John Herschel discovered NGC 5168 = h3512 on 16 Jun 1835 and recorded "A small but very rich milky way cluster; 3.5' l, 3' br; st 13...16m. Place that of a double star [HJ 4591]." On a later sweep he added "place of a double star in centre of a rich, much comp but vF cluster; gbM; 4' diam; st 15m; a remarkable object."

400/500mm - 18" (7/11/05 - Magellan Observatory, Australia): gorgeous low power Milky Way field at 76x, but it was difficult to identify the cluster not having a finder chart. Initially I was drawn to a striking asterism in the field consisting of a distinctive group of four stars forming a rough 8'x3' parallelogram with brightest member mag 7.8 HD 117356, but the enclosed stars were no richer than the surrounding field and it looked like an asterism. Then I noticed that 12' S of the mag 7.8 star was an eye-catching double star = HJ 4591 (10.2/10.4) at 7" surrounded by a few faint stars over unresolved haze. Increasing the magnification to 228x, this rich knot resolved into three dozen mag 14 and fainter stars in a 2'x1.5' roundish cluster with the double star on the following side and this was clearly NGC 5138.

Notes by Steve Gottlieb