Minimum-cost 4-m telescope developed at October 1979 Nanjing study of telescope design and construction

A lightweight 4-m telescope with a 6400-kg primary mirror of f/11.5 was developed during a two-week workshop at the Nanjing Astronomical Instruments Factory sponsored by the Purple Mountain Observatory, Academia Sinica, Nanjing, People's Republic of China. A central column supports the secondary mirror, thus eliminating all structures around the periphery of the primary mirror. The altazimuth mounting has the elevation axis behind the primary mirror and cell, requiring a counterweight. The Cassegrain focal position coincides with the elevation axis. A single secondary mirror and appropriate field correctors enable operations at the Harland Epps-Dan Schulte (HEDS), Cassegrain, Nasmyth, and coudé foci. Relay of the Cassegrain beam to the coudé is via an elliptical relay mirror. Cost scaling law considerations indicate that this 4-m design will have a cost comparable with that of a conventional 2.2-m telescope. A discussion of the double-tapered lightweight Cer-Vit-type mirror is included.