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Last updated: 8 May 2011
Subject:	ETX 125 Observing Doubles
Sent:	Saturday, May 7, 2011 10:48:15
From:	John Farrant (
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

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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