NGC 6645 – Open Cluster in Sagittarius – Also Known as The Ringlet Cluster

JULY 2011 OBSERVERS CHALLENGE – NGC-6645

NGC 6645 is a fabulous open cluster that is often overlooked by many amateur astronomers.  You can find this cluster about 2 degrees north of the much brighter cluster, Messier 25.  This summer time cluster is well positioned during the months of August, and September.  I could easily see NGC 6645 with my 102 mm refractor at 50x, but  appeared only as a faint haze, west of a fairly bright chain of five stars.  When increasing the magnification to 90x, an obvious grouping of stars making a small ring, devoid of any stars.  

I have observed this cluster on many occasions, but had never noted this most unusual feature.  Others have reported seeing this small circlet of stars.  From the Observing Handbook and Catalog of Deep-Sky Objects by Brian Skiff and Christian Luginbuhl:  “The denser core is about 10′ across and dominated on the N side by a 2′.5 circlet of a dozen stars with an empty center.”  

In Deep-Sky Wonders by Sue French, P-221:  “My 10-inch scope at 115x reveals a lovely group of 70 irregularly strewn stars with a sable void near its center.”  

I could count about 20 of the brighter members with the 102 mm refractor, however, with a night of better conditions the transparency would have allowed a better resolve of some of the fainter members. A couple of nights later, with excellent conditions, using my 10-inch f/4.5 reflector and a magnification of  208x, the “ringlet of stars” and the central void could be seen very easily.  About 60 stars could be counted, with about 12-15 stars comprising the central ring.  The overall shape of the cluster is mostly irregular, and a fairly bright double star was noted SSW of the center.  The following sketch was made using a No. 2 pencil, and a blank 5 x 8 note card, with the colors being inverted via computer.  

My observations of this cluster were made from my moderately light polluted backyard in the foothills of North Carolina.  I thought this cluster needed a name:  The “Ringlet Cluster” sounded pretty good.  

If you have not observed NGC 6645, please give it a try.  It very well may become one of your favorite summertime open clusters.    

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