Moving Target Ephemerides
Details of how JWST and APT obtain and use moving target ephemerides, how a proposer can obtain ephemerides using JPL Horizons, and how JWST's orbit affects ephemeris accuracy.
Using the orbital elements specified in APT and the relative positions of the guide star and the moving target, a 5th-order polynomial is constructed that describes the path of the guide star across the field of view of the Fine Guidance Sensor (FGS). The FGS then tracks the guide star along this path; this keeps the moving target fixed in the science instrument reference frame (see the Moving Target Acquisition and Tracking article for more details). Ephemeris data will not be included in the FITS headers, but pointing information obtained every 64 ms will be included, enabling a reconstruction of the target's motion across the sky (in RA/Dec coordinates). The heliocentric and observer distances will also be included in the FITS headers.
JWST in JPL Horizons
The observatory orbit will be updated after launch to reflect the actual launch date and the L2 halo orbit that is chosen. The halo orbit can differ substantially depending on launch date and course corrections, so ephemeris predictions even for main belt asteroids can be highly uncertain (up to 20') until the final JWST orbit has been determined. For trans-Neptunian objects, the initial ephemeris uncertainty will be around 1", but will become much smaller once the orbit of JWST is known.
Station-keeping to maintain the observatory's orbit about the L2 point will be performed every 21 days. Therefore, it is expected that the updated JWST orbit from each station-keeping procedure will be sent to JPL and incorporated in Horizons roughly every 21 days. A description of the orbit and the station-keeping procedures can be found on the JWST Orbit page.
A nominal JWST orbit is available in the JPL Horizons ephemeris generation system. There are 2 ways to specify JWST as the Observer Location* in Horizons: @jwst, or the observatory code, 500@-170. Figures 1–3 provide a quick walkthrough for specifying the Observer Location in Horizons.
Step 1. JPL Horizons home page
On the main page of the JPL Horizons web interface, locate the Observer Location row, as highlighted in Figure 1. Click on change.
Step 2. Observer Location page
After clicking on change, you will be redirected to the Specify Observer Location page. Locate the Specify Origin: Named Body or Site section and the search box. Input either @jwst or 500@-170, and click the Search button. Then, you will be redirected back to the main page.
Step 3. Check that Observer Location was updated
After being redirected to the main page, the Observer Location should have updated to read "James Webb Space Telescope (JWST) Spacecraft [500@-170], as highlighted in Figure 3.
Ephemerides and the APT Visit Planner
Main article: APT Visit Planner
The Visit Planner in the Astronomer's Proposal Tool (APT) uses orbital elements directly from JPL Horizons to construct ephemerides and determine observing windows. See the Tutorial on Creating Solar System Targets in APT for step-by-step instructions on obtaining orbital parameters for any moving target.