This fine globular star cluster straddles the border between Serpens and Ophiuchus. 4.8 magnitude Tau Ophiuchi lies 40' to the south-southwest. The sketch at left presents NGC 6539 as it appears at 82X in my 10-inch Starfinder Newtonian. The globular cluster covers a 4' diameter area, is quite bright at magnitude 8.9 but shows no individual members. The horizontal branch stars are faint at 18.3 magnitude and the brightest cluster members weigh in at 15.9 magnitude. Twelve field stars frame the globular. NGC 6517 is located 1.5 degrees to the southwest.