J.L.E. Dreyer discovered NGC 4023 on 24 Apr 1878 on a late observation of the NGC 4005 group at Birr Castle. He described the nebula as "pF, pL, diffuse" and placed it at 358.1" in PA 117.8° with respect to NGC 4015. It was labeled "Gamma" on the final compiled sketch of the cluster. At this offset is UGC 6977 = PGC 37732. This was apparently the last night that nebulae were discovered at Birr Castle.

400/500mm - 17.5" (3/19/88): faint, small, slightly elongated ~N-S, weak concentration. Member of the NGC 4007 group with NGC 4015 7' WNW.

600/800mm - 24" (3/22/14): fairly faint, fairly small, round, weak concentration, slightly irregular surface brightness. Situated 5.9' SE of NGC 4015.

Notes by Steve Gottlieb