June 2020 Challenge Object - NGC 6366

Original article appears in GuideStar June, 2020.

Ed Fraini


The purpose of the visual challenge object is to encourage visual observation and to help each other improve our observational skills.    This month's target is a small dim globular cluster in Ophiuchus that will take good observational skills.  It will help us reach this goal by comparing our observations. The HAS VSIG would love to hear about your visual observations.  Send them to Ed Fraini at [email protected] and he will get them posted to the VSIG list server, or just share them to the VSIG list server directly (contact Ed to subscribe to that list also). 

The month of June brings us some beautiful globular clusters in Ophiuchus, both M10, M12, plus M14, we know well. There is a lesser observed globular cluster on the eastern edge of Ophiuchus that is challenging because it is dim and loose. When you first observe it, you might not think it is a globular at all.  NGC 6366 has a magnitude of 9.5 and is small, only 13 arcmin in size.  A globular cluster will have a density classification ranging from 1 to 12, and NGC 6366 is on the loose end with a designation of 11 on this scale.  You will be best served to use low power to observe this particular globular cluster, perhaps about 100X.  With your low power eyepiece, place your field of view midway between Gamma Oph and Sabik, which form a line along the east side of Ophiuchus.  You should readily see Oph 47 to the right of this midpoint, at a magnitude of 4.5, you may well see this star naked eye.  At 1/2 degree to the east of Oph 47 lies a wide pair of matched stars in brightness and color.  These two stars mark the western boundary of our target object. On June 13th, Skytools shows that the best time for making an observation will begin at 2330, as NGC 6366 crosses above 2X air mass.

When making your observation note your impression of how dense the core is, consistency of star color and how well you can resolve stars across the cluster.  Observe if going to a higher power enhances your view.  Lastly, notice how well the target stands out from its background.

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