Procedures for acquisition of guide stars and tracking of moving targets with apparent rates of up to 75 mas/s are described in this article.

See also: Fine Guidance Sensor (FGS)

JWST's scheduling is 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 75 mas/s can be tracked by JWST with stability comparable to fixed targets. Observations of objects moving at apparent rates of up to 100 mas/s will only be considered in special situations due to the effort required to plan such observations and their extreme inefficiency.

Guide stars for moving target observations

See also: JWST Guide Stars

Due to the nature of JWST scheduling, a moving target will have multiple guide star candidates available for each window. 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. The smaller number of available guide stars will not prevent a moving target observation from executing. 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). Loosening geometric or timing constraints will improve schedulability.

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 38" for moving targets, meaning that if a target moves more than 38" 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. The Astronomers Proposal Tool (APT) will warn users if the scheduling duration for a particular visit exceeds the usable guide star duration. Both numbers are reported in the visit form, which is available under the observation form in the tree editor on the left hand side of the APT GUI. This will largely affect observations of very fast-moving targets (e.g., long-period comets, interstellar objects, near-Earth asteroids) and will therefore not be a cause for concern for the majority of moving target observations.

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.

Shadow observations will be a future capability 

Shadow observations will not be implemented in APT for the Cycle 4 proposal period.

Observations of faint, extended sources, e.g., comets with faint comae, can benefit from "shadow observations." These are observations of the background field at a later time when the science target has moved out of the field; the shadow observation is then subtracted from the original science observation. The target must be completely out of the field when the shadow observation occurs. Additionally, even for a target moving at the maximum tracking rate, a non-zero time interval is required between the science and shadow observations for the target to move fully out of the field. The time between observations can be shorter for faster-moving targets; shorter times between observations are preferable and will prevent the background from changing significantly.

The shadow observation must be tracked in the exact same way as the science observation so that the star streaks are replicated and can be appropriately subtracted. This requires the science observation to occur before the shadow observation so that the exact track from the science observation can be used to plan the shadow observation; this ordering is due to the event-driven nature of JWST scheduling.

Non-sidereal tracking

JWST will support tracking rates of up to 75 mas/s. Nearly any target, including comets and near-Earth asteroids (NEAs), in the field of regard can be tracked. The excellent tracking rate and pointing stability observed on-orbit effectively renders 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 are supported.

References

Milam, S., et al. 2016, PASP, 128, 959 
The James Webb Space Telescope’s Plan for Operations and Instrument Capabilities for Observations in the Solar System
ADS  arXiv