Double stars are interesting, and sometimes beautiful, objects and can be observed under adverse viewing conditions, e.g. in strong moonlight or in areas badly affected by light pollution. When two double star systems are found close together the effect is even more striking, and the famous double double epsilon Lyrae is one of the most impressive targets in the night sky. There are several other well-known double double systems, but, surprisingly, there does not seem to be a compilation of such objects (apologies if I have overlooked an earlier list).
Over the last year I have been working on a compilation of "double doubles" (and higher multiples) that are visible in medium sized scopes. This is intended as a list of visually impressive combinations, and the great majority of potential double doubles in the Washington Double Star catalogue (
ad.usno.navy.mil/wds/
) were excluded. This list is restricted to primaries of at least 8th magnitude and secondaries of 9th magnitude (though a few are fainter), and to doubles having a separation between 1.5" and 50", though a small number are outside this range. Also, highly unequal doubles have been omitted, the brightness difference between the components of each double is generally less than 3 magnitudes. All the pairs are within 50' of each other, and most are much closer than that. Double doubles located in open clusters were generally excluded because the presence of numerous field stars greatly reduces the visual impact of the double double. Most can be observed using eyepieces of "normal" afov (50deg), but a few require the use of wide angle or ultra wide angle eyepieces to achieve the necessary magnification, and still accommodate both doubles in the fov. The minimum magnification required to split a double star is approximately 150/separation in arcseconds, but visual acuity varies so many observers will find that a higher magnification will be needed. Magnifications of 750/separation have been recommended for best viewing, and much higher magnifications can often be used. The separation of fainter stars, and of very unequal pairs is more difficult. Most observers would agree that the best magnification is one that gives clear separation of the components, but keeps them in the broader context of the surrounding star field. All of these double doubles should be within the reach of 200mm scopes and most should be observable even using much smaller apertures. However, some of the narrower pairs will not be resolved unless the seeing is reasonably good.
Some of these double doubles, such as epsilon Lyrae, are physically connected, others are just chance alignments. Some have additional components that are either too close to be resolved using medium sized telescopes, or are too faint and/or distant to contribute to the visual spectacle. In many such cases, notes on the additional components are provided for the sake of completeness.
The double doubles were located by searching the WDS, using AstroPlanner (
www.astroplanner.net/
), and searching the web (especially Alain Gerard's excellent site:
www.astrosurf.com/agerard/observ.html
) and Sissy Haas' book (Double Stars for Small Telescopes: More Than 2,100 Stellar Gems for Backyard Observers, Sky Publishing, 2006). The descriptions are based on my own observations and on reports from a wide variety of other observers, in books and posted on the web. There double doubles are presented roughly in order of increasing Right Ascension, but targets in the same constellation are generally listed together. All the data listed were obtained from the 2012 version of the WDS catalogue. Of course, physical binaries are mobile systems, so the data will become out of date relatively quickly. In the case of binaries with short periods, the data may already be significantly outdated, and no attempt has been made to calculate more current data using the published orbits.
For convenience, the list has been split up into sub-lists containing the targets that are best viewed within two-month periods. The first list, for July & August, is now attached as a zipped pdf file. I hope some of you will check out these objects, and any comments, corrections or suggestions would be most welcome.
Clear skies.
Mike
  