Key to the Messier Guide Tables

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.

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