Visual observer Roger Ivester tells how to find the open cluster NGC 6645 in Sagittarius.
NGC 6645 is a fabulous open cluster that is often overlooked by many amateur astronomers. You can find it about 2º north of the much brighter cluster, Messier 25. This summertime cluster is well positioned during the months of August and September.
I could easily see it with my 102 mm refractor at 50x, but it 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 made 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 resolving 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" or the central void could be seen fairly easily. About 60 stars could be counted with 12-15 stars comprising the central ring. The overall shape of the cluster is mostly irregular, and I noted a fairly bright double star south-southwest of the center.
The above sketch was made using a No. 2 pencil, and a blank 5 X 8 note card, inverting the colors via my scanner. My observations of this cluster were made from my moderately light-polluted backyard in the foothills of western North Carolina. I thought this cluster needed a name, so I'm now calling it the "Ringlet Cluster." If you have not observed NGC 6645, please give it a try, and see if you too can see the ring of stars. Roger Ivester
Were you able to find the "Ringlet Cluster"? Could you make out the small ring devoid of stars? Tell us in the comments!
Roger Ivester has enjoyed the wonders of the night sky since he was 12 years old. He is a visual observer and enjoys sketching and writing about what he sees. In 2009 he helped start the Las Vegas Astronomical Society Observers Challenge, and works with Fred Rayworth on a monthly basis to compile the report. Roger and his wife Debbie live in the foothills of western North Carolina.