JWST Moving Target Observing Procedures

Procedures for acquisition of guide stars and tracking of moving targets with apparent rates of up to 30 mas/s (108 "/hour)

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Main article: Fine Guidance Sensor (FGS)

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.



Guide stars for moving target observations

Main article: JWST Guide Stars

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 the figure below), 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 can be 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 arcseconds for moving targets, meaning that if a target moves more than 30 arcseconds 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.

Figure 1. Example scheduling window of a moving target observation

The scheduling window is filled with multiple sets of guide stars in order to maintain scheduling flexibility. The large green bar at the top represents the window for the observations to be made, with the distance between the latest start time and latest end time equal to the length of the visit. The position of the latest start time in a given guide star window is determined by the length of the visit and the duration of the guide star's availability. If the visit time is longer than the selected guide star's observability window, it may be split into multiple visits using multiple guide stars, and incur the necessary additional overheads.


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 on-board system identifies appropriate guide-star(s) for that start time, and the observatory slews to place one of those 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.

Figure 2. Schematic showing guide star acquisition in the Fine Guidance Sensor (FGS) for moving target observations

Schematic for Moving Target Observation

Steps 1 and 2 are the same as for fixed targets. The slew after step 2 is different, in that the guide star is moved to a point slightly ahead of the science target in its motion, and then tracking is engaged at the time when the science target reaches the correct position in the science instrument. Note that the FGS and associated software are tracking the guide star, not the science target itself. The 32 × 32 pixel guide box moves in the FGS field of view as the telescope tracks, such that the science target remains fixed in the reference frame of the science instrument for the entirety of the observation.


Non-sidereal tracking

JWST will support tracking rates of up to 30 milliarcseconds/second (108 arcseconds/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 below). Models show JWST's pointing stability for moving targets (<10 milliarcseconds over a 1000-second 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.

Figure 3. Near-earth asteroid rates

Apparent rates of 11,467 near-Earth asteroids (NEAs) observable in 2019. The dark curve is the histogram of the number of days that NEAs are observable within different rate bins (1 milliarcsecond/second bin width), only considering dates when the objects are within JWST's field of regard (elongation angles between 85° and 135°). The gray curve is the cumulative histogram normalized to fit in the same plot area (the values on the y-axis do not apply to the gray curve). The vertical dot-dashed line is the average rate of the NEAs and the vertical dashed line marks the maximum 30 milliarcsecond/second rate observable by JWST. On a date when a given NEA is within JWST's field of regard there is a 91% probability that it can be tracked by JWST. (Milam et al., 2016)
Figure 4. Main-belt asteroid rates

Apparent rates of 305 main belt asteroids (MBAs) observable from 2019 to 2020. The dark curve is the histogram of the number of days that MBAs are observable within different rate bins (1 milliarcsecond/second bin width), only considering dates when the objects are within JWST's field of regard (elongation angles between 85° and 135°). The gray curve is the cumulative histogram normalized to fit in the same plot area (the values on the y-axis do not apply to the gray curve). The vertical dot-dashed and dashed lines are the average and median rates of the MBAs, respectively. All MBAs can be tracked by JWST on any given date that they are in the field of regard. (Milam et al., 2016)
Figure 5. Comet rates

Apparent rates of 170 comets observable from 2019 to 2020. The dark curve is the histogram of the number of days that comets are observable within different rate bins (1 milliarcsecond/second bin width), only considering dates when the objects are within JWST's field of regard (elongation angles between 85° and 135°). The gray curve is the cumulative histogram normalized to fit in the same plot area (the values on the y-axis do not apply to the gray curve). The vertical dot-dashed line is the average rate of the comets and the vertical dashed line marks the maximum 30 milliarcsecond/second rate observable by JWST. Only 1 comet cannot be tracked during the time period, and 6 are only trackable for a fraction of the time period. (Milam et al., 2016)
Figure 6. KBO and centaur rates

Apparent rates of 130 Kuiper Belt Objects (KBOs) and Centaurs observable from 2019 to 2020. The dark curve is the histogram of the number of days that the KBOs and Centaurs are observable within different rate bins (1 milliarcsecond/second bin width), only considering dates when the objects are within JWST's field of regard (elongation angles between 85° and 135°). The vertical dot-dashed and dashed lines are the average and median rates of the KBOs and Centaurs, respectively. All KBOs and Centaurs can be tracked by JWST on any given date that they are in the field of regard. By extension, all of the giant planets, which have semi-major axes between the MBAs and KBOs, can be tracked by JWST on any date that they are in the observatory's field of regard.


References

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




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