For NGST, a technology called image-based wavefront sensing control is being considered to provide a feedback loop to keep mirror elements aligned. This technique electronically infers distortions that are occurring on the mirror surface by analyzing the focal plane of light hitting the telescope. The information is used to introduce a correction to the mirror surface. It's done iteratively, and it takes a lot of computing power, said Geithner. But it's a proven technique that was used to identify the problem with the main mirror of the Hubble, he added.
Vibration is a special concern.
Cryogenic temperatures exacerbate the problem because structures become very stiff, and damping is greatly diminished at very cold temperatures, he explained. Trying to make the whole spacecraft very rigid may make its launch and deployment in space impractical, he said. The attitude of the craft will be controlled by big momentum wheels. To take the jitter out of the optical path, a small mirror will be located in the optical train of the telescope. The mirror will steer the beam with a high degree of precision before the light reaches the detectors, to compensate for any vibration.