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Aims. We investigate the properties of astronomical images made by combining the best images selected from a sequence of shortexposure frames (Lucky Imaging); we assess the match between modelling and observation and discover what variations in seeing occur on very short timescales. Methods. Numerical simulations of a random phase-changing screen passing across a telescope aperture with ideal optics are used to determine the expected point spread function and isoplanatic properties for a range of seeing conditions for comparison with observations. Results. All the model images comprise a diffraction-limited core with Strehl ratios from 0.05–0.5 and an underlying broad disk. The isoplanatic patch sizes are large and coherence times long. The observations are a close match to the models in most respects. Large variations in seeing occur on temporal scales as short as 0.2 s and spatial scales as small as 1 m.

The point spread function in Lucky Imaging and variations in seeing on short timescales