Different Types of Telescopes
There are three main types of telescope – refractor, reflector, and catadioptric.
The most common type of telescopes are called refracting telescopes. This name also covers most binoculars, camera lenses and even glasses. Refractors work by bending light through a primary convex lens known as the ‘objective’ lens, and forcing the redirected light rays to converge at a space known as the ‘focal point’. From here, the light rays are focused – often by a prism or a second lens – into the eyepiece, where they are usually refracted again to form a prefect replica image. The telescope made famous by Galileo was a 30x magnification refractor telescope, albeit a very simplistic one. Refractor telescopes are great starter scopes, and are ideal for using in urban or heavily light-polluted environments.
A reflector telescope works on an entirely different principle. Instead of using lenses to capture and focus light, reflectors feature large-aperture “light buckets” or tubes that funnel light down the shaft towards a mirror, which then reflects the light into an eyepiece consisting of several different types of lenses. Reflector telescopes need lots of space and little background light, so they make ideal scopes for using in rural environments.
A catadioptric telescope, also known as a ‘compound’ telescope, uses a combination of mirrors and lenses to increase the scope’s focal range and power while keeping weight, length and bulkiness to a minimum.
The most popular type of catadioptric telescope is the Schmidt-Cassegrain, used by amateur astronomers the world over. Based on Bernhard Schmidt’s original camera lens design, scopes with this configuration feature a corrector plate as the first optical element, which is then figured by placing a vacuum on the internal side, and grinding the lenses and mirrors to the exact specification and angle required to correct the spherical aberration caused by the primary mirror. Many variations of the Schmidt-Cassegrain configuration exist, but in general they can be divided into two principal design forms: compact and non-compact.
In the compact form, the corrector plate is located at or near the focus of the primary mirror. In the non-compact version, however, the corrector plate is moved back towards the center of the primary mirror. This has the effect of keeping the corrector at the center of curvature of the primary mirror.
Catadioptric scopes are expensive, but operate well in many different environments. They make great entry-level scopes, and many people find that they stick with catadioptrics throughout their astronomical careers.
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