Active Optics on the Baade 6.5-m (Magellan I) Telescope
  The Magellan active optics system has been operating continuously on the Baade 6.5-m since the start of science operations in February 2001. The active optical elements include the primary mirror, with 104 actuators, and the secondary mirror, with 5 positional degrees of freedom. Shack-Hartmann (SH) wavefront sensors are an integral part of the dual probe guiders. The probes function interchangeably, with either probe capable of guiding or wavefront sensing. In the course of most routine observing stars brighter than 17th magnitude are used to apply corrections once or twice per minute. The rms radius determined from roughly 250 SH spots typically ranges between 0.05" and 0.10". The spot pattern is analyzed in terms of a mixture of 3 Zernike polynomials (used to correct the secondary focus and decollimation) and 12 bending modes of the primary mirror (used to compensate for residual thermal and gravitational distortions). Zernike focus and the lowest order circularly symmetric bending mode, known affectionately as the "conemode," are sufficiently non-degenerate that they can be solved for and corrected separately. 
Paul L. Schechter (1), Greg Burley (2), Charles L. Hull (2), Matt   Johns (2), Buddy Martin (3), Skip Schaller (4), Stephen ...