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A Newtonian telescope is a type of reflecting telescope where a large single parabolic mirror acts as the objective. The light from the parabolic mirror is reflected out the side at the top of the telescope to where it comes to a focus and may be observed.
Sir Isaac Newton invented the Newtonian telescope in 1668.  Of the reflector designs the Newtonian is the simplest as only one optical surface needs to be ground. The secondary mirror is simply a flat mirror held at a 45 degree angle in front of the primary mirror.
This design simplicity has made the Newtonian not only the oldest reflecting telescope design, but a very popular one with amateur observers. In general terms a Newtonian telescope will be the least expensive per unit aperture than other types of telescope. The simplicity also allows many amateur astronomers to build their own telescope.
The Newtonian isn't particularly popular with professional astronomers as larger telescopes with longer focal lengths become unwieldy as the eyepiece and sensor mount are at times high above the ground.
- ↑ Freedman, Roger; Geller, Robert; Kaufmann III, William Universe 10th ed.