Ernst-Dieter Knohl

VLT primary support system

A large, flexible mirror like the VLT Primary needs several sophisticated support systems, one for each operation during its lifetime. Typical mirror specifications, design loads and design requirements are given. For astronomical use ZEISS proposes a hydraulical passive support system for all 6 DOF of the mirror. For active correction an actuator with high dynamic bandwidth is connected to the axial passive support. Design features are discussed. Particularly the dynamic behavior of the support system is studied in an ACSL simulation. Active damping is foreseen as backup feature.

Status of the secondary mirrors (M2) for the Gemini 8-m telescopes

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.

Segmented mirrors for DGT

The ArbeitsGemeinschaft Deutsches GrossTeleskop (AG-DG1) is concerned with the realization of a telescope in the 12 m class and has charged Carl Zeiss with a feasibility study concerning the optical figuring of large mirror blanks. The state of the art technology of spin-cast ZERODIJR mirror substrates at Schott Glaswerke allows fabrication of thin meniscus blanks up to 8.2 m in diameter. Therefore the primary is to be assembled from several single segments and should be optimized for the segmentation and the polishing process. Two methods of segmenting the primary mirror are considered: Combining either four 90-degree segments or a central 8 m monolith surrounded with 12 segments of 3Odegree each. Several methods of figuring and testing of out-of-axis segments are discussed. Techniques for a stiff connection of single segments are examined. Basic considerations of the axial support system of nonrotational segments are presented. The printthrough effect on the mirrors surface is calculated with varying pad distances and with varying geometry of the support points. The Membran Tool Processing (MTP) developed by Carl Zeiss will be used for polishing the mirror blank. With the active controlled strip tool large areas can be asphencal shaped and polished in a much shorter time than with the conventional technology.