Heinrich d'Arrest discovered NGC 558 on 1 Feb 1864 with the 11-inch refractor at Copenhagen, while measuring nearby H III-441 = NGC 560 and H III-442 = NGC 564. He also accurately placed the nearby mag 12 star (called mag 10) as 5 seconds preceding.

400/500mm - 17.5" (1/1/92): very faint, very small, very small bright core, faint stellar nucleus, elongated 2:1 WNW-ESE in direction of a mag 12.5 star 1.3' WNW of core. Member of AGC 194 with NGC 560 4' NNE.

600/800mm - 24" (9/23/17): at 375x; fairly faint, small, slightly elongated WNW-ESE, ~20" diameter. Appears to have a brighter bar oriented WNW-ESE [confirmed on the SDSS]. A mag 12.0 star is 1.3' WNW. NGC 560 lies 4.2' NNE. In AGC 194.

Notes by Steve Gottlieb