R.J. Mitchell discovered NGC 3993 on 25 Apr 1854 and it was labeled "Eta" on the constructed sketch of the group made at Birr Castle. In a later observation, a very faint star was noted close south-following and two mag 11-12 stars north-preceding. The micrometric position is very accurate.

Harold Corwin notes that WH's III-324 refers to NGC 3997 and not NGC 3993 as Dreyer assigned in his 1912 "Scientific Papers of WH". But Wolfgang Steinicke lists WH as the discoverer of NGC 3993 on 6 Apr 1785 (sweep 393), probably due to the better match in position.

400/500mm - 17.5" (3/19/88): faint, fairly small, very elongated NW-SE, broad concentration. Forms a pair with NGC 3989 2.7' WSW. Member of the NGC 4007 group.

600/800mm - 24" (3/22/14): fairly faint, moderately large, very elongated 3:1 or 7:2 NW-SE, 1.2'x0.35', weak concentration. Two mag 11.5-12 stars are off the NW end. Sandwiched between NGC 3997 3.0' NE and NGC 3987 4.7' SE. Also NGC 3989 is 2.5' W.

Notes by Steve Gottlieb