Persian astronomer Al-Sufi first mentioned IC 2391 = Lac II-5 in his "Book of Fixed Stars" (964 AD) as a "nebulous star." Nicolas-Louis de Lacaille observed it in 1751-1752 during his journey to the Cape and described a "small heap of stars" in his 1/2-inch refractor. Solon Bailey also found the cluster on a photographic plate in 1896 using a 1" (f/13) Cook lens at the Arequipa station. The discovery was reported in "A Catalogue of Bright Clusters and Nebulae" (Annals of Harvard College Observatory, Vol LX, No. VIII) and Bailey was credited with the discovery in the IC.

300/350mm - 13.1" (1/30/06 - Costa Rica): very bright and large naked-eye cluster surrounding mag 3.6 Omicron Vel (just north of the False Cross). Resolved in 9x50 finder and my 15x50 IS binoculars. Includes 7 bright stars with a wide (1.3') bright pair of mag 5 stars on the east side. The border of this scattered group is not well defined but the fainter stars fill out over a degree in the 20mm Nagler 66' field.

13.1" (2/17/04 - Costa Rica): this very large naked-eye cluster appears as a fuzzy glow surrounding mag 3.6 Omicron Velorum (1.8° NNW of mag 2.0 Delta Velorum in the False Cross). This scattered 50' group contains perhaps three dozen stars and barely fits in the field of the 20mm Nagler. Perhaps a better view was obtained in the 9x50 finder or 10x30 IS binoculars which did a nice job of resolving the brighter stars, but the group was really too sparse and scattered to look impressive in the 13".

10x30mm Canon IS (3/28/19 - Tasmania): very bright naked-eye cluster with a few stars seen unaided. Excellent resolution in 10x30 binoculars but still scattered as spread out over 50'. Nice wide double star at edge. Includes a half-dozen brighter stars.

Naked-eye - (3/16/18, West Texas): I noticed the Omicron Vel cluster was fairly easily visible naked-eye as a fuzzy patch, though only 7° (or less) elevation at a latitude of +30. It was partly resolved in 10x50 binoculars.

Notes by Steve Gottlieb