Like many branches of astronomy, the observation of double stars can be appreciated at several levels. For those who enjoy the night sky, double stars offer some of the most attractive sights around and they are particularly good in small telescopes where the colours are much more obvious. For a good list of the most impressive pairs, consult the list of 100 best pairs on the Astronomical League Double Star Club website1 or lists of pairs in Sky & Telescope and other journals.2-8
Some observers use double stars as a test object to see what their telescope is capable of in terms of angular resolution. Tables 2.1-2.6 give a range of test pairs for both binoculars and telescopes with a range of apertures from 9 cm to 60 cm.
A few observers find double stars to be so endlessly fascinating that they wish to make useful contributions to the subject. This may be by making measures of p and Q for the binary systems using a micrometer, doing photometry of wider pairs with a CCD camera or calculating orbits from the observed positions. Most of this book will be dedicated to the description of such techniques and opportunities for useful work are discussed further in Chapter 19.
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