Sky coverage

Observations at the NOT in July 2001 indicated that the isoplanatic angle for observations using the Lucky Exposures method was $30$ $as$. This represents a substantial improvement over the typical values expected for I-band adaptive optics. Images with FWHM as small as $130$ $as$ were obtained at this separation from a reference star. Observations in May 2000 were suggestive of a smaller isoplanatic angle at that time. Future analysis of data taken at the NOT in 2003 may give a better indication of the typical range of isoplanatic angles at the NOT site.

Observations of M13 in July 2001 indicate that good image quality can be obtained using reference stars as faint as $I=15.9$. Observations analysed using faint reference stars from June 2003 had image quality which was consistent with that obtained in July 2001 using reference stars of similar magnitude.

The limiting magnitude and isoplanatic angle, combined with models for galactic star counts at I-band indicate that approximately $25\%$ of the night sky should be within range of a suitable reference star for the Lucky Exposures method.