A walk-through of a simple APT mosaic example is described for MIRI and NIRCam in this article.

 Specifying Mosaics in APT Video Tutorial

This example walks the user through a simple APT mosaic program. An accompanying APT example file is available within APT, using the File → JWST Demonstration Proposal option. Grab the file called Mosaic Example. Two targets have been entered in the program: M83 and M82. A simple pair of NIRCam and MIRI mosaics are created using galaxy M83 as an example, followed by an example for a non-symmetrical mosaic case (M82), where the NIRCam field orientation must be restricted using special requirements. The user is encouraged to download the accompanying APT file, follow along in APT, and experiment with the various control features on their own. A view of the targets in Aladin using the DSS and no overlays, is shown below. Figure 1 shows the Aladin visualization display for these 2 galaxies, displayed using the DSS (Palomar Digital Sky Survey) display option.

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Symmetrical mosaics covering M83

The example file contains 2 simple mosaic definitions covering the bright disk of M83. Observation 1 is a NIRCam 4 × 2 mosaic that covers an approximately square region, using the 3-TIGHT dither pattern, while observation 2 shows a MIRI imaging 5 × 9 mosaic covering approximately the same region. Overlays of the instrument fields of view are shown in Figure 2. Note that for NIRCam, a simplified version of the field overlay is shown here, selected using the Single Aperture control option in the Aladin control panel. This feature is new as of APT 25.1.1.

Since no PA constraints have been placed on these observations, Aladin simply shows a default orientation. (You should be aware of whether you are viewing a fixed orientation you have chosen. If the observation is unconstrained, the pattern of tiles shown may rotate with time.) No position angle constraints have been placed on these proposed observations, and so a run of the Visit Planner is free to rotate the pattern to assess if there are times when all of the proposed tiles can obtain guide stars simultaneously. 

The NIRCam mosaic has 8 visits (one for each tile), and the Visit Planner's assessment returns with a green check on the observation, indicating there are times when all tiles can get guide stars simultaneously and all other visibilities are good. A check of the guide star situation against the V3PA is shown in Figure 3, and indicates there are 2 solid windows of schedulability.

Likewise, the MIRI mosaic in observation 2 passes the Visit Planner check, even though in this case there are 45 visits involved. Figure 4 confirms that there are 2 good windows where all 45 visits can get guide stars, and the green check in the VP confirms schedulability against all constraints.

If an observer was happy with all other observation settings for these observations, they could be submitted.

What if all of the tiles had not been able to get guide stars at the same time?

APT provides a way for splitting off problem tiles to a separate associated observation.

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A non-symmetrical case: M82 with NIRCam

The situation in the previous section worked well because both the regions to be observed and the shape of the mosaic itself were symmetrical, so that the angle of the mosaic at the time of actual scheduling was not critical to the success of the observation. This is ideal, as it provides the most scheduling flexibility. However, as the situation with M82 demonstrates (Figure 1, bottom), this is not always the case.

In observation 3, a mosaic has been defined that is long and narrow, to cover the edge-on disk of the target. However, if this mosaic is rotated, it will no longer cover the region of interest. Hence, one must restrict the permitted position angle of this mosaic in order to force the mosaic into the desired position, as shown in Figure 5. 

Whenever possible, however, it increases scheduling flexibility significantly if a range in position angle is allowed, instead of a fixed angle. In this example, the special requirement set on the observation allowed a 20° range centered on APA = 157°, which still maintains the alignment of the mosaic on the object well enough. Once the special requirement is set, one can run the VP to check the schedulability at the angles allowed. In this case, the VP returns a green check, indicating all is good with schedulability at these angles. A check of the V3PA diagnostic confirms a good window for guide star availability, as shown in Figure 6.

As a double check, one can return to the Aladin display and turn on the Orient ranges tab, which returns the display shown in Figure 7, where the green part of the circle indicates the amount of APA range allowed. If it turned out that the desired optimal position angle range was not available for your target, the area of interest can still be covered by skewing the mosaic and using the mosaic overlap controls to position the tiles. Figure 8 shows an example, where offsetting the columns and allowing more tile overlap have covered approximately the same region of M82.

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Practice exercises

Here are some things to try with the mosaic examples.

1. Display M83 and the observation 1 (NIRCam) field overlay in Aladin. Toggle between the full FOV display and the Single Aperture display option to see the difference.
   
   
2. Select an observation in the form editor and open the Mosaic Properties tab. Adjust the percentage of allowed overlap in one or both directions and compare the appearance in the Aladin display. Change the number of tiles in one or both directions of the mosaic and note the change in the Aladin display.
   
   
3. In the form editor, choose observation 3, open the Special Requirements tab, and edit the allowed range of APA. Then run the VP and check the schedulability.
   
   
4. Select an individual visit in observation 3 and display it in Aladin. In the Aladin GUI, select the GS check option, and potential guide stars should be indicated with green symbols. If the FoV option is checked in the Aladin control panel, a display showing all of the JWST instrument footprints should appear. Verify there are one or more green guide stars in at least one of the boxes that represent the position of the FGS guider.

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