Generally these objects are not visible with the naked eye, but the brightest can be seen with a small telescope or even with a good pair of binoculars.

The Messier index (1774) lists a total of 110 deep sky objects and NGC (New General Catalogues, 1880) list nearly 8000 objects. They are especially used by astronomer amateurs as simple collections of objects to observe or to test their equipment and their observation skills.

The Messier marathon consists in the observation of the 110 objects in a single night (possible only in March or April). More difficult and dedicated to larger telescope owners is the observation of the William Herschel 400 objects catalog (1864).