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.
Field of regard
The thermal design of the observatory constrains its allowed solar orientations. In order to keep the mirrors and instrument suite passively cooled behind the sunshield, the solar elongation (the Sun-JWST-target) angle must remain between 90° to 130° with an additional roll of ±5° about the line of sight. This results in a field of regard spanning 85° to 135° in solar elongation angle, as illustrated in the figures below. Therefore, objects at the anti-solar point (opposition, elongation angle equal to 180°) cannot be observed by JWST.
Constraints on moving targets
The constraints on elongation angle define the time of year a target can be observed. The JWST field of regard 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, as described above. This shape results in two observing windows each year for objects along the ecliptic, each ~50 days long. Again, due to the shape of the field of regard, the time between the end of the first window and the beginning of the second is ~90 days. The end of the second window is followed by ~170 days until the first window opens again. These numbers are rough estimates for objects near the ecliptic; actual window lengths and spacings depend on the distance from the ecliptic and the magnitude of the apparent motion. Generally, objects further from the ecliptic will have larger observing windows and the slower the apparent motion of an object, the larger its observing window. We recommend that proposers determine the observing windows for their objects using JPL Horizons (there is an option on the "Table Settings" page to specify a range of solar elongation angles).
The following Solar System bodies can never be observed by JWST: the Sun, Mercury, Venus, Earth, and the Moon. These constraints also mean that Mars is observable only every other year, starting in Cycle 2.
Moving target articles