  It is widely believed that adaptive optics only has a role in correcting
turbulent wavefronts on large telescopes using very bright reference stars.
Unfortunately these are very scarce and many astronomical targets require
wavefront correction to work over much of the sky. We therefore need to be able
to use very much fainter reference objects. Laser guide stars in principle can
allow 0.1 arcsecond resolution but have a number of severe technical problems
that limit their application. Our aims are to provide imaging at even higher
resolution than Hubble. Lucky Imaging completely eliminates the tip-tilt errors
in astronomical wavefront detection. Most of the power that remains is in low
order, large scale structures. These may be detected with high sensitivity
using photon-counting EMCCD detectors working at high frame rate, up to ~100Hz.
With a new design of curvature wavefront sensor, wavefront errors may be
measured and corrected to give near diffraction-limited performance on large
groundbased telescopes in the visible. Reference stars (and reference compact
galaxies) fainter than I~17.5 mag may be used routinely. This paper will
describe how these work, what detector and other hardware is needed and what
software should be used to measure the wavefront errors and drive deformable
mirror hardware. The software techniques that are used are those routinely
applied for MRI and CT imaging. They are fast and relatively easy to implement.
The net effect is that imaging systems can be constructed that improve
substantially over Hubble resolution from the ground for a relatively modest
sum of money.
