Field-of-Regard and Sky Coverage

This schematic figure shows the sunshield architecture for the JWST, with the direction of the Sun (and approximately Earth and Moon) indicated by the arrow. The sunshield permits the observatory to pitch toward and away from the Sun by approximately 45 degrees, while still keeping the telescope in the shade.

The figure above shows the visibility during a single visibility window in red and the total visibility in blue. Objects at an ecliptic latitude below 45 degrees have two visibility windows with a minimum duration of 50 days each on the ecliptic and increasing with increasing latitude. At a latitude of 45 degrees or higher the two visibility window merge and the total visibility continues to increase up until a latitude of 85 degrees where it reaches the full year. The areas within five degrees of he ecliptic poles define the continuous viewing zone and are visible all year around.

The observatory can roll freely about the Sun line resulting in the instantaneous field-of-regard shown in this figure. The direction of the Sun is indicated by the Solar symbol at the bottom of the sphere. The field-of-regard covers almost 50% of the sky. Targets in the ecliptic plane become accessible for approximately 69 continuous days twice a year. Near the ecliptic poles, targets may be viewed for half a year; those within 5 degrees of the poles are viewable throughout the year.

The restricted viewing opportunities also translate into restricted orientations of the focal plane with respect to the celestial sphere. For Hubble, astronomers can specify any orientation of the cameras and spectrographic slits, although such details might delay their observations. 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 69 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.