Words in bold italics are buttons or parameters in GUI tools. Bold style represents GUI menus/ panels & data software packages.
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).
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.
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.
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.
Revised figure layout
Added figure illustrating effect of AperturePA on tile locations.