Guide Stars for Webb

Sherie Holfeltz,

Diffraction-limited telescopes in space demand high pointing stability to ensure sharp images. Great effort and expense was paid to ensure the quality of Hubble images—with outstanding success, due to the low jitter when Hubble is locked onto guide stars. Now, as we plan operations for the James Webb Space Telescope, we are drawing on the Hubble experience to ensure the efficient selection of guide stars to stabilize Webb’s pointing.

Hubble has three fine guidance sensors (FGSs). Their fields of view (FOVs) arc around the apertures of the other science instruments. Hubble observes pairs of guide stars simultaneously, with each star in the FOV of a different FGS. The typically long FGS-to-FGS lever arms allow Hubble’s pointing control system to tightly control the telescope’s roll around the line of sight, as well as the transverse pitch and yaw.

On Webb, there are two FGS FOVs, which are more centrally located than on Hubble (see Figure 1). The short FGS-to-FGS lever arm would not be effective in controlling the roll of the telescope, and therefore Webb uses separately mounted star trackers to control roll. The Webb attitude control system uses the FGSs in closed-loop mode to control the telescope’s pitch and yaw. Thus, only one guide star is needed for the fine guiding of Webb.

The Webb FGS is a dual-channel, near-infrared camera (like NIRCam) with two adjacent 2.4′ × 2.4′ FOVs. Each focal plane array is a 2048 × 2048 mercury-cadmium-telluride sensor-chip assembly. The central 2040 × 2040 pixels are sensitive to light, while the four outermost rows and columns are reference pixels, for bias measurements. The Webb FGS has neither a shutter nor a filter wheel.

The Webb FGS operates with a pass band from ~0.6 to 5 microns. The camera can reach 58 µJy at 1.25 µm (J = 18.6) in 63 milliseconds using in 8 × 8 pixel sub-arrays. This combination of sky coverage and sensitivity ensures that the FGS will be able to meet the requirement that it have a 95% probability of finding a useful guide star in the range 12.1 ≤ J ≤ 18.6 anywhere on the sky, even at sparsely populated, high galactic latitudes.