Deep-sky object

[1][2] The classification is used for the most part by amateur astronomers to denote visually observed faint naked eye and telescopic objects such as star clusters, nebulae and galaxies.

This distinction is practical and technical, implying a variety of instruments and techniques appropriate to observation, and does not distinguish the nature of the object itself.

[3] As telescopes improved these faint nebulae would be broken into more descriptive scientific classifications such as interstellar clouds, star clusters, and galaxies.

[7] Observing faint objects needs dark skies, so these relatively portable types of telescopes also lend themselves to the majority of amateurs who need to travel outside light polluted urban locations.

The Herschel 400 Catalogue is also a popular list with observers and is considered more challenging; it was designed for larger telescopes and experienced amateur astronomers.