CAOSP abstracts, Volume: 43, No.: 3, year: 2014

Abstract: Lucky imaging is a competitive alternative for high-resolution imaging with a possibility of applications on small telescopes. The advantage of this technique is that small telescopes are not time expensive, therefore long observing runs, lasting for several hours or nights can be planned, enabling for time-resolved observation of sources in crowded fields. In the ideal case, a lucky image is diffraction-limited, while the actual resolution (in the order of several 0.1 arc seconds) is still close to the diffraction-limit of small telescopes. However, occulted optical systems, such as Cassegrain or RCC (Ritchey-Chretien-Coude) perform a poor imaging near the diffraction limit, because the secondary mirror significantly decreases the contrast. By basic optical calculations one can conclude that a typical Cassegrain-system has similar PSF to that of an unocculted telescope with a 40% less aperture, while the Strehl ratio is decreased to about 30% simply due to the secondary mirror. Since the profile is widened and the precious signal decreases significantly, a well-constructed unocculted telescope can perform at least as well as a Cassegrain system which is twice as large. My conclusion is therefore that "dropping out the secondary mirror makes the aperture double" — at least in Lucky Imaging applications.