Composite pictures of selected recentered short exposures to improve the angular resolution

The gain in resolution achieved by selecting frames in a sequence of many short exposures is calculated. The angular resolution as a function of the selection is given. Published values are in good agreement with measurements obtained by several observers. A gain in resolution by a factor 2 can be achieved by selecting 10% of short exposures then adding, each being recentred, up to D/r0 approximately=15. At optical wavelengths the technique should be effective when the seeing is good, but not exceptional and with telescopes of one meter diameter.