Visualizing the instrument field of view for a Solar System observation along a Solar System moving target trajectory, including dithers, in the JWST Astronomer's Proposal Tool (APT), is covered in this article.

Viewing a Solar System target track with instrument FOV and dithers in Aladin

Once you have created a Solar System target and an observation of that target, you can visualize the on-sky track of the target with the instrument field of view (FOV) superposed at intervals along the track. For the center-most position along the track, the instrument FOV is displayed including all dither pointings specified in the observation. The visualization capability is coupled with APT's ability to generate a simple ephemeris for moving targets as viewed from JWST. Both capabilities are accessed via the Solar System tab in the Form Editor for the observation, as shown below.

From the Solar System tab, add any constraints on the observing circumstances for your target. The fewer constraints and the wider the ranges you specify, the easier it will be to schedule your observations. Also, unless the V3PA constraint is scientifically necessary, it should be removed from the observation prior to final submission: it's purpose is simply to force Aladin to show the observation at a valid Orient.
Once you have specified your constraints, switch to the Visit Planner (VP) view, using the button at the top of APT. Once there, follow these steps (and see Figure 3):

1. Click on the Update Display button at the bottom to make the system compute the observability windows for your target and observation. That button will show red text if the observability windows don't reflect the current Solar System Windows, Special Requirements, or overall observation parameters you have specified. Once that calculation is complete (it can take up to a few minutes), the Update Display button will be grayed-out, and you'll see the available observing windows displayed in the VP view.
2. In the VP view, click on the ">" symbol next to the observation and then next to the visit(s) you wish to visualize to expand full detail of the calculation.
3. Hover your mouse over the green bars in the "Field of Regard" row of the display, taking care to also examine dates that are within the overall observability windows (black bars) at the "Observation" level in the VP view.
4. Note the ranges of valid dates ("Observation" row) and PA range ("Field of Regard" row) you wish to use for visualization. Note that the PA range reported is for the observatory V3 axis, not the science instrument aperture PA.

Once you have chosen a valid range of position angles (and the corresponding date range) for your observation, as described above, follow these steps (see Figure 4):

1. Switch to the Form Editor using the button at the top left of the APT GUI
2. Return to the observation you wish to visualize in the left-hand panel
3. Open the Special Requirements tab
4. Add a PA Range special requirement, entering the range you determined above. At any given time the allowed PA range is approximately ±10°, but the allowed PA range over a span of days will be larger.

These steps are necessary because APT does not know when your observation will be scheduled (even if you specify so many constraints there is only a single time at which it can be scheduled). Adding the PA range special requirement will also force the correct orientation of the instrument field of view when you visualize your observation.

Once the observation has been completely defined, including adding the PA Range window described above, the target track and instrument field of view can be visualized in Aladin. The first step is to generate the ephemeris (see Figure 5):

1. In the APT Form Editor, return to the Solar System tab on the observation
2. Click the Generate New Ephemeris button at the bottom of the Solar System tab
3. Enter the date range (or dates within the range) that you noted from above
4. You can select JWST or Earth as the Ephemeris Center in the dialog; use JWST
5. Enter a Step Size for the ephemeris, i.e., the time between points along the ephemeris.
6. Click Generate.
   NOTE: Initially this button is grayed out. Observers will need to click somewhere in the APT window, then Generate will become active.

Observers primarily interested in the target's track may wish to use a fairly large range of dates; if the interest is in seeing the dither pattern or mosaic coverage, only a single date is really needed. Setting the Step Size to provide a few points along the track is a good starting point. Note that for instrument apertures that are very small (like the NIRSpec IFU or MIRI MRS) it can be hard to see them plotted along the ephemeris track. Starting with a small range of dates and about 10 ephemeris points may help (see Figure 7).

Ephemeris generation is usually fairly quick (< 1 min). Once complete, the observer-supplied parameters used to generate the ephemeris, as well as the start and end on-sky position will be shown below the Generate New Ephemeris button. Clicking the View in Aladin button at the top of the APT GUI will open the Aladin window, with the instrument FOV displayed at the date range and spacing specified in the Generate New Ephemeris step, above. Points along the ephemeris are not connected by lines (the orange lines in Figure 7 were added). For this NIRSpec IFU observation, the  FOV is so small as to be nearly invisible at the zoom level imposed by the date range and total target motion. The simplest way to add images of the sky to the Aladin display is with the Load DSS button, highlighted in Figure 7. In particular, if Load DSS is used, it is easy to update the background image when the target is changed, or if the interval of the ephemeris is updated (simply hit the button after changing target or interval). If an image is displayed from within the Aladin window itself the process is more complicated, but is addressed in this knowledge base article. The Aladin viewer is discussed in the APT Aladin Viewer article.Once the view is displayed in Aladin dither parameters can be adjusted in the Form Editor view in the APT window; changes to the dither (or mosaic) pattern will be reflected in real time in the Aladin display. If the ephemeris date range is changed significantly and a new ephemeris generated, it will be necessary to update the Aladin display by clicking the Load DSS button again. It also sometimes necessary to click a different item in the APT hierarchical tree (left panel) and then back on the observation of interest to get the footprints to update in the Aladin viewer.

The Clear Ephemeris button will simply clear the ephemeris currently in memory (including deleting the ephemeris table data), but will not change the Aladin display.

Limitations:

• APT / Aladin does not support visualizations of planets and their satellites, neither in their apparent size nor their relative positions. Observers with approved programs can request assistance from their program coordinator, or use online tools such as NASA's Planetary Data System Ring-Moon Systems Node visualization tools (https://pds-rings.seti.org/tools/).
• Start and end dates for ephemeris generation are not required to be consistent with periods when the target is within the JWST field of regard. The ephemeris table and on-sky visualization can, then, include dates when the target is not actually observable from JWST. Observers should be sure to specify date ranges that are consistent with the observation windows in the Visit Planner.
• Dithers are only shown for the middle time and position along the ephemeris track (or the time just following the mid-time, if an even number of steps are specified). Dithers are executed the same way regardless of pointing or track rate, so this is a fairly minor limitation.
• Observers must re-enter all fields presented after clicking the Generate New Ephemeris button.
• A valid proposal ID (number) must be present on the cover sheet of the proposal. See the note above Figure 5 for more information.

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Viewing dithers with a fixed target  proxy for a moving target (legacy method)

The example below defines a fixed target, M35 (an open cluster on the ecliptic), as a proxy for Callisto. Picking a proxy on the ecliptic near RA = 6hr  (as for M35) or 18hr is useful because the allowed V3 position angles are parallel, to within ±5°, with lines of constant Dec (i.e., PA near +90 or -90). Observers may instead wish to pick a fixed target near the expected positions of their moving target, but then the evaluation of the valid PA values is more complex (see above). Visualization of the instrument FOV, placement of the FOV for various dither and mosaic-tile offsets, and depth of coverage is then possible by changing (temporarily) the target of the observation from Callisto to M35.

In order to visualize the dithers, click on the View in Aladin button at the top of the window. The Aladin viewer is discussed in the APT Aladin Viewer article.

In the Aladin window, as shown in Figure 9, any observations you have highlighted in the left sidebar will be displayed over the sky catalog of your choice (DSS, 2MASS, Simbad, etc.). 

Pan and zoom to display at the level you want. Figure 10 shows a screenshot of a NIRCam subpixel dither. 

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Links

Download the Astronomer's Proposal Tool

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