During the years from 1758 to 1782 Charles Messier, a French astronomer (1730 - 1817), compiled a list of approximately 100 diffuse objects that were difficult to distinguish from comets through the telescopes of the day. Discovering comets was the way to make a name for yourself in astronomy in the 18th century -- Messier's aim was to catalog the objects that were often mistaken for comets. (from the SEDS Messier Catalog)
The Messier list serves as an, admittedly imperfect, best deepsky objects list for viewers in the northern hemisphere. It leaves certain objects out that should be included. It includes objects that probably shouldn't be there. Still, beginning amateur astronomers who complete this list earn the right to drop the term "beginner."
"This list is used by members of the Saguaro Astronomy Club of Phoenix, AZ. for the Best of the NGC achievement award." Complete this list, and you are a seasoned observer. Again, biased for northern viewers.
The definitive southern list, Bennett, like Messier, established the list to aid comet hunters. It leaves out the Keyhole Nebula, but covers most of the other significant deepsky objects south of the celestial equator. Of the 150 objects in the list, 26 overlap with the Messier listing.
This is a somewhat quirky list by Patrick Caldwell-Moore. Numbered north to south, the objects visible to northern observers overlap surprisingly little with the 110 Best of the NGC. As the 110 Best is a well thought out list, this makes the Caldwell list more difficult than the all the lists mentioned above. The last 40, or so objects represent a kind of "greatest hits" of the deep south.
This list makes no claims to usefulness. I began it as a list of interesting objects not in the any of the other lists, but it evolved into a listing of all the other objects in the Deepsky Atlas. Thus, it includes many objects like faint galactic companions to brighter galaxies. You need a 12" telescope to see some of these.
If you have any questions about the Hawaiian Astronomical Society