BASIC OBSERVING SERIES

by Greg Burnett (gburnett)

Double Star Observing

[This article was first published in Star Stuff, the newsletter of the Ford Amateur Astronomy Club, in August, 1993.]

In an earlier Star Stuff article (v.2, n.1, January, 1993) I discussed double stars in general, including their classifications (visual, astrometric, spectroscopic, eclipsing, and optical), and some fundamental information on observing. This article will take up several other aspects of double star observing, in an attempt to avoid redundancy.

For serious (but still amateur; read "satisfying") double star observing, your telescope must deliver the goods with respect to resolution. Double stars are often used as test objects to determine the true resolving power of a telescope (See the January article for a table of theoretical resolving powers of telescopes of various apertures.), and that's fine if you're paranoid about the quality of your telescope's optics. The fact is that amateur scopes are always a compromise of several factors, including aperture (i.e. light gathering power), resolution, and portability. Most amateur scopes reflect a compromise that leans toward one particular factor: "Dobs" toward light-gathering, SCTs toward portability, and refractors (and Questars) toward resolution and image quality. [Digression: There is always alot of discussion about choosing your first telescope. Perhaps someone should author an article on choosing your last telescope; which compromise is best for your long-term observing interests? Hmmm... stay tuned to Star Stuff!] For double star observing there is no substitute for crisp, high-resolution images, regardless of aperture. There is a endless range of double stars suitable for telescopes of every size. No matter how much aperture you have, there will still be a zillion doubles beyond your reach. The important thing is that your scope delivers an image that permits observation of doubles with separations down to (and perhaps below) the theoretical limit for its aperture. Otherwise, your double star observations will not be interesting or satisfying.

In response to a point source of light (e.g. a star) a telescope produces an "Airy disk" (named for George Airy, optician and astronomer) that is considerably larger than what the true geometric image of the star would be, typically by a factor of 1000 or more. It is the Airy disk that you see in the eyepiece, not the star. The Airy disk is surrounded by a series of concentric diffraction rings. The Airy disk and its associated diffraction rings represent the best star image your telescope can theoretically produce. If you can see the Airy disk in your telescope, you can make satisfying observations of double stars.

The ability of a telescope to "split" a double simply means that the Airy disks of the two stars are discernably separate. Below the theoretical limit of separation, the Airy disks will overlap and form a figure "8" or an oval. In this case the star is certainly seen to be double, even though the images of the components are not distinctly separated. Obviously, a high-resolution image is required to make these observations.

Given excellent optics, one remaining obstacle to double star observing is the "seeing" conditions on any particular night. Atmospheric turbulence can prevent your telescope from performing at its best. On nights of poor seeing, doubles that are normally resolvable will be seen as boiling, indistinct knots of glare. On marginal nights, doubles higher above the horizon usually give better results. The nearer your viewing angle is to the zenith, the less air you're looking through.

Double stars can be challenging targets for two reasons: separation and brightness difference. Some of the most difficult doubles for my 6-inch scope are not particularly close, but they exhibit large magnitude differences between the components. For example, Antares has a comfortable 2.7 arcsecond separation, but the 5.4 magnitude secondary can be very difficult to see in the glare of the 1.2 magnitude primary. This difference of over 4 magnitudes makes Antares resolvable only on nights of steady seeing. (Its -17 degree declination doesn't help matters either!) Sirius is an extreme example. The primary is -1.5 magnitude and the secondary is 8.5, giving a difference of 10 magnitudes. Even though the separation is a generous 4 arcseconds, Sirius is impossible to resolve in amateur scopes and is a serious challenge for observatory-class equipment.

There are interesting doubles suitable for any telescope, even binoculars. Most star atlases include lists of popular and interesting double stars. Norton's Star Atlas and volume 1 of the Webb Society Deep-Sky Observer's Handbook are both good sources. Serious observers should consult Sky Catalogue 2000.0, volume 2, which lists over 8,000 double and multiple stars.