  We present the novel design, laboratory verification, and on-sky performance
of our advanced triplet atmospheric dispersion corrector (ADC), an important
component of the Magellan Adaptive Optics system (MagAO), which recently
achieved first light in December 2012. High-precision broadband (0.5-1.0
microns) atmospheric dispersion correction at visible wavelengths is essential
both for wavefront sensing (WFS) on fainter guide stars, and for performing
visible AO science using our VisAO science camera. At 2 airmasses (60 degrees
from zenith) and over the waveband 500-1000 nm, our triplet design produces a
57% improvement in geometric rms spot size, a 33% improvement in encircled
energy at 20 arcsec radius, and a 62% improvement in Strehl ratio when compared
to a conventional doublet design. This triplet design has been fabricated,
tested in the lab, and integrated into the MagAO WFS and the VisAO science
camera. We present on-sky results of the ADC in operation with the MagAO
system. We also present a zero-beam-deviation triplet ADC design, which will be
important to future AO systems that require precise alignment of the optical
axis over a large range of airmasses in addition to diffraction-limited
broadband dispersion correction.
