As part of the overall programme concerning high resolution imaging, a
wavefront sensor has been designed and constructed at MRAO (St.Jacques
et al.,
submitted) to provide rapid and continuous measurements of the atmospheric
turbulence at the WHT as part of the Joint Observatories Seeing Evaluation
(JOSE) programme. The instrument uses an 
lenslet array to provide an
array of stellar images whose instantaneous positions give a measure of the
wavefront slope across 
cm subapertures of the WHT pupil. A
fast-readout CCD is used to give frame rates up to 866Hz, easily fast enough
for measurements of all significant atmospheric turbulence. Arrangements for
precise and very rapid alignment and a simple user interface make it possible
for regular seeing measurements to be made by telescope operators in less than
30 min of override time. Results so far confirm that there are no
significant telescope problems, that the turbulence corresponds very closely to
the theoretical Kolmogorov models and that an adaptive optic system for 2.2
microns would need correction update rates of no more than 100Hz to achieve a
Strehl ratio as high as 0.6 in 0.7 arcsec seeing.