Location of the elevation axis in a large optical telescope

Proposed designs for the next generation of large optical telescopes favor a tripod or quadrupod secondary support, and a primary supported from the back, but it is not yet clear whether the elevation axis should be in front of the primary or behind it. A study is described of the effect of elevation-axis location on key performance parameters (fundamental frequency, blockage, and wind-induced secondary decenter) for a 30-m Cassegrain telescope with a mount configuration that is typical of the new designs. For a fast (e.g., f/1) primary, the best location for the elevation axis is behind the primary. The penalty for moving the elevation axis in front of the primary is roughly a 40% decrease in fundamental frequency and a corresponding reduction in the control bandwidth for pointing and optical alignment.