Some Double Star Details

The list on the following page is my compilation of the prettiest doubles (and multiples) visible from the Northern and/or Southern Hemispheres when Orion is riding high in the sky. The list has a couple of novel columns: color difference and optimum magnification. The color-difference rating is the number of letters between stars on the well-known OBAFGKM scale of spectral classification. For example, Almach's components are B and K, so Almach rates a 4. (In "More Pretty Double Stars" I present 54 double or multiple stars visible during summer in the Northern Hemisphere.)

I've found that doubles look their best at a magnification that is approximately 750 divided by the separation in arcseconds. For example, Eta (η) Cassiopeiae's components are separated by 12.5" — 750/12.5 yields an optimum magnification of 60x. Similarly, since the unaided eye can resolve stars as close as about 300", the minimum magnification for a split is 300 divided by the separation. In the case of Eta, that would be around 24x (300/12.5). Note that some of the stars on the list have optimum magnifications that fall within the range of binoculars.

It has been stated that the maximum magnification is about 50 times the aperture in inches. But you'll often benefit by going higher when viewing tight doubles. I begin by finding the star with my scope set at low power (which is also the maximum field of view). Then I work up to the optimum magnification.

The theoretical resolving limit of a high-quality telescope is about 5.5 divided by the telescope's aperture, in inches. But in all my years of double-star observing, I've never achieved this limit. So don't be disappointed if you don't either. Atmospheric turbulence will also play its part in reducing your ability to resolve tight doubles — save the closest pairs for nights of steady seeing.