At an altitude of 2804 m, the National Solar Observatory 1 at Sacramento Peak is in a prime location for high-resolution imaging of the Sun. The Dunn Solar Telescope (Figure 3.1) at Sacramento Peak was originally dedicated the Vacuum Tower Telescope in 1969 and rededicated in 1998 after Richard Dunn who was the main driving force behind it. Its basic design consists of a large 41 m tower with a 0.76 m window at the top. Sunlight is reflected by a pair of movable 1.1 m mirrors down a 1.2 m diameter, 100 m long evacuated tube. Light reflects off a concave 1.6 m main mirror at the bottom of the telescope, 57 m below ground, and then back up to the observing room. The two mirrors at the top act as a heliostat, therefore compensating for proper motion. The entire set-up hangs from a low-friction tank that contains about 10 tons of mercury and hence it is easy to rotate the 200 tons of tube and instruments to compensate for apparent image rotation.
1 The National Solar Observatory is operated by the Association of Universities for Research in Astronomy under a cooperative agreement with the National Science Foundation