JWST Target Viewing Constraints

JWST has time-variable viewing constraints, imposed by a combination of observatory safety concerns and target position in ecliptic coordinates.  

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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.

Field of regard

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 determined by the shape of the sun shield, as shown in Figure 1. Pointing constraints imposed by the attitude control system allow the telescope to point towards targets between solar elongation of 85° and 135°, thus creating a large annulus on the sky where JWST can safely point. 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. The shape of the sun shield is also responsible for the narrow range of permitted roll orientations around the telescope boresight (the optical or V1 axis). This instantaneous roll flexibility is about ±5° but varies with time and look direction. For more on allowed position angles as a function of time and target positions, refer to JWST Position Angles, Ranges, and Offsets.

Figure 1. The JWST field of regard

JWST can point at solar elongations between 85° and 135°, as shown in the figure above. It can also observe at any location in the 360° circle perpendicular to the sun line, which defines a large annulus where JWST can observe at a given time. This defines the field of regard (FOR).

Target observability

Observability with JWST is very dependent on a given target's ecliptic latitude Below 45° ecliptic latitude, JWST can observe targets in two visibility windows per year centered about six 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 will be 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.

Figure 2. Target observability as a function of ecliptic latitude

The number of days per year that targets are observable by JWST, as a function of ecliptic latitude. The graph shows the total number of days, but below 45° ecliptic latitude, this total visibility comes in the form of two smaller time periods separated by approximately six months. Above 45°, one longer viewing period is available for targets, lengthening until the continuous viewing zone is reached at approximately 85° ecliptic latitude. Available position angles are also limited by ecliptic latitude.



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