by Anthony Ayiomamitis
Open cluster NGC 6494 in Sagittarius above is a rich dense cluster comprised of over 100 member stars which are of uniform brightness and well detached from the rich background sky. The cluster spans 30 arc-minutes in diameter or 15-20 light-years across. M23 lies at a distance of 2,150 light-years away and is dated at a mere 220-300 million years-old and as evidenced by the predominance of white hot stars in the image below. The cluster is best observed using low magnifications (50-100x) during mid-summer as it appoaches the southern meridian near midnight. NGC 6494 was discovered by Charles Messier in 1764.
Technical Details:
Date: May 10, 2010 @ 02:40 - 04:50 UT+3
Location: Athens, Greece (38.2997° N, 23.7430° E)
Equipment: AP 160 f/7.5 StarFire EDF, AP 1200GTO GEM, SBIG ST-10XME, SBIG CFW10, SBIG LRGB filters
Integrations: LRGB @ 36:30:30:30 using 3-/6-min subs, 1x1 binning, 1.17"/pixel, -17.5d C
Further details are available here.