  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.
