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.

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Primary channel selection and mosaics

See also: MIRI MRS Field and Coordinates

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, 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).

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.

Figure 1. MRS mosaic dependence on Primary Channel

Blue/red rectangles illustrate the locations of the Channels 1 and 4 fields of view in the first and second tiles (solid and dotted lines respectively) of a two-tile mosaic.  The left panel illustrates a case in which Channel 1 is selected as the Primary Channel (thus defining the pointing origin, tile angle, and nominal tile offset distance), while the right panel illustrates a case in which Channel 4 is selected as the Primary Channel.  Note that in the latter case the fields of view for Channel 1 data in the tiles do not overlap.  The crosses represent the center of the fields for each tile. The tiles are shown undithered for visual illustration purposes.
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-hand 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-hand 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.

Figure 2. MRS field of view angle

Channel 1 mosaic with 4 columns and 1 row as shown by APT/Aladin with an Aperture PA of 0° (left) and an Aperture PA of 360° - 8.4° = 351.6° (right) to roughly align the mosaic tile placement with the RA/DEC coordinate frame.

Dithering and Mosaics

See also: MIRI MRS 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.

Figure 3. Example dithered MRS Mosaic

Same as Figure 1, but with the 2-Point Extended Source dithering patter at each mosaic location. The blue cross denotes the (undithered) central location of channel 1 in the first mosaic tile. 



Latest updates
  • (07/08/19)
    Added figure illustrating effect of AperturePA on tile locations.
  • (11/21/19)
    Revised figure layout

Bold italics style indicates words that are also parameters or buttons in software tools (like the APT and ETC). Similarly, a bold style represents menu items and panels.