
Subject: ETX 125 Observing Doubles Sent: Saturday, May 7, 2011 10:48:15 From: John Farrant (johnfarrant@gmail.com) With light pollution becoming an increasing problem here in Eastern Spain, I have taken a bigger interest in observing double stars up to the theoretical limit of my ETX 125. As you know, seperation and the relative magnitude of the primary and secondary components present the main two obstacles to this type of astronomy. There are a couple of Windows programs which assist in determining whether a particular double is easy or difficult. The January 2002 edition of Sky & Telescope contains further information on the subject. What I wanted was a far simpler method of estimating the difficulty of enjoying unequal double stars which didn't involve lengthy calculations or inputting data into a computer. So I examined my log of doubles that I had observed or missed over the past five years. Pulling out my calculator I began playing about with the figures. I came up with a formulae  not very precise  but useable none the less. To find the difficulty or otherwise of a particular double, you simply divide the doubles separation by the difference between the magnitudes of its primary and secondary components. If the answer is less than 1 the double is extremely difficult. If the result is 1 point something, this double is difficult but duable. A figure greater than 1 means that this double is easy. For example, Castor with its twin gives us a figure of 2.83  therefore an easy double. Izar produces an answer less than 1  this double is difficult. The formulae is preety sloppy and doesn't work in all cases, but it gives a rough guide and can be performed at the telescope. Thank you Mike for your site. John Farrant
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