Double jewels in Cancer.
I keep hearing adults say that anyone will be disappointed by a telescope so small. I've even heard some say that a 2.4-inch isn't enough to do "real" astronomy. Nothing, in my opinion, could be further from the truth.
Just consider the constellation Cancer. Here, in a relatively small space, lie seven easy targets for the smallest telescope: not only the giant cluster M44 (Praesepe or the Beehive) and the lesser cluster M67, but also the double stars Iota ([Iota]), [Phi.sup.2] ([[Phi].sup.2]), 24, and Zeta ([Zeta]) Cancri, and Struve 1311. Personally, I enjoy these doubles just as much as the clusters.
But make no mistake about it: a small refractor from a department store does [TABULAR DATA OMITTED] not come ready to use. Its cut-rate eye-pieces and flimsy mount head must be replaced, and, like any telescope, it needs a good finder. I decided to add these to my own 2.4-inch refractor before facing the higher cost of another whole telescope.
One thing I learned in the process is that almost anything is better than the mounting that comes with such a scope. Amateurs have a long tradition of making heavy, stable, low-cost mounts using threaded plumbing pipe. When I made a pipe mount for my 2.4-inch (greasing the threads with old lipstick), I found the little refractor's usability much improved. If you're unfamiliar with this once-popular mount design, you can write to me at the address below for specifics.
Iota Cancri is my very favorite double star for the 2.4-inch. It is bright and wide and appears double even in a finderscope. This pair, in my opinion, is the most striking color contraster in the sky. It's natural to compare it with Albireo, but I think Iota is better. Why? Because the color of its primary looks truer. I see Albireo's as orangish yellow but Iota's as pure yellow. Each has a striking sapphire companion of similar brightness inequality. Am I the only one who likes Iota better?
Struve 1311 (usually abbreviated with a capital Greek letter sigma as [Sigma] 1131) is a pretty pair of reddish orange twins. They are easily separated by my little refractor. In noncontrasters like this pair I sometimes enjoy an indecisive hue. A star's color is a product of its temperature, as is its spectrum, but color is also affected by other factors including the seeing. That's why, when the seeing is unsteady, this pair might first look more red than orange and then reverse itself moments later.
Also, double stars are notorious for showing exaggerated colors due to how the eye deals with contrasts in color and brightness. In the illustration on the facing page the stars are tinted according to my own impressions.
[Phi.sup.2], 24, and Zeta Cancri are all yellowish pairs. Each, however, is its own shade of yellow. Since these pairs fall nearly in a north-south line, they can easily be swept up for comparison. I see [Phi.sup.2] as whitish gold, 24 as orangish yellow, and Zeta as pure mustard.
Zeta Cancri is famous as a triple star, but component B is now well beyond the limits of a small telescope. This year it is separated from A, the bright primary, by a mere 0[double prime].7. With my 5-inch refractor I can see that A looks slightly elongated rather than perfectly round - but that's about it. I did receive a letter from John Newsom of Paragould, Arkansas, who says he can elongate the A-B pair with his 6-inch f/12 doublet refractor. Can anyone get a complete split with an 8-inch? Component C, on the other hand, is easy.
Zeta's three stars are notable for being, unlike most, rather similar to our Sun in luminosity, temperature, color, and size. The A-B pair completes an orbit in 60 years at a mean separation of 19 astronomical units (the size of Uranus's orbit); C circles them in perhaps 1,200 years at a distance of about 175 a.u. In addition, C displays gravitational evidence of having an unseen white-dwarf companion circling it at a distance of 5 a.u. The whole system is about 70 light-years away.
Compared to Zeta AB, 57 Cancri is a piece of cake. This is one of my favorite picks for a medium aperture. In my 5-inch at 200x it's a pair of pale orange twins absolutely in contact. I admit to being fussy about couples of this sort. Few sights can make my breath catch as quickly as a touching pair of perfect diffraction disks, but blobs of fuzz or glare leave me less than captivated. It is for "bright tights" like 57 Cancri that atmospheric steadiness (rather than clarity) is so important, because what you need first and foremost is a sharp image.
And if you like crisp diffraction patterns the way I do, small apertures are best. The smaller the aperture, the larger the diffraction disk and the less it will be fuzzed up by imperfect atmospheric seeing. One trick for a large scope on a tremoring night is to reduce its aperture with an off-axis stop, a circular hole cut in a card taped over the front. That way most of the atmospheric eddies that distort starlight will be bigger than the scope's objective. This keeps them from breaking up the diffraction pattern. In addition, an off-axis stop will reduce the overpowering glare of a bright star and improve the apparent quality of imperfect, "light-bucket" optics. Of course, you lose any chance to get the higher resolution that the larger aperture ought to provide.
I also need my 5-inch to resolve the close pair 66 Cancri. This one is moderately wide but very unequal, with a beautiful color contrast. To me it looks orangish white and subtle blue.
Struve 1177 is a delight in my 5-inch just for its brightness. If only it were a bit wider it might look nice in my 2.4-inch. It just barely appears double in the 5-inch at 50x. At such low magnification it shines bright whitish gold, the companion looking about a magnitude fainter.
Alpha Cancri requires still more aperture. This is one of my favorite sights in the entire constellation. Normally I wouldn't save a personal favorite until nearly last, but this is the one that demands the biggest scope. Its companion is semiwide, 11[double prime] from the primary, but 7.5 magnitudes (1,000 times) fainter.
For this kind of double, aperture and sky clarity are especially important. So is high power. And the atmospheric seeing still plays a role too. You can see faint stars better when they are tiny, concentrated points rather than big, vague blurs that have a lower surface brightness.
It was for pairs of this sort that I bought my 13-inch Dobsonian. I'd buy one again just to see Alpha Cancri. In this light-grabbing monster Alpha is an overpowering gold fireball, almost dazzlingly bright and bordered on its edge by the ghostliest speck. Fantastic! Can anyone spot the companion with a 6-inch? How would you rate your sky at the time?
While you're here, don't pass up the open cluster M67 located just under 2 [degrees] due west of Alpha. Its total magnitude is 7; its stars are magnitude 10 and fainter.
Struve 1254 in M44 is wider and less uneven than Alpha, but it's still not a pair to recommend for a small scope. Perhaps a 4-inch under an ideal sky might do, yet I doubt anything smaller will resolve it. Like Iota and Alpha, this is one you can locate with the naked eye - sort of. If you can see Praesepe without optical aid, that's where to point your scope. Struve 1254 lies just about at the center of the cluster. Look for two bright, wide pairs along a straight east-west line on the south edge of the cluster's brightest part. Struve 1254 is the brightest star just north of the eastern pair. In my 13-inch at 98x, it is brilliant pale orange with a tiny bluish pinpoint beside it to the northeast. A similar pinpoint lies about four times as far in roughly the same direction, and another is at three times the pair's separation toward the north-northwest.
SISSY HAAS, a Pennsylvania homemaker and double-star enthusiast, wrote "Jewels in Lynx" in the April 1994 issue.
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|Title Annotation:||double stars|
|Publication:||Sky & Telescope|
|Date:||Mar 1, 1995|
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