People have used telescopes since the early 1600's.
A reflecting telescope uses mirrors to reflect light, as opposed to refracting telescopes that use lenses. Although this makes the reflecting telescope more expensive, they also tend to experience fewer technical problems. Reflecting telescopes come in various designs, some of which use flat mirrors to reflect light or the focal plane back to the user.
Design Types
Reflecting telescopes come in one of three designs. In the prime focus, the astronomer sits inside the telescope where the reflected light is focused. The Cassegrain focus uses a hole in the primary mirror to reflect light. The Coud focus uses a flat mirror near the bottom to reflect light up and to the side.
Coud Focus Types
Coud Focus reflecting telescopes come in four types. The Newtonian uses a parabolic mirror, with a flat mirror to reflect the focal plane up the tube and to the side. The Cassegrain and Maksutov don't use a flat mirror, but rather use a spherical mirror to reflect light and correct distortion caused by the primary mirror. The Celestron uses a flat mirror to reflect light back to a finder scope that runs with a main scope to the same eyepiece.
First Use of the Flat Mirror
Sir Isaac Newton is credited with putting the first flat mirror inside a reflecting telescope in 1668. Newton realized that this could lead to a more comfortable viewing position.
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