Segmented mirror

Figure 9. Dense mirror segmentation with an annular pattern...

Mirror segments (like Keck) will concentrate the diffracted energy into lines perpendicular to the edges, thus producing a diffraction pattern that is brighter or darker in some places than that of a circular aperture. 

Low cost and fabrication risk for long-lead items (primary mirror segments) ... 

Chanan, G., Troy, M., Ohara, C., 2000, "Phasing the primary mirror segments of the Keck telescopes a comparison of different techniques", SPIE 4003, 188... 

Keywords optical testing, mirror segments, aspheric mirrors... 

Mauna Kea in Hawaii. A total of 36 hexagonal Zerodur mirror segments with a thickness of 75 mm form the primary mirror of 10 m diameter (see Fig. 4.63). 

A sophisticated computer-controlled support system keeps the mirror segments in the desired position. The finishing of the off-axis aspheric segments is performed by stressed mirror polishing, a newly developed process for which the initially round meniscus-shaped blanks of 1.9 m diameter are mechanically deformed in such a way that a desired aspheric surface results after the stress is relieved. 

Fig. 4.54. Production of the XEUS X-ray telescope (a) Zerodur mandrel, (b) electro-formed mirror segments, (c) telescope segment, (d) model of XEUS...

Another possibility for optimizing the diameter-to-thickness ratio of a substrate for a mirror of large diameter is being realized by the California Association for Research in Astronomy (CARA) with the Keck Telescope on Maima Kea in Hawaii. A total of 36 hexagonal Zerodur mirror segments with a thickness of 75 mm form the primary mirror of 10 m diameter (see Fig. 4.68). 

The process uses the implantation of inert gases in the mirror surface to create a thin layer of compressively stressed material that acts to flatten the mirror segment. This approach is similar to the stress reduction that occurs due to phosphorous dopant diffusion from the PSG layers during the PolyMUMPS high-temperature anneals, as described in Section 1.2.1 on surface micromachining. In contrast to the anneal step, where dopants diffuse in from both sides, the ion bombardment process only occurs on the top side of the mirror surface. 

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