Moving Target Field of Regard
The JWST field of regard will constrain the time of year a target can be observed. Because Solar System targets and the observatory are moving with respect to each other, the duration of the visibility windows could differ significantly from the standard durations.
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Field of regard
See also: JWST Target Observability and Observatory Coordinate System, JWST Position Angles, Ranges, and Offsets
See JWST Target Observability and Observatory Coordinate System and JWST Position Angles, Ranges, and Offsets for a more detailed look at the field of regard (FOR) and observatory pointing constraints.
Constraints on moving targets
Observatory constraints on solar elongation angle (85°–135°) define the time of year a target can be observed. The JWST field of regard (FOR) can be thought of as a hemisphere with a hole in it. The hole is due to the need to avoid observing at or near the anti-solar point (opposition). This shape results in 2 observing windows each year for objects along the ecliptic, each ~50 days long for targets with low apparent rates of motion. Again, due to the shape of the FOR, the time between the end of the leading window and the beginning of the trailing window is ~90 days. The end of the trailing window is followed by ~170 days until the leading window opens again (see Figure 1).
These numbers are rough estimates for fixed targets and slow-moving objects near the ecliptic; actual window lengths and spacings depend on the distance from the ecliptic, the magnitude of the apparent motion, and for objects in or closer than the main asteroid belt on their orbital motion. Generally, objects further from the ecliptic will have larger observing windows. Once a target is specified in APT, the Visit Planner will correctly compute the times of apparition within the FOR. JPL's Horizons tool also allows solar elongation constraints to be entered, and yields results consistent with those from the Visit Planner.
The following Solar System bodies can never be observed by JWST: the Sun, Mercury, Venus, Earth, and the Moon, and minor bodies within about 1 AU of the Sun.
Instrument FOV orientations
See also:
JWST Field of View; JWST Instrument Ideal Coordinate Systems; JWST Position Angles, Ranges, and Offsets
To evaluate the allowable position angles for the science instruments when observing moving targets, use the Moving Target Visibility Tool (MTVT) or follow directions in Tutorial on Visualizing Dithers of a Solar System Observation in APT.
The Aladin feature in APT does not present accurate orientations for the instrument FOVs; this is the case for observations of both fixed and moving targets. The default orientation in Aladin has the V3 direction parallel to equatorial north, which is never valid for observations of targets near the ecliptic. See Tutorial on Visualizing Dithers of a Solar System Observation in APT for a workaround.
Figure 1. JWST focal plane orientation on the ecliptic when pointing is in (leading) the direction of motion or opposite of that (trailing)
Click on the figure for a larger view.
Orientation of the JWST focal plane and instrument FOVs for observations in each section of the field of regard. The orientations shown above are the only 2 options for observations of objects along the ecliptic.
When JWST is pointed into the leading FOR as shown above, it is also pointed into the micrometeoroid avoidance zone (MAZ). Observing in the MAZ is generally but not universally discouraged. Further information is available in JWST Micrometeoroid Avoidance Zone
When elongation range is specified in JPL Horizons, the output includes a flag indicating whether the target is leading ("/L" – east of) or trailing ("/T" – west of) the Sun. That convention is the opposite of the convention in the context of Figure 1.