  Astronomers working with faint targets will benefit greatly from improved
image quality on current and planned ground-based telescopes. At present, most
adaptive optic systems are targeted at the highest resolution with bright guide
stars. We demonstrate a significantly new approach to measuring low-order
wavefront errors by using a pupil-plane curvature wavefront sensor design. By
making low order wavefront corrections we can deliver significant improvements
in image resolution in the visible on telescopes in the 2.5m to 8.2m range on
good astronomical sites. As a minimum the angular resolution will be improved
by a factor of 2.5 to 3 under any reasonable conditions and, with further
correction and image selection, even sharper images may be obtained routinely.
We re-examine many of the assumptions about what may be achieved with faint
reference stars to achieve this performance. We show how our new design of
curvature wavefront sensor combined with wavefront fitting routines based on
radon transforms allow this performance to be achieved routinely. Simulations
over a wide range of conditions match the performance already achieved in runs
with earlier versions of the hardware described. Reference stars significantly
fainter than I 17m may be used routinely to produce images with a near
diffraction limited core and halo much smaller than that delivered by natural
seeing.
