JWST Moving Target Ephemerides

This page contains information on how JWST and APT obtain and use moving target ephemerides, how the proposer can obtain ephemerides using JPL Horizons, and how JWST's orbit affects the accuracy of the ephemerides.


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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 Tutorial on Creating Solar System Targets in APT for step-by-step instructions on obtaining orbital parameters for any moving target.



Moving target ephemerides and JWST

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 JWST Moving Target Observing Procedures page 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

A nominal JWST orbit (computed on April 3rd, 2018, for planning purposes) is available in the JPL Horizons ephemeris generation system. This JWST ephemeris is valid from 2018-Oct-01 through 2024-Dec-31. There are two ways to specify JWST as the Observer Location in Horizons: "@jwst" (without quotes) or the observatory code "500@-170" (without quotes). The images below 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 the figure below. Click on "change."

Figure 1. JPL Horizons home page



  • Step 2. Observer Location page. After clicking on "change," the user will be redirected to the Observer Location page. Locate the "Lookup Named Location" section and the search box. Input either "@jwst" (without quotes) or "500@-170" (without quotes) and click the "Search" button. The user will be redirected back to the main page.

Figure 2. Specifying the observer location



  • Step 3. Check that Observer Location 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 the image below.

Figure 3. Observer location set to JWST.



JWST orbit 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 arcminute) until the final JWST orbit has been determined. For trans-Neptunian objects, the initial ephemeris uncertainty will be around 1 arcsecond, 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.



References

JPL Horizons web interface




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