Obj is the object’s name. In cases where the object was observed with a narrowband filter, there are two entries, one under the normal name and one with an appended f, like this:
| Obj | S178 | S70 | U178 | U70 | Type | Con | RA | Dec | Mag | PBrt | SBrt | Size |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| M97 | 3C | 3C | 5D | — | PLN | UMa | 11:14.8 | +55:01 | 9.9 | 21.1 | 21.1 | 3.3 |
| M97f | 2B | 2C | 3C | 4C | PLN | UMa | 11:14.8 | +55:01 | — | —- | —- | 3.3 |
The first entry gives my ratings for M97 as seen (or not seen) without a filter. The entry under M97f gives my ratings for the object as seen through a Lumicon UHC filter.
S178, S70, U178, and U70, are my ratings for each object in under suburban and urban skies, using my 178-mm and 70-mm telescopes. The number indicates the difficulty of seeing the object, as follows:
- 1 – very easy, obvious even to a beginner
- 2 – easy, immediately obvious to an experienced observer
- 3 – moderate, may take a little looking
- 4 – hard, need to know where in the field to look
- 5 – very hard, borderline observation, intermittently visible
- A – spectacular
- B – beautiful or unusual
- C – unspectacular but interesting
- D – detectable but nearly featureless
Type is one of the following:
- C/N – cluster with nebulosity
- GAL – galaxy
- GCL – globular cluster
- NEB – bright nebula
- OCL – open cluster
- OTH – other
- PLN – planetary nebula
- SNR – supernova remnant
The distinction between open cluster, nebula, and cluster with nebulosity is fairly arbitrary in many cases; all of the diffuse nebulae in the Messier list have embedded stars, and in some cases (like M8 and M16) it’s not clear whether the Messier designation applies to the nebula alone or to the nebula and the contained cluster.
Con is the official abbreviation for the constellation containing the object.
RA and Dec are the celestial co-ordinates for the object, derived from a variety of sources.
Mag is the object’s integrated visual magnitude (also known simply as visual magnitude or magnitude). This data was obtained from a variety of sources in 2002, and does not necessarily reflect the latest estimates. Magnitudes for nebulae in particular are notoriously unreliable.
PBrt and SBrt are the object’s peak surface brightness and average surface brightness respectively, both in units of magnitude per square arcsecond. These data are described in more detail in my Surface Brightness page.
The inclusion of peak surface brightness was suggested by Brian Rachford, then at the University of Colorado in Boulder, now associate professor at Embry-Riddle Aeronautical Univerisity. It is currently given only for galaxies and globular clusters. For globular clusters, it is Part III, item 10 in the database maintained by William E. Harris of McMaster University. For galaxies it is Rachford’s calculation of the brightness of the central arcminute based on data from the Third Reference Catalog of Bright Galaxies.
Size is the object’s size in arcminutes, again derived from a variety of sources and not necessarily reflecting the most recent estimates.
Tony Flanders Astronomy 
Thanks for the guide. It has been most helpful as I star hop my way through the Messier Objects.
Quick question: What were the approximate Bortle Classes and/or sky quality magnitudes of the sky at your urban and suburban observation sights?
I’ve read through all of your site, I think, and have failed to locate that information.
I appreciate the time and detail you have put into your guide.
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I rate my urban site (Danehy Park in Cambridge) as Bortle 8 by most measures. Though Bortle’s article states that “only the bright Messier objects are detectable with a modest-size telescope,” which obviously isn’t true! But the Milky Way is completely invisible, and M31 is very tough to spot naked-eye. Back then when my eyes were better my limiting naked-eye magnitude was about 4.7, and my SQM typically reads between 17.7 and 18.0.
The suburban observations were done at various locations in Lincoln, MA, which matches Bortle’s description for Class 6 better than his description for Class 7. The summer Milky Way is consistently visible but severely faded, SQM measurements typically around 19.5.
I will try to work this information into the guide in some more natural place. Note that the guide was published before Bortle’s article, which was in turn published before the Sky Quality Meter was invented.
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