JWST's scheduling will be event-driven (see JWST Observing Overheads and Time Accounting Overview). This requires flexibility in the selection of guide stars over the scheduling window because not all guide stars will be usable for the entire window due to the motion of the target.
Once an appropriate guide star is selected, guiding on the moving target is performed by treating the guide star as a moving target in the Fine Guidance Sensor (FGS) and keeping the moving target stationary in the frame of reference of the science instrument (NIRCam, NIRISS, NIRSpec, or MIRI). Targets moving up to 30 mas/s (108 arcseconds/hour), the maximum rate of Mars, can be tracked by JWST.
Due to the nature of JWST scheduling, a moving target will have multiple guide star candidates available that can be used during different windows. At the time of the observation (the "Visit" in Figure 1), the first usable guide stars will be selected for tracking. The faintest guide stars that can be used for moving targets are ~1 mag brighter than those used for fixed targets. For observations without tight geometric or timing constraints (or no constraints), this will not prevent the moving target from being observed. However, in rare circumstances involving observations with very tight constraints, suitable guide stars may not be available within the constraint windows, particularly for targets far from the galactic plane (high galactic latitudes).
Long observations (>1 hour) can use multiple guide stars, but must be broken into multiple visits for each guide star. The visit splitting distance is 30" for moving targets, meaning that if a target moves more than 30" during an observation it will be split into additional visits with new guide star acquisitions. A new visit and separate guide stars, of course, will be required for any observations with a different instrument.
Telescope pointing for moving targets
The start time of a moving target observation (and therefore the target position) is not known ahead of time due to the event-driven nature of JWST scheduling. To allow for this flexibility, and as described above, multiple guide star candidates are identified during the scheduling process on the ground such that one or more will be available regardless of when the observation actually begins.
Once a moving target observation reaches the front of the event-driven schedule queue, the onboard system identifies appropriate guide star(s) for that start time, and the observatory slews to place one of the guide stars in the Fine Guidance Sensor (FGS) field of view. The system then performs guide star identification, finds the position of the selected guide star in the FGS, computes the slew needed to put the science target at the appropriate location (ambush point) in the science instrument field of view, and then executes that slew. A small extra amount of time is included such that the slew to the science pointing is guaranteed to complete before the science target reaches the ambush point.
JWST will support tracking rates of up to 30 mas/s (108 "/hour), the maximum rate of Mars. Nearly any target, including comets and near-Earth asteroids (NEAs), in the field of regard can be tracked (see Figures 3–6). Models show JWST's pointing stability for moving targets (<10 mas over a 1,000 s period) is comparable to the pointing stability for fixed targets (Milam et al. 2016). This excellent tracking rate and pointing stability will effectively render moving targets into fixed targets on the detector frame during individual exposures and leave background sources (stars, galaxies, slower moving Solar System targets, etc.) streaked. Dithers and mosaics will be supported.