Wavefront Sensing and Control on the James Webb Space Telescope

To align the telescope, each primary mirror segment is mounted on three bipods (a hexapod) to form a kinematic attachment to the backplane. Six actuators with both coarse- and fine-positioning capability adjust the lengths of the hexapod’s legs. This arrangement permits six degrees of freedom to be adjusted independently: x- and y-position, piston, tip, tilt, and clocking. The secondary mirror also has six actuators, permitting the same kind of control. For each of the primary mirror segments, an additional (seventh) actuator controls the radius of curvature.

In total, there are 132 controllable degrees of freedom in aligning the OTE. The process by which these degrees are used to optimize the Webb image quality is called wavefront sensing and control (WFS&C). Unlike the case for ground-based adaptive optics systems, WFS&C is not a real-time process that runs autonomously on the telescope. For ground-based observations, the short characteristic timescales of atmospheric turbulence dictate rapid sensing and correction. By contrast, Webb operates at the second Sun-Earth Lagrange point, L2, in an environment with no atmosphere and little gravity. The relevant timescales for WFS&C are dictated by slow thermal drifts, which occur on day-to-week timescales. Therefore, the cheapest and most reliable approach is to obtain data through the regular, pre-planned observing methods, downlink of the data, analysis on the ground, and uplink to the telescope of any commands to move actuators.

Immediately after OTE deployment, the telescope point-spread function (PSF) consists of 18 separate, out-of-focus images, one from each primary mirror segment. The first OTE commissioning steps use a bright star to determine and calibrate the telescope line of sight, and to locate and identify the image of each individual segment. The mirror segments can then be tilted to align the 18 separate images into a single image. The wavefront superposition thus obtained is incoherent. As a result, the PSF at this stage has a width typical of a single (1.32 m flat-to-flat) mirror segment.