Open Cluster in Ophiuchus

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This very large open cluster is just 1.5 degrees northeast of Beta Ophiuchus, and it is easy to find if you know where to look. The cluster is often overlooked by amateur astronomers, perhaps because it is so big that it does not show itself well in typical telescopes at medium or high power. But it is a great object for binoculars or a small telescope with a wide field of view. A few dozen stars of varying brightness can be seen in an area that is about twice the diameter of the full Moon.

The cluster appears large because it is relatively close, at a distance of about 1100 light years. For comparison, the well-known Pleiades (M45) are about 444 light years away, and the Beehive Cluster (M44) is about 510 light years away.

Magnitude 4.7
Apparent Size 70'
Distance (light yrs) 1,100
Right Ascension 17:46.2
Declination +05 43
Field of View 124' x 93'

Image details:  Exposure times of 33 minutes luminance and 15 minutes each of red, green, and blue, taken with an SBIG ST-8300M imager and a 500-mm Tokina lens at f/8.

June 2017