NASA's James Webb Space Telescope

Pointing and Guiding

JWST's observations are constrained by the necessity to keep the observatory constantly in the shadow of the sunshield. The left panel shows the accessible field of regard of the observatory. At any given time it can view about 40% of the sky. As JWST and the Earth orbit the Sun, the field-of-regard moves across the sky. The right-hand panel shows the approximate overall sky coverage accumulated over a year. There is a continuous viewing zone within 5 degrees of each ecliptic pole. Thirty percent of the sky is viewable for at least 197 days per year, and all of the sky will have at least 51 days of continuous visibility each year. (Left figure from Gardner et al. Space Science Reviews (2006) 123: 485–606.)

Field of Regard

JWST's observations are constrained by the necessity to keep the observatory constantly in the shadow of the sunshield. 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. Within this pitch range, the observatory can access targets about the Sun line resulting in the instantaneous field-of-regard shown in figure above. 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.

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.

The Attitude Control System

The JWST attitude control system (ACS) uses a variety of equipment and measurements to achieve the exquisite pointing accuracy and stability needed.  JWST uses six reaction wheels to rotate the observatory. These are flywheels that can trade angular momentum with the observatory, allowing it to rotate without using fuel. A typical slew between astronomical targets will take 20 to 30 minutes.  Over time, reaction wheels may build up stored momentum that needs to be cancelled. This will require firing thrusters (and using fuel). Hence the overall scheduling of the observatory will involve careful planning to try to balance the fuel use while executing an efficient sequence of observations.

The reaction wheels work in combination with three star trackers and six gyroscopes that provide feedback on where the observatory is pointing and how fast it is turning. This enables coarse pointing sufficient to keep the solar array pointed at the Sun and the high-gain antenna pointed at the Earth. To take images and spectra of astronomical targets, finer pointing is needed. This information comes from the Fine Guidance Sensor (FGS). The FGS is an infrared camera with two adjacent fields of view, each spanning 2.4' x 2.4'. It operates in a broad wavelength band from 0.6-5 μm, and can guide on stars as faint as JAB=19.5 (58μJy). This will give JWST a 95% probability of finding suitably-bright guidestars at any point in the sky. The FGS on-board software locates stars in the image, identifies the pattern from an uplinked catalog, and sends a signal to the attitude control system to allow it to place the target precisely where it belongs for the observations. Once in fine-guidance mode, the FGS will report an error signal 16 times per second with an uncertainty of less than 4 milliarcsec on each axis. For additional information about the FGS, please refer to JDox

JWST will be capable of relative pointing offsets with an accuracy of 5 milliarcsec rms, which will enable subpixel dithering and coronagraphic acquisition. Absolute astrometric accuracy will be limited to 1 arcsec rms by the accuracy of the guide star catalog. JWST will have the capability to observe moving targets with apparent rates up to 0.030 arcsec/second.