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.
See also: JWST Astronomers Proposal Tool Overview, MIRI MRS APT Template, APT Mosaic Planning, JWST Mosaic Overview
See also: Specifying Mosaics in APT
Primary channel selection and mosaics
See also: MIRI MRS Field and Coordinates
Words in bold are GUI menus/
panels or data software packages;
bold italics are buttons in GUI
tools or package parameters.
Choosing
Channel 1 (or
ALL) 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, left panel). 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 1, right panel). A 10%
Row and
Column Overlap is recommended to allow the resulting data to be combined together.
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 tiles.
Since the MRS ideal coordinate frame is defined to be aligned with V2/V3, an Aperture PA of 0° will result in a series of mosaic tiles that are rotated with respect to the RA/DEC coordinate frame by about 8° (see Figure 2, left panel). In order to align a series of mosaic tiles with an astronomical object the rotation angle between the MRS channel of choice and the V2/V3 coordinate system should be subtracted from the desired astronomical PA (Figure 2, right panel). Note that due to distortions in the MRS FOV it is always best to confirm the locations of your mosaic tiles visually in Aladin.
Dithering and mosaics
See also: MIRI MRS PSF and Dithering
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.