Spin-casting of a 6.5-m Honeycomb Sandwich Mirror for the MMT Conversion

1. Introduction The Steward Observatory Mirror Lab has recently completed the spin-casting of the largest structured mirror ever made, a 6.5-m f/1.25 honeycomb sandwich mirror of borosilicate glass. This mirror will be installed in the Multiple Mirror Telescope operated by the University of Arizona and the Smithsonian Institution, replacing the six 1.8-m mirrors that currently make up the MMT and thereby doubling its collecting area. The casting of this mirror represents the final step in development of technology to produce 8-meter-class lightweight mirror blanks. The honeycomb sandwich structure provides improved mechanical and thermal performance compared to solid blanks. The sandwich is ten times stiffer than the same mass of glass in a solid meniscus, leading to lower deformation under wind forces and lighter telescope structures. Ventilation of the honeycomb with air at ambient temperature reduces the mirror’s thermal time constant to less than an hour, allowing the mirror to accurately track the changing ambient temperature and reduce mirror seeing. Most of the large mirrors to be cast at the Mirror Lab will be extremely fast, in the range f/1.14 to f/1.25. For larger telescopes there is added incentive to keep focal lengths short, in order to minimize enclosure costs and wind-induced motion of the secondary mirror. The MMT Conversion has a particular need for short focal length: the new telescope must fit in the existing enclosure, designed for six 1.8-m f/2.7 mirrors, with only minor modifications.