Repairing the position angle scale

WJ Macdonnell made a number of studies of double stars, where two stars are so close to each other that they appear as one. Through a series of observations over time, an astronomer can determine whether a double star is an optical double star, with two stars sitting in a single line of sight but at greatly differing distances from Earth, or a physical double star, where two stars are close to each other at roughly the same distance from Earth. These observations involve the measurement of each star’s position angle, or its position relative to the celestial pole, on the telescope’s position angle scale.

On Macdonnell’s telescope, the position angle scale is made up of a thin ring of glass. It was still in its housing when the Museum acquired the instrument, but was broken in several places. The conservation team carefully cemented the broken pieces together, using a glue that would not interfere with the optical qualities of the glass.