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Deep sky object

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.

A deep sky object: the peculiar spiral galaxy ESO 510-13 in Hydra. Its equatorial dust plane is considerably warped. The galaxy is 170 million light-years from earth and 100,000 light-years across.
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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.