phy1020.DVI

A relatively new trend in large-mirror technology is to create large astronomical mirrors not as a single
large mirror, but as a collection of segments (typically hexagons) that are fitted together to form one large
mirror. The smaller mirrors are easier to deal with (although they must be formed to complex asymmetrical
shapes), and if one breaks, it can be replaced much more easily than replacing an entire large mirror. A dis-
advantage is that the segments are subjected to various deformations due to temperature changes, mechanical
stress, etc. that can easily place the segments out of alignment with each other. Keeping all the segments
properly aligned requires that each segment’s position be controlled by a computer, in a system calledactive
optics.
Several ground-based segmented-mirror telescopes have already been built, and the upcoming James
Webb Space Telescope will incorporate segmented mirrors.