Segmented mirror telescopes

Abstract

Constructing the primary mirror of a telescope out of segments, rather thanfrom a monolithic piece of glass, can drastically reduce the mass of themirror and its material costs, thereby making possible the construction ofoptical/infrared telescopes with very large diameters. However, segmentation alsointroduces a host of complications, involving the fabrication of off-axis optics, novel diffraction effects, active segment control systems, in situ aberrationcorrection, and the optical alignment of large numbers of degrees of freedom.Progress in these latter areas over the last 25 years has led to the successfuldevelopment of the two Keck telescopes, as well as several other segmentedtelescopes in the 10-m class. Giant segmented telescopes of similar design, butwith mirror diameters of 30-40 m, are now in the planning stages, withfirst light expected around the end of the decade. Segmentation has alsomade possible the 6.5-m James Webb Space Telescope, which is currentlyunder construction. In this work, the technical issues associated withsegmentation are discussed and reviewed in detail. Particular attention ispaid to the properties of arrays of hexagonal segments (the segmentationpattern of choice for these telescopes), including their diffraction patternsand algorithms for their active control. Optical alignment issues are alsodiscussed.