Friedrich August Winnecke discovered NGC 6366 = Au 36 on 12 Apr 1860 with a 3-inch comet-seeker at the Pulkovo observatory and noted "faint, 2 to 3' diam, no significant central brightening." Auwers included it in his 1862 catalogue of new nebulae.

200/250mm - 8" (6/27/81 and 5/21/82): large, very diffuse, unresolved. Located 15' E of a mag 4.5 star that detracts from viewing.

300/350mm - 13.1" (7/5/83): large, diffuse, very weak concentration. About a dozen faint stars are resolved over a hazy background. Located 17' E of 47 Ophiuchi (V = 4.5).

13.1" (6/19/82): few faint stars resolved over a large, hazy region.

400/500mm - 17.5" (7/20/98): at 220x appears as a diffuse irregular glow, ~4' diameter, with only a weak concentration. Two mag 9 and 10 stars are off the west side, the nearest is less than 4' from center and a closer pair of mag 11.5-12 stars [45" separation] is at the south edge. There are perhaps a half dozen faint but obvious stars visible over the ill-defined glow including a couple of mag 14 stars 2' S of center, one a similar distance east of center and an addition pair on the SE side. About a dozen stars are visible with careful viewing. At 280x, the cluster is pretty clumpy and starting to really break up into numerous very faint stars. Roughly two dozen stars can be glimpsed with averted vision, many near the threshold of visibility. The full extent of the cluster is difficult to trace but extends beyond the central 4' region.

Notes by Steve Gottlieb