Lucky Sevens

Late September through early November are, for much of the US, the best times to observe the night sky. Temperatures are cooling, the stifling humidity of the Midwest is dissipating for the year, summer’s annoying flying/crawling vermin are dying off. The sky itself is transitioning between the rich galaxy fields of Spring, sitting low in the west as the sun sets, and the rising galaxy fields of the autumn skies—and never the twain shall meet—with the cluster-and-nebula-rich swath of the Milky Way separating them like a vaporous gash, which is itself sliding inexorably toward the east to spill its contents across the horizon before sinking out of sight.

And so it was that your humble australopithecine found himself assembling his primitive telescope gear—a 12.5” Discovery truss-style Dobsonian, as more-modern technology like Schmidt-Cassegrains and apochromatic refractors confuses and frightens me—at the visitor’s center of Giant City State Park for a near-to-last roundup of globular clusters (which are most prevalent in the late Spring/summer sky). Needing only nine more to finish the Astronomical League’s Globular Cluster Program, he found himself racing against the change of seasons and some very uncooperative weather, to catch whatever clusters were possible in the given span of sky; having already conquered most of the autumn globulars visible with the 12.5”, targets that might be visible in the coming months were becoming few and far between.

This night—Sunday, September 21—conditions were quite good. I sought, in particular, to find Palomar 7 (a.k.a. IC 1276) in the constellation Serpens Cauda, which had thwarted me the previous night under somewhat lesser conditions. I also aimed to do a sketch of Terzan 7, an extremely-faint globular in Sagittarius, which I had observed back in July but wasn’t 100% sure I had seen. Judging from the sketch—and from comments from experienced observers on the CloudyNights forums–I had indeed observed this very difficult target. While waiting for conditions to darken enough for observing Palomar 7 and Terzan 7, I went after a couple of other globulars of varying degrees of difficulty. Observing with me this night was Bob M., fellow AASI member and deep-sky observer for more than a half-century.

Notes from the 9/21 session are as follows; all observations were made with a 14mm 82* eyepiece yielding 113x (P = preceding; F = following; CC = Shapley-Sawyer Concentration Class):


MOON: 27 days, absent
SEEING: 5-6 (Pickering)
NELM: 5.9
WEATHER CONDITIONS: temps in 50s-60s, moderate humidity (some dew), somewhat breezy

Others present: RM 

M69 (Sgr)—7th mag—about 5’ across—nicely resolved—almost like M15 with larger core—core is 3/4-2/3 of diameter—doesn’t have stellar nucleus—to SF side are brightest couple of cluster stars, halfway between core and halo—bright (7th) field star 10’ to NP—100 or so resolved stars visible w/averted—not totally dark yet—CC 7—to SE:

NGC 6652 (Sgr)—not unlike M69—quite a bit fainter; mag 9—more concentrated—3’—star-like central point—to SF and due P sides are brightest cluster stars, makes core look elongated within halo—brightest, to P side, maybe 12th mag—core makes up middle 60%—outer edges granular, a few stars visible—to NP is bright 7-8 field star—satellite through N side of field P-F—cluster CC 6?

Palomar 7 (IC 1276) (SerCau)—really indistinct—2 on averted scale; maybe 13th mag—4’ diam? —really coming and going—has on direct P side a 13th mag star, cluster member?—about 6’ N of cluster is 10th star, next to it on P side 11th mag star—little triangle, longest side 8’, faintest triangle star to S, which has cluster to F side—cluster definitely there—just a glow—no real central concentration, CC 10? too hard to tell—to S of field, about 18’ from cluster another 10-11 mag star—sparse field, near a lot of dust??

My estimates of M69, both in size and CC, were a bit off; it’s about 3-4′ visually in mid-sized scopes like mine, and a CC of 5.  Concentration class is always tough to estimate, although my estimates have largely been pretty good (I was pretty much on the money with NGC 6652).

The sketch of Terzan 7 was accomplished in the interregnum between NGC 6652 and Palomar 7; the arrow indicates the direction of field motion (toward the preceding side). I only had a narrow “window” between two trees to find and sketch the cluster. In this timeframe, I also observed NGC 6517 in Ophiuchus and NGC 6539 in Serpens, both of which “lead up” to Palomar 7. This is part of the reason why there’s such a long gap between 6652 and Pal 7.

We didn’t stay long; I had to work the next day.  (Those Eohippus won’t hunt themselves.)

In honor of Palomar 7 and Terzan 7—two of my better accomplishments in observing—here’s a marvelous and appropriate Chris Squire track, featuring three of my favorite musicians (Squire, drummer Bill Bruford, and the magnificent sax/flute virtuoso Mel Collins):

Next up: a recap of Saturday the 20th, a slightly-busier night.