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.
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