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Adeep-sky object (DSO) is anyastronomical object that is not an individual star orSolar System object (such asSun,Moon,planet,comet, etc.).[1][2] The classification is used for the most part byamateur astronomers to denote visually observed faint naked eye andtelescopic objects such asstar clusters,nebulae andgalaxies. 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.
Classifying non-stellar astronomical objects began soon after the invention of the telescope.[3] One of the earliest comprehensive lists wasCharles Messier's 1774Messier catalog, which included 103 "nebulae" and other faint fuzzy objects he considered a nuisance since they could be mistaken for comets, the objects he was actually searching for.[3] As telescopes improved these faint nebulae would be broken into more descriptive scientific classifications such asinterstellar clouds, star clusters, and galaxies.
"Deep-sky object", as an astronomical classification for these objects, has its origins in the modern field of amateur astronomy. The origin of the term is unknown but it was popularized bySky & Telescope magazine's "Deep-Sky Wonders" column, which premiered in the magazine's first edition in 1941.[4] Houston's columns, and later book compilations of those columns, helped popularize the term,[5] each month giving the reader a guided tour of a small part of the sky highlighting well-known and lesser-known objects for binoculars and small telescopes.
There are many amateur astronomical techniques and activities associated with deep-sky objects. Some of these objects are bright enough to find and see in binoculars and small telescopes. But the faintest objects need thelight-gathering power of telescopes with largeobjectives,[6] and since they are invisible to the naked eye, can be hard to find. This has led to increased popularity ofGoTo telescopes that can find DSOs automatically, and largereflecting telescopes, such asDobsonian style telescopes, with wide fields of view well suited to such observing.[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 outsidelight polluted urban locations.[8] To cut down light pollution and enhance contrast, observers employnebular filters, which are designed to admit certain wavelengths of light and block others.
There are organized activities associated with DSOs such as theMessier marathon, which occurs at a specific time each year and involves observers trying to spot all 110 Messier objects in one night. Since the Messier catalog objects were discovered with relatively small 18th-century telescopes, it is a popular list with observers, being well within the grasp of most modern amateur telescopes. TheHerschel 400 Catalogue is also a popular list with observers and is considered more challenging; it was designed for largertelescopes and experienced amateur astronomers.[9]
There are many astronomical object types that come under the description ofdeep-sky objects. Since the definition is objects that are notSolar System objects or individual stars, examples include:[10]
