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James Webb Space Telescope
Field-Of-Regard And Sky Coverage

relative attitudes of JWST

This figure shows the relative attitudes of JWST with respect to the Sun at the limits of its pitch range. The sunshield design permits the observatory to pitch toward and away from the Sun over a range of approximately 50 degrees, while keeping the telescope in the shade.

visibility during a single visibility

Within this pitch range, the observatory can access targets about the Sun line resulting in the instantaneous field-of-regard shown in this figure. The field-of-regard covers about 40% of the sky. As JWST and the Earth orbit the sun, the field-of-regard moves across the sky. This limits when a particular target can be observed and for how long. Targets in the ecliptic plane are accessible for approximately 53 continuous days twice a year. Near the ecliptic poles, targets may be viewed for over half a year; those within 5 degrees of the poles are viewable throughout the year.

visibility during a single visibility

The figure above shows the target visibility during a single visibility window in red and the total yearly visibility in blue. Targets within 45 degrees of the ecliptic have two visibility windows per year with a minimum duration of 53 days each on the ecliptic. The visibility window durations increase with increasing latitude. At an ecliptic latitude of 45 degrees or higher the two visibility windows merge and the total visibility continues to increase up until a latitude of 85 degrees where it reaches the full year. These areas within five degrees of the ecliptic poles define the continuous viewing zone.

The restricted viewing opportunities also translate into restricted orientations of the focal plane with respect to the celestial sphere. For Hubble, astronomers can specify nearly any orientation of the cameras and spectrographic slits, although such details will restrict when their observations can execute. JWST observers will have less choice, except for targets near the ecliptic poles. At zero ecliptic latitude, the orientation range is small (10 degrees) for both of the 53 day viewing periods during the year. For intermediate latitudes (say, the galactic poles), the range increases to approximately 90 degrees if the ranges for the two viewing periods are combined.