  Since its beginnings, diffraction-limited ground-based adaptive optics (AO)
imaging has been limited to wavelengths in the near IR ({\lambda} > 1 micron)
and longer. Visible AO ({\lambda} < 1 micron) has proven to be difficult
because shorter wavelengths require wavefront correction on very short spatial
and temporal scales. The pupil must be sampled very finely, which requires
dense actuator spacing and fine wavefront sampling with large dynamic range. In
addition, atmospheric dispersion is much more significant in the visible than
in the near-IR. Imaging over a broad visible band requires a very good
Atmospheric Dispersion Corrector (ADC). Even with these technologies, our AO
simulations using the CAOS code, combined with the optical and site parameters
for the 6.5m Magellan telescope, demonstrate a large temporal variability of
visible ({\lambda}=0.7 micron) Strehl on timescales of 50 ms. Over several
hundred milliseconds, the visible Strehl can be as high at 50% and as low as
10%. Taking advantage of periods of high Strehl requires either the ability to
read out the CCD very fast, thereby introducing significant amounts of
read-noise, or the use of a fast asynchronous shutter that can block the
low-Strehl light. Our Magellan VisAO camera will use an advanced ADC, a
high-speed shutter, and our 585 actuator adaptive secondary to achieve
broadband (0.5-1.0 micron) diffraction limited images on the 6.5m Magellan Clay
telescope in Chile at Las Campanas Observatory. These will be the sharpest and
deepest visible direct images taken to date with a resolution of 17 mas, a
factor of 2.7 better than the diffraction limit of the Hubble Space Telescope.
