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Optical telescopes are divided into three types.
- Galilean refracting telescopes (also known as dioptrics)
- Newtonian reflecting telescopes (also known as catoptrics)
- Catadioptrics (i.e. Schmidt-Cassegrain, and Maksutov-Cassegrain)
Galilean or refracting telescopes employ the refractive properties of light, an d are constructed of lenses. These can be used for both terrestrial and astronomical viewing.
Newtonian or reflecting telescopes employ the reflective properties of light, using a concave paraboic primary mirror to collect and focus incoming light onto a flat secondary (diagonal) mirror that in turn reflects the image ot of an opening at the side of the main tube and into the eyepiece.
Catadioptrics (generally referred to as Cassegrains) use a combination of mirrors and lenses to fold the optics and form an image.
Binoculars are two identical or mirror-symmetrical telescopes mounted side-by-side and aligned to point accurately in the same direction, one to be viewed through each of the user's eyes to present the viewer with a greater sense of depth and distance than a single-lens telescope allows.
Relatively small, single-tube telescopes are often called monoculars to contrast with binoculars. Unlike a monocular telescope, a binocular gives users a seemingly 3-dimensional image: the two views, presented from slightly different viewpoints to each of the viewer's eyes, merge to produce a single perceived view with a sensation of depth, allowing distances to be estimated. A binocular is also more comfortable for viewing, as it negates the need to close or obstruct one eye to avoid confusion. It is also easier and more comfortable to steadily hold and move a pair of binoculars than a single tube, as the two hands and the head form a steady 3-point platform.
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