Deep sky object
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Deep sky object (DSO) is a term used often in amateur astronomy to denote objects in the night sky other than solar system objects (such as planets, comets and asteroids), single stars and multiple star systems. Generally these objects are not visible with the naked eye, but the brightest of them can be seen with a small telescope or even with a good pair of binoculars.
Types of DSO's:
These are classified by the Messier catalogue of 110 objects and the much more comprehensive New General Catalogue which contains nearly 8000 objects. Many sets of these and other objects from more specialised catalogues such as the UGC are used by amateurs as a test of their observing skills and their equipment. The so called Messier marathons occur only at a specific time of year when observers try to spot all 110 objects in one night. A much more demanding test known as Herschell's 400 is designed to tax larger telescopes.es:Objeto del espacio profundo fr:Objet du ciel profond it:Oggetto del profondo cielo nl:Deep Space Object zh:深空天體