JWST Target Viewing Constraints
JWST has time-variable viewing constraints, imposed by a combination of observatory safety concerns and target position in ecliptic coordinates.
At all times during the operational phase of the mission, the JWST telescope and science instruments must remain shielded from the sun. To not do so would endanger the entire functionality of the observatory. The geometry of the JWST sun shield limits where JWST can point at a given time and for how long. It also impacts the observatory's ability to observe the celestial sphere at certain position angles, especially for target positions at low ecliptic latitudes.
JWST field of regard (FOR)
The JWST field of regard (FOR) is the region of the sky where scientific observations can be conducted safely at a given time. The FOR is defined by the allowed range of boresight pointing angles for the observatory relative to the sun line, which must remain in the range 85° to 135° at all times to keep the telescope behind the sun shield. Thus, the FOR is a large torus on the sky that moves roughly 1° per day in ecliptic longitude, following the telescope in its path around the sun. Over time, this annulus sweeps over the entire celestial sphere. As a result of the FOR, JWST can observe about 39% of the full sky on any given day and can access 100% of the sky over 6 months. Figure 1 shows a schematic of the FOR.
Beginning in late 2022, NASA has imposed a micrometeoroid avoidance zone to minimize potential impacts from space debris coming from the direction of orbital motion. This zone overlaps a significant portion of the orbital leading side of the FOR torus. Observations in this overlap region with the FOR are not disallowed, but must be minimized for the long term safety of the observatory. Starting with Cycle 2, APT contains warnings and errors related to proposed observations that require observations in this restricted region.
Observability with JWST is very dependent on a given target's ecliptic latitude. Below 45° ecliptic latitude, JWST can observe targets in 2 visibility windows per year centered about 6 months apart, with each window lasting at least 50 days. Above 45° and below 85° ecliptic latitude, the visibility windows transition to one much longer visibility period. As Figure 2 shows, ecliptic latitude determines the number of days per year that targets are observable by JWST. Also, the allowed field of view position angles on the sky available for a given target are affected by the target's ecliptic latitude. These windows and allowed position angles can be calculated for a particular target using one of the JWST target visibility tools.
JWST has a relatively small continuous viewing zone (CVZ), located within 5° of the ecliptic poles. The CVZ is important for some science programs that involve monitoring throughout the year and is also useful for calibration observations. Although the roll flexibility is still about ±5°, the JWST field of view rotates around the V1 axis (boresight) through the entire available 360° over the course of the year.