Visual Double Star Measurements with Equatorial and Alt-Azimuth Telescopes

Professional astronomers have carried out visual double star measurements for over 200 years. These scientists measured the separation between double stars in arc seconds and the position angle in degrees that defined the orientation of pairs with respect to celestial north. There are two types of double stars: binary stars and optical doubles. A binary star consists of two stars revolving around a common center of mass. Over time, the orbital motion of each star can create an elliptical change in the observed separation and position angle if the pair proves to be binary in nature. Conversely, if the two stars are an optical pair, meaning they do not orbit around a common center of mass, there will be a linear change in separation or position angle over time.Today’s astronomers continue to observe these changes with fairly simple equipment. The typical setup for visual observations includes an equatorial mounted telescope with tracking motors, a laser etched astrometric eyepiece, and an inexpensive digital stopwatch that reads to the nearest 0.01 seconds. Not only is this procedure used by professional astronomers but it can be performed accurately by amateur astronomers and students as well. In fact, this type of research is particularly well suited for students for several reasons, including experience collecting and analyzing data, working as a team, and opportunity to promptly publish scientific papers (Johnson 2008). Figure 1 (next page) shows a student observing the double star Alberio.