A Star-Hop that Finally Worked!
September 2010 :
I’m starting this piece by confessing to you that, even after nearly 20 years’ involvement in amateur astronomy, there are still a couple of objects on the Messier list that I’ve not observed. No good excuses come to mind, but the number was reduced by one on the evening of Saturday, July 31. I was doing some casual observing at the home of my friend Tim Dube in East Douglas, Massachusetts; we were using his 16-inch Dobsonian reflector for viewing objects to the south. The time was roughly around midnight when I began to consider trying to locate the globular cluster M75 in eastern Sagittarius, almost on the border of Capricornus.
Several times over the past 17 years I’ve made unsuccessful attempts at finding M75 and have always felt that perhaps I just didn’t try quite hard enough-this particular object had confounded me for no good reason, and it seemed high time to track it down. The cluster is magnitude 8.6 and about 6 arcminutes in diameter, a rather compressed and fairly conspicuous deep-sky object that should be easy enough to pinpoint in 6-inch scopes, let alone a 16-inch. The main problem for anyone not using a computerized scope or a mount having setting circles is finding M75 to begin with, due to its placement in a “blank” section of the sky with no bright stars anywhere nearby from which an easy star-hop can be done. The position, by the way, is RA 20h 06.1m, Dec -21° 55’ and a glance at a star atlas will immediately show you why I consider this a challenging “job” for most amateurs. A magnitude 6.5 star lies 1.1° to the southwest; this star is one of a very wide pair oriented roughly ENE to WSW. The star at the WSW end of the pair, about ½° away, is of magnitude 6.0 and has the potential of making this wide pair show up reasonably well in a finder scope as a simple asterism by which to get within a degree away from M75. At 2.75° due east of the globular is the magnitude 5.9 star 4 Cap, but I don’t think it’s of much help for this star-hop. (Check a detailed atlas to see what I mean, one that shows stars to 6.5 magnitude or fainter.)
I seldom use optical finder scopes and Tim doesn’t on any of his scopes-we nearly always use a red-dot, “reflex” (zero power) type for fast and easy alignment on a given point in the sky, then it’s a matter of “sweeping” to a desired location by using the lowest power, widest field eyepiece available. Still, I must admit that a good optical finder is a huge plus factor when star-hopping your way to any given object that isn’t all that bright or prominent, particularly when using a fairly long focal length scope that simply won’t permit observations at extreme low power even when using a 40 or 45mm eyepiece. A quality optical finder scope would’ve quickly enabled us to zero-in on M75, I’m sure. I started my star-hop by repeating exactly the same mistake I’ve made in the past, meaning that the guide stars chosen as the initial step in trying to locate the cluster are a little too far away, especially since no optical finder was involved. Also, the direction of the sightline extended through the star pattern I’ve traditionally used just does not work for picking up M75.
Approximately 4° to the SSE of Beta 1 Capricorni-”Dabih”, magnitude 3.1 and paired with magnitude 6.1 Beta 2 to its immediate west-lies a triangular asterism of reasonably bright, easily seen stars ranging in magnitude from 5.0 to 6.1, each of which is a double star. The 3 stars are, from west-to-east, 10 (Pi), 11 (Rho), and 12 (Omicron) Capricorni. Rho is a wide binocular double and the brightest in the triangle, located at its northern tip. (The magnitude 6.7 wide companion is about 250” to the SSE.) Pi is a close pair of magnitudes 5.3 and 9.0 at the west tip. Omicron, at the southern tip, is a fine pair of magnitudes 6.1 and 6.6 at an attractive separation of 22”. I should mention here that Rho Cap is actually a multiple star. There’s a magnitude 9.5 companion separated from the primary by only 1.0” and a much dimmer third star of magnitude 13 at roughly 55” away. The wide binocular companion, plus the magnitude 5.0 primary, are the only 2 stars usually seen. My hat’s off to anyone who manages to split that very uneven tight pair!
Positioned about 2.0° to the WSW of Pi (which is at the west tip of the triangle I’ve described) is yet another double star, 7 (Sigma) Capricorni, a pair of magnitudes 5.5 and 9 separated by 56”. I’ve long thought that this star might be considered as the point of an arrow, with the triangular pattern to its ENE serving as the feathers. The trouble with using this arrangement of stars without benefit of a finder scope is the 2.0° separation between Sigma and Pi and the fact that my “arrow” doesn’t really point southwest to M75 at all, but to a spot due north of the cluster by 2.0° or so, nearly on the ecliptic. Add to this problem the distance between Sigma Cap and M75 (4.25°) and you can begin to see my dilemma. “Sweeping” off the triangular asterism was simply not working for me, just as it hadn’t in any of my past attempts. I determined to try finding a completely new approach for a star-hop, but what would it be?
Whenever I reach an impasse like this, I figure it’s time to take another good, hard look at a star atlas. As I scanned Tim’s Sky Atlas 2000.0 back in the house under decent lamplight (red flashlights on an atlas are often too tough for me), something jumped out at me in a way I’d not noticed before, and I can’t explain why-it just “clicked”, so to speak. It wasn’t so much that I was unfamiliar with a certain prominent asterism of 4 fairly bright stars to the SSW of M75, the center of this group being roughly 5.25° from the globular; I’d seen this grouping many times over the years and noted the rather striking appearance in an otherwise blank region of Sagittarius. For whatever reasons, it simply never occurred to me that this star pattern was, in fact, nearly ideal for use in star-hopping to M75, even without a finder scope! (Look at an atlas and you’ll easily see why.)
The asterism in question was noted by Ptolemy and named by him the “Terebellum”; it resembles a little kite or cross with its cross-bar (arm) skewed a bit, meaning not at a 90° right angle to the long axis. Still, the pattern formed by the 4 stars is unmistakable and quickly recognized by any observer familiar with this particular area of the sky. The long axis is not oriented due north-south or east-west; the “bottom” of the cross/kite is southeast of its “top” member at the northwest end, and the figure therefore is angled with respect to the southern horizon when at its culmination (transit point). The 4 stars involved are, from west-to-east, 58 (Omega) Sgr, mag 4.7; 59 Sgr, mag 4.5; 60 Sgr, mag 4.9; and 62 Sgr, which has a slightly reddish hue due to its spectral class of M4. The mean magnitude of this slightly variable star is about 4.54; 62 Sgr is also listed as V3872 Sgr. After I noted that the cross-bar/arm formed by 59 and 60 Sgr seemed to point almost directly NNE to M75, I hit upon the idea of starting my new star-hop to the globular by using these 2 stars as a “jumping-off” point and direction indicator, even though the cluster lies over 5.0° distant from the mid-point of 59 and 60.
We hurried back to the scope so as to take advantage of M75’s fortuitous placement in a gap between a tall pine tree to the east and a large house to the west, looking south across a side street. Our timing was perfect, because the Terebellum’s cross-bar/arm happened to be oriented perpendicular to the horizon. This enabled me to slew the Dobsonian straight up in the altitude axis after centering 59 and 60 Sgr in a wide field eyepiece, with no difficulties involving simultaneous azimuth motion-no side motion at all was necessary to track north to the wide pair of magnitude 6.0 and 6.5 stars I described near the end of paragraph 2. Once I located the fainter of these stars, it was a quick and simple matter to navigate 1.1° northeast and there it was-after so many years, I had finally found my “missing” Messier. This is one of the things I enjoy most about amateur astronomy-the “trip” is often as good as (or even better than) the “destination”, particularly when you’ve managed to find a great object without the aid of a computerized scope or a mount with setting circles. This was an evening to remember.
In closing, here’s an interesting coincidence about the Terebellum asterism: 2 stars within the group are anchor points for one of the official constellation-shape outlines, as based on modern, simplified, geometric representations of the centaur-archer figure. (Not all charts show this line, but many do.)
