Heinrich d'Arrest discovered NGC 1645 on 31 Oct 1864 with an 11" refractor at Copenhagen while observing NGC 1643 (10' northwest). He noted it was double the size of NGC 1643 and his position is accurate.

400/500mm - 17.5" (12/23/89): faint, very small, slightly elongated, almost even surface brightness. Forms a pair with NGC 1643 10' NW. Located 7.8' ESE of a mag 9.5 star. Appears fainter than V = 12.2.

Notes by Steve Gottlieb