Last week I observed M101 with my 16" from Cherry Springs and spent quite some time marveling at this object, which may be the most detailed one outside the Local Group. One of its 10 regions that bear individual NGC designations, NGC 5453, eluded me on that night. This compact knot is a Lord Rosse discovery, and as such would require looking for it specifically using a photographic chart. I was moving generally from east to west, and in the manner of a Messier marathon forgot to look for this little knot when I was in the area. Unlike the Herschel discoveries such as NGC 5461 and ’62, the Rosse regions do not jump out at you. 5451 was bordering on difficult even once I located it using the chart. The strongly elongated region that marks the southern end of the chain that is the western arm should be the combined glow of NGC 5447 and 5450. The latter is counted as a Lord Rosse discovery and the former as a Herschel discovery. In photographs, they are separated by a narrow dark lane, NGC 5447 being the smaller, brighter northern component, which by itself is not nearly as elongated north to south. My observation makes me think that the original Herschel discovery more likely was the combination of the two. Also I observe that three perfectly nice star clouds, one of them even farther away from the core than NGC 5458, did not earn NGC designations.
As the chart, I was using the annotated negative photo from the Stoyan Messier Atlas (Cambrigde University Press). It identifies a number of starlike Hodge HII regions, which the author could detect visually with his 14”. I was, in retrospect, somewhat unprepared to properly read this chart. I could only detect two Hodge regions unambiguously, as became clear later when I compared my observations with the high-resolution photographic charts in the original publication, Hodge et al. ApJ 1990. These are labeled on my sketch as H1216 and H327. These are particularly bright and isolated individual HII regions. A typical NGC object in M101 corresponds to or contains an agglomeration of a very large number of individual Hodge HII regions. The same can be said of at least two of the three anonymous star clouds in my sketch. The vast majority of the Hodge regions are starlike even on high-resolution photographic charts; the exceptional ones, such as 1216 and 327, which are not dots are nonetheless completely round and look like photographic images of bright stars, i.e. they could actually be all starlike even photographically. What I label as H327 using the catalog number from the 1990 paper is the same as what is labeled HK371 on the Stoyan chart, 371 being its number in the older, smaller catalog by Hodge and Kennicutt. I am imagining that a serious, dedicated hunt for Hodge regions in M101 would be a nice way to kill time.