William Herschel discovered NGC 6645 = H VI-23 = h2013 on 27 Jun 1786 (sweep 576) and recorded "a beautiful cluster of vS stars; very rich, the stars are of various sizes and some of them form an almost circular row in the middle. About 15' diameter." John Herschel made a single observation; "a rich, p comp cluster; irreg figure having a vacancy in the middle; stars = 11...15m."

200/250mm - 8" about 50 stars resolved, moderately large, unresolved haze, including a close triple star, rich in faint stars, several dense patches.

400/500mm - 17.5" (8/1/92): at 82x, this is a striking, rich cluster of 20' diameter! Quite rich in faint stars but a noticeable "hole" in center is devoid of stars. A rich knot of stars is on the south side of void and a close triple star is on the north side. A string of stars extends out of the cluster to the east. Three mag 7-8 stars are in the field to the NE. At 220x on the south side of the void are 8 stars in a "V" asterism. Roughly 75 stars are resolved within the central 10' diameter. A rich clump of 8 stars is just resolved at the south edge of the circular hole.

Notes by Steve Gottlieb