<title>Design and support of the 1.7-m f/5 secondary mirror for the MMT conversion</title>

Abstract

The f/5 focus of the converted Multiple Mirror Telescope (MMT) has been designed for optical fiber spectroscopy over a 1 degree(s) diameter field as well as superlative imaging over a 0.5 degree(s) diameter field. The secondary mirror required for this purpose is nearly as large as one of the existing MMT primary mirrors (1.7 m). To attain the specified performance of the telescope support structure, this secondary must be made quite light (approximately 300 kg), without excessively compromising its stiffness. Our design calls for removing excess weight by machining hexagonal cells into a near zero-expansion materials such as ULE or Zerodur. The completed blank will be approximately 20 cm thick at the center, tapering to approximately 13 cm at the edge. The support of a secondary of this size must be considered as part of the blank design. We describe two possible axial support techniques: a vacuum support or a multiple actuator support similar to that adopted for the primary mirror of the converted MMT. Tangent flexures at the edge of the blank appear to be the most attractive radial support option.