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 68 degrees, while still keeping the telescope in the shade.

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.