Common names give for deep-sky objects are, like constellation figures, not official. Many objects have several different popular names.
For example, Messier 17, an emission nebula in Sagittarius, has the following popular names:
- Omega Nebula
- Checkmark Nebula
- Swan Nebula
- Horseshoe Nebula
- Lobster Nebula
Many of the same popular names are also given to different objects. There are several pinwheel galaxies, as well as several pinwheel clusters. There are several butterflies, both clusters and nebulae.
Anyone can come up with a fanciful name for an object since they are not official.
Most deep-sky objects are "officially" designated by their catalog numbers. Most objects are listed in multiple catalogs and have multiple designations.
For example, Messier 17 has the following catalog designations:
- M 17
- NGC 6618
- OCL 44
- LBN 60
Since Charles Messier's catalog is the most famous, one of the earliest to list deep-sky objects, and because it contains some of the brightest and most spectacular examples, those objects in this list are usually referred to by their Messier number first. The New General Catalog (NGC) usually comes next in preference when designating an object, and then the Index Catalog (IC). Objects that are not in the Messier, NGC or IC catalogs usually take the designation of the catalog that listed them first, although there are many exceptions.
In this book, abbreviations are usually used for catalog designations, such as "M" for Messier catalog objects, and "NGC" for objects in the New General Catalog.
In wide-field and constellation images where objects are identified, the NGC is dropped from the identification and just the numbers in the object designation are used. For example, in the image map for Orion, NGC 2264 is designated as just 2264. Messier 42 is designated as M42. Sharpless 2 - 264 is designated as Sh2-264. Melotte 111 is designated as Mel 111. And so on.
Here is a list of the catalog abbreviations used in this book, and links, where available, to the original catalogs.