Globular clusters were classified by Shapley and Sawyer as being on a scale from 1 to 12 where 1 is very loosely packed central area and 12 is highly packed central area. As a visual astronomer, it seems difficult to judge the concentration of a globular cluster.
Has anyone tried to classify globular clusters concentration classes for something like the Astronomical League Club for Globular Clusters? http://www.astroleague.org/al/obsclubs/globular/globular1.html
It seems that to judge concentration you first need to know what percentage of the diameter of the globular cluster corresponds to the packed core. Visually, knowing the official stated size seems to be required since without that, it is hard to say how concentrated the core is as a glow percentage.
Can any human eye get anywhere near detecting all twelve steps of concentration? It is easy enough to distinguish 3 or 4 levels, but twelve seems daunting.
How can I attempt to properly judge globular clusters on the Shapley and Sawyer scale?