Theodor Johann Christian Brorsen discovered NGC 6539 = Au 39 in 1856 (probably Sept) at the Senftenberg Observatory in the present-day Czech Republic, probably using a 9.4-cm comet-seeker. Arthur Auwers observed this globular on 10 Oct 1860 with the Konigsberg 6-inch refractor and reported (in the notes to his 1862 list of new nebulae) that it "looked faint, but pretty well at 65x; it appears to be a faint star group of about 3' diameter, centrally surrounded by numerous stars 12m." (translation by Wolfgang Steinicke). NGC 6539 is one of the brighter southern objects that John Herschel missed.

200/250mm - 8" (6/22/81): faint, moderately large, very diffuse, no resolution. This globular straddles the Serpens/Oph border, 44' NE of Tau Ophiuchi, an excellent 1.5" double star.

400/500mm - 17.5" (8/10/91): fairly faint, moderately large, 3' diameter, round, diffuse, broad weak concentration. A number of stars are very near including a mag 12.5 star off the NW edge and a fainter mag 13.5 star off the SE edge. In addition, an extremely faint 15th mag star is resolved near the NW edge and a mag 15 star is visible near the center but no other resolution was evident.

600/800mm - 24" (7/30/16): at 260x; fairly faint, moderately large, round, ~3' diameter, very weak concentration. A brighter mag 12.8 star is just off the northwest edge and a mag 13.5 star is off the southwest edge. The cluster was very lively at 432x and several faint to extremely faint stars were resolved around the edges of the halo including a few easy ones just off the west side. Several very faint to extremely faint stars scintillate over the core and main portion of the halo, popping in and out of view with the seeing, though only a couple of these were consistently visible.

Notes by Steve Gottlieb