The Discovery Channel Telescope (DCT) is a 4.3-meter telescope with a thin meniscus primary mirror (M1) and a
honeycomb secondary mirror (M2). The optical design is an f/6.1 Ritchey-Chrétien (RC) with an unvignetted 0.5°
Field of View (FoV) at the Cassegrain focus. We describe the design, implementation and performance of the DCT
active optics system (AOS). The DCT AOS maintains collimation and controls the figure of the mirror to provide
seeing-limited images across the focal plane. To minimize observing overhead, rapid settling times are achieved
using a combination of feed-forward and low-bandwidth feedback control using a wavefront sensing system.
In 2011, we mounted a Shack-Hartmann wavefront sensor at the prime focus of M1, the Prime Focus Test Assembly
(PFTA), to test the AOS with the wavefront sensor, and the feedback loop. The incoming wavefront is decomposed
using Zernike polynomials, and the mirror figure is corrected with a set of bending modes. Components of the
system that we tested and tuned included the Zernike to Bending Mode transformations. We also started open-loop
feed-forward coefficients determination.
In early 2012, the PFTA was replaced by M2, and the wavefront sensor moved to its normal location on the
Cassegrain instrument assembly. We present early open loop wavefront test results with the full optical system and
instrument cube, along with refinements to the overall control loop operating at RC Cassegrain focus.