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A Newtonian telescope is a reflector telescope with a flat diagonal secondary mirror (termed a fold mirror) reflecting the light at right angles out of the cylinder of incoming light (it is essentially a prime focus telescope with a fold mirror). It is a simple designed now used for some small telescopes. Isaac Newton used a spherical primary mirror, but the term is also used today if it is parabolic.
The term Newtonian (or name Newton) is also incorporated in the common terms of other telescope designs that include such a secondary fold mirror, such as the Schmidt-Newtonian telescope.