SIMBAD references

We describe a new wavefront sensor based on the efficient Shack-Hartmann quad cell system. The key improvement to existing designs is a modified lenslet array. The traditional refractive lenslet array is replaced with a segmented reflective system that allows individual control of each subaperture. This system has produced diffraction limited slope correction of the six 1.83 m mirrors of the MMT using a 9.9 V magnitudeguide star and significant image improvement on guide stars as faint as 11.3 V magnitude, limited by the readout noise of the CCD quad cell detector. Recent experiements with an improved detector diffraction limited imaging on guide stars as faint as 15th magnitude. This optimized wavefront sensor, equally applicable to filled aperture telescopes, promises to extend the amount of sky coverage available for adaptive correction in the near IR.