Fabrication of large secondary mirrors for astronomical telescopes,”

ABSTRACT A new class of telescope is being built with primary mirrors as large as 8.4 meters in diameter and as fast as f/i . Fabricating the secondary mirrors for these telescopes has presented tough challenges because of their large sizes, up to 1.7 meter diameter; their aspheric departure of more than 300 microns; the required figure accuracy of a few tens of nanometers; and the fact that they are typically convex and difficult to measure. We have developed tools and techniques to meet these demands to produce secondary mirrors efficiently and accurately. A dedicated facility was constructed in the Mirror Lab that integrates a 1 .8-m stressed-lap polishing machine with interferometric and mechanical measuring systems. This paper presents data from a 1 .15-rn secondary that was finished in our shop, and from two other large mirrors that are currently being fabricated