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Proceedings Paper

Status of the secondary mirrors (M2) for the Gemini 8-m telescopes
Author(s): Ernst-Dieter Knohl; Armin Schoeppach; Michael A. Pickering
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Paper Abstract

The 1-m diameter lightweight secondary mirrors (M2) for the Gemini 8-m telescopes will be the largest CVD-SiC mirrors ever produced. The design and manufacture of these mirrors is a very challenging task. In this paper we will discuss the mirror design, structural and mechanical analysis, and the CVD manufacturing process used to produce the mirror blanks. The lightweight design consist of a thin faceplate (4-mm) and triangular backstructure cells with ribs of varying heights. The main drivers in the design were weight (40 kg) and manufacturing limitations imposed on the backstructure cells and mirror mounts. Finite element modeling predicts that the mirror design will meet all of the Gemini M2 requirements for weight, mechanical integrity, resonances, and optical performance. Special design considerations were necessary to avoid stress concentration in the mounting areas and to meet the requirement that the mirror survive an 8-g earthquake. The highest risk step in the mirror blank manufacturing process is the near-net-shape CVD deposition of the thin, curved faceplate. Special tooling and procedures had to be developed to produce faceplates free of fractures, cracks, and stress during the cool-down from deposition temperature (1350 C) to room temperature. Due to time delay with the CVD manufacturing process in the meantime a backup solution from Zerodur has been started. This mirror is now in the advanced polishing process. Because the design of both mirrors is very similar an excellent comparison of both solutions is possible.

Paper Details

Date Published: 25 August 1998
PDF: 10 pages
Proc. SPIE 3352, Advanced Technology Optical/IR Telescopes VI, (25 August 1998); doi: 10.1117/12.319292
Show Author Affiliations
Ernst-Dieter Knohl, Carl Zeiss (Germany)
Armin Schoeppach, Carl Zeiss (Germany)
Michael A. Pickering, Morton Advanced Materials (United States)

Published in SPIE Proceedings Vol. 3352:
Advanced Technology Optical/IR Telescopes VI
Larry M. Stepp, Editor(s)

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