2008PASP..120..465R

We examine the historical evolution of the image quality delivered by astronomical telescopes since the days of William Herschel. This is done by analyzing double-star discovery data from nine celebrated observers who used 12 telescopes at 11 sites. Complementary information is derived from errors in the separation measurements of the Sirius system between 1862 and 1960. The best delivered image quality steadily improved with time, from a full width at half maximum intensity of 5{Prime} in 1780 to 0.5{Prime} in 1970. This appears to have resulted largely from increasingly better optics. Selection effects and limitations that have affected these double-star surveys are briefly discussed, as is the driving role toward telescope improvement played historically by the study of double stars.