Two Centuries of Image Quality Estimates from Double-Star Data

Abstract

ABSTRACT. 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″ in 1780 to 0.5″ 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.

Why Was the Companion of Sirius Not Seen Prior to 1862?

“…Given the low elevation of Sirius at upper transit from Europe and North America, a seeing disc 50 of 2-3″ would typically be had at nineteenth-century observatories because of their low altitudes and thermal imbalance in their environment. Signal to noise calculations 51 show that, to have sufficient contrast, the image of the Sirius companion could then only be detected in significantly better, hence quite rare, conditions even at its apoapsis. The serendipitous sighting of the companion by the Clarks in 1862 and Rutherfurd's measurements, among others, must have been facilitated by such conditions.…”