Lucky Exposures: Diffraction limited astronomical imaging through the atmosphere

Abstract

Through selection of short exposure images we achieve diffraction-limited I-band imaging from well-figured ground-based telescopes as large as 2.5 m diameter. The faint limiting magnitude and large isoplanatic patch size for the Lucky Exposures technique at the Nordic Optical Telescope means that 25% of the night sky is within range of a suitable reference star for I-band imaging. Typically the 1%--10% of exposures with the highest Strehl ratios are selected. When these exposures are shifted and added together, field stars in the resulting images have Strehl ratios as high as 0.26 and full width at half maximum flux (FWHM) as small as 90 milliarcseconds. Within the selected exposures the isoplanatic patch is found to be 60 arcseconds in diameter at 810 nm wavelength. Images within globular clusters and of multiple stars from the Nordic Optical Telescope using reference stars as faint as I~16 are presented. A new generation of CCDs (E2V L3Vision CCDs) were used in these observations, allowing extremely low noise high frame-rate imaging with both fine pixel sampling and a relatively wide field of view. The theoretical performance of these CCDs is compared with the experimental results obtained.