Double Stars

Here are notes on my experiences viewing a few double stars. I consider
refractors to be the best telescopes for this sort of viewing. Their star images
are almost always neater, tighter, and steadier than those produced by other
kinds of telescopes. Since they are unobstructed, their diffraction rings are
fainter, making it easier to see faint companions near bright stars.

All but impossible unless seeing is excellent. In typical seeing, the sparkling orange primary simply eats up the close blue companion. I have rarely seen it from my usual site in upstate New York, where Antares never rises very high. My best views have been from Grand Canyon. On one especially steady night, using my 155mm refractor, both stars were rock steady for as long as I cared to look, two perfect Airy disks nestled together, a very beautiful sight. On another occasion my 92mm refractor was also able to split the pair from Grand Canyon, but not nearly as easily or well.

Easy in almost any telescope, this pair does not present a meaningful challenge.

Also easy, I used to split this regularly with a 70mm telescope. In my 92mm, the split is very easy indeed. Any night that fails to show it is a poor one for double stars.
Zeta Bootis
Until a few years ago, my 155mm regularly split this nearly equal pair when its separation was around 0.75 arc seconds. Since then it has closed up considerably and won't be available to me again for some time. As of mid-2011 the pair is at about 0.5", and can still be elongated with my 155mm. Smallest separation will take place around 2021.
Pi Aquilae (Rebchis)
This nearly equal pair has a separation of around 1.5 arc seconds, making it a good test object for an 80mm scope. When I was a kid I spent too much time staring at it with my 3" refractor. It's easy in my 92mm and not too difficult in my 89mm Questar.
Gamma Andromedae
This pair is one of the most beautiful in the sky, with a golden primary and an indigo companion. It can be seen with almost any telescope. The secondary, Gamma Andromedae B, is itself a notoriously close and difficult binary. Using a 6-inch refractor at high power, in 1986 I was able to distinguish its binary nature as two overlapping Airy disks of somewhat dissimilar size, a figure eight or peanut configuration. The separation then was about 0.5 arc seconds. A few years after that I was able to get a true split using a 14.5-inch reflector from the Florida Keys. I doubt that scope could have achieved that feat from a site with lesser seeing conditions. Since then the pair has tightened to around 0.3 arc seconds, a very challenging object indeed.
Epsilon Lyrae
Some observers use the Double Double as proof of the quality of their scopes. I look at it differently: any telescope that can't readily split both pairs, with their separations of over 2 arc seconds, is not worth having. A good 70mm will split them handily, while anything larger should have no difficulty. These stars are a beautiful sight in my 155mm using a binoviewer.
I saw Sirius B for the first time at the 2006 Winter Star Party, which is noted for having some seriously good seeing. The first scope I saw it though was an 8" TEC refractor at 500X. It was pretty easy. At that power, the current 7 arc second separation looks like quite a distance. I then tried it in my 155mm A-P refractor, but could not see it. The next night I tried again with my scope using 470X and a binoviewer. This time it was intermittently visible, sometimes pretty easily. I was set up next to Sue French, and we used her 10-inch f/6 reflector to also see it. It was a bit easier in that scope despite the flary stray light coming off the supernally brilliant primary star. This 10-inch is a very nice scope despite the very different nature of its high-power star images compared to those of a refractor. For some reason, Sue was unable to see B through my scope, though a number of other people did. I saw it often enough so that I have no doubt. In every case the star was seen at the correct position angle.

I have never seen B from here in New York, nor had any suspicion of seeing it. Nor had I ever seen it in previous tries from WSP. I did see it again from the 2007 WSP, using that very same 10" reflector, and also an 8" refractor. I also saw it from the 2011 WSP using my 155mm.

When attempting this observation with a telescope that has a star diagonal, wait for Sirius to reach the meridian, and rotate the diagonal so the eyepiece sticks straight up. B will then be near the 4 o'clock position relative to Sirius itself. Still widening, this double will get a little easier every year for a few more years to come.

Adhara (Epsilon Canis Majoris)
This "Mini-Sirius" is a good warmup for the real thing. With a separation similar to the current value of Sirius (as of 2007), the magnitudes are disparate, but not as much as Sirius A and B. If conditions prevent you from resolving this pair, you can forget about Sirius for that evening. In my 92mm refractor it resolves neatly at 120X with good seeing from Florida, but is rarely resolvable from my New York site, which is eighteen degrees farther north.

Bird 3
This curious triple star is located about three quarters of a degree northwest of the Cat's Eye Nebula in Draco. It consist of two similar tenth magnitude stars roughly 30" apart, with a fainter star almost exactly between them, creating a short line. It looks quite odd, almost artificially perfect, and is well worth a look.

Propus (Eta Geminorum)
This tricky pair is similar to a miniature version of Antares, being an orange star with a much fainter companion hard against it. I have not clearly resolved it with my 92mm refractor, but my 155mm scope gives a neat, pretty view.

Porrima (Gamma Virginis)
This classic binary was an easy target throughout the first decades of my observing career, until a few years ago when its orbital motion reduced its separation to almost nothing. By Spring of 2007 it was rapidly widening, and stood at about 0.8". My 155mm scope at very high power revealed two little balls leaning against each other. As of 2011 this pair is at about 1.5", within reach of any decent scope of 3" aperture or greater. It will get easier every year for a long time to come.