An investigation of lucky imaging techniques

We present an empirical analysis of the effectiveness of frame selection (also known as Lucky Imaging) techniques for high resolution imaging. A high-speed image recording system has been used to observe a number of bright stars. The observations were made over a wide range of values of D/r0 and exposure time. The improvement in Strehl ratio of the stellar images due to aligning frames and selecting the best frames was evaluated as a function of these parameters. We find that improvement in Strehl ratio by factors of 4 to 6 can be achieved over a range of D/r0 from 3 to 12, with a slight peak at D/r0 ~ 7. The best Strehl improvement is achieved with exposure times of 10 ms or less but significant improvement is still obtained at exposure times as long as 640 ms. Our results are consistent with previous investigations but cover a much wider range of parameter space. We show that Strehl ratios of >0.7 can be achieved in appropiate conditions whereas previous studies have generally shown maximum Strehl ratios of ~0.3. The results are in reasonable agreement with the simulations of Baldwin et al. (2008).