J.L.E. Dreyer discovered NGC 4021 on 26 Apr 1878, the last session "nebulae" were discovered at Birr Castle. He recorded, "F, S, R or lE and placed this object 58.2" in PA 312.1° with respect to NGC 4015. It was labeled "Delta" on the final compiled sketch of the cluster. At Dreyer's micrometric offset is CGCG 127-124 = PGC 37730. Dreyer made an incorrect assumption that William Herschel's III-323 = NGC 4015 and III-324 = NGC 4021, based on their separations - these numbers apply to NGC 3987 and NGC 3997.

400/500mm - 17.5" (3/19/88): very faint, very small, slightly elongated ~E-W, 0.4'x0.3'. Forms the northern vertex of an equilateral triangle with NGC 4015 5' SW and NGC 4023 6' SSE. Member of the NGC 4007 group.

600/800mm - 24" (3/22/14): faint, fairly small, round, 24" diameter, slightly brighter core. Located 5.2' NE of NGC 4015 in the NGC 4005 cluster.

Notes by Steve Gottlieb