Harold Corwin

IC 2091 is one of ten new "nebulae" claimed by Isaac Roberts on a photograph of the field around NGC 1665. Seven of these (IC 2094, IC 2097, IC 2098, IC 2099, IC 2101, and IC 2102) actually are galaxies, and are indeed new.

IC 2091, however, is a group of five faint stars, apparently blurred together into a single "Stellar nucleus surrounded by faint nebulosity" on Roberts's plate. Since he was using a 20-inch reflector, the plate scale must have been rather small. This, poor seeing, and a long exposure on a grainy plate may account for the asterisms that he saw as nebulous. The same thing happened with IC 2100 (which see), a double star also on Roberts's plate though discovered by Bigourdan and properly credited to him by Roberts.

Rather frustratingly, Roberts gives no details about exposure times or emulsion types -- just the sort of thing that would help us to better understand what he was describing on his plates. He does go on about the nebulae on this particular plate, however, noting many of them as spirals, and saying that William Herschel did not see this feature or that in those that were known previously.

I believe that Roberts's plates are now at the Observatoire de Paris. If so, it may be possible to examine them to see if -- as I suspect in the cases of IC 2092 and IC 2096 -- defects are masquerading as nebulae.

Roberts also has an interesting note ending his short paper, saying that many of the faint "nebulae" being discovered on photographic plates by other astronomers are nothing more than stars blurred by seeing ("atmospheric tremors").

― IC Notes by Harold Corwin

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