundaries of this field will be used to determine the effective footprint for purposes of determining mosaic tile boundaries and overlaps. Similarly, choosing channel 4 will place the target at the pointing origin of the channel 4A field of view, and the boundaries of this field will be used for determining mosaic tile positions. Since the channel 4 field of view is much larger than that of channel 1, the mosaic tile offsets will be commensurately larger. With channel 1 (or ALL) as the primary channel, each channel 1 tile will overlap the others to a specified percentage, while each channel 4 tile will overlap the next more substantially (Figure 1). If channel 4 is selected as the primary channel, each channel 4 tile will overlap the others to the specified percentage, while there will be gaps between the channel 1 tiles (Figure 2).
Note that tiles generated by APT are produced symmetrically about the astronomical target. Therefore, for an even number of tiles, the astronomical target will be centered between two tiles, while for an odd number of tiles it will be centered in a tile.
Dithering and Mosaics
See also: MIRI MRS Dithering
Note that although the figures above only show a single pointing for each mosaic tile, these tiles should also be dithered in order to optimize spatial and spectral sampling throughout the field of view (e.g., Figure 3). The extended source dither patterns described on the MRS dithering page are best suited for this purpose as they are designed to optimize spatial sampling while maximizing common field overlap area. Usage of such a pattern will require additional specification of a dedicated sky exposure.
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