John Herschel discovered NGC 6558 = h3731 on 3 Aug 1834 and recorded "globular cluster; not vB; R; glbM; 2'; resolved. The stars barely discernible." On sweep 794 (June 1837) he logged "globular; pB; irreg R; gmbM; composed of st 16m, on a milky way ground of mixed L and vS stars." This object may be a compact open cluster.

Pietro Baracchi observed the cluster on 5 Oct 1885 with the 48" Melbourne telescope and wrote "pB, pS, round, gbM, resolvable clluster of very minute stars. A multitude of small stars in the field." It was described as "probably be classed as a globular" in the 1921 Helwan list (based on photos taken in 1914-16).

200/250mm - 8" (7/31/86): faint, small, round, six faint stars lie to the south.

400/500mm - 17.5" (7/29/92): moderately bright, small, 1.5'-2.0' diameter, irregular outline. Has a fairly smooth halo with a small bright core offset to the north side with a stellar nucleus sometimes visible. Five or six very faint mag 14.5-15 stars are embedded within the halo. Located in a rich star field.

Notes by Steve Gottlieb