Telescopes are of two main types – reflectors and refractors. The fundamental difference between both types is the kind of glasses used for light collection. Refractors use lenses while reflectors use mirrors. Going back to school physics, it is obvious that light from distant bodies, like celestial objects, comes at us as a parallel beam. Due to this parallel nature of the light rays, reflector telescopes use parabolic mirrors.
Such a parabolic mirror focuses each ray of light it receives from the original parallel beam at one single point. Because of this nature of the mirror, a reflector type telescope is far more advantageous in comparison to a refractor. A singularly important point to be noted is that reflectors do not suffer from chromatic aberrations which are a common problem for refractors. This is because, unlike refraction which alters the way in which different wavelengths are refracted, mirrors do not affect the wavelengths at all.
A second advantage is that they are relatively less expensive for the same quality of magnification. Furthermore, since light is reflected, only the reflecting surface needs a perfect finish. In the case of refractors, light has to pass form one side of the lens to the other, as a result of which both refracting surfaces need to be perfectly polished.
Astronomers generally prefer reflecting telescopes; however, even these have some disadvantages. The most glaring of these is the difficulty faced in correct alignment of the mirrors. Reflectors are equipped with an open tube, thus the inner optical elements require regular maintenance and cleaning. Without this care, images produced by the telescope end up blurred and indistinct. There are also instances of requiring secondary mirrors to correctly focus light at particular spots. These secondary mirrors can also adversely affect the image quality.
The major disadvantage faced with both refractors and reflectors is that the final image is an inverted image. An easy solution for this fault is the placement of a relay or prism in the passage of light, so that an already inverted image reaches the eyepiece, which again inverts it to produce an erect image. This is because the relay or the prism as the case may be, acts as a light ray inverter resulting in a final upright image.
The above paragraphs mention the use of secondary mirrors in reflectors to change the spot of light focus. These secondary mirrors are smaller than the primary mirror. According to historical accounts, Sir Isaac Newton used a mirror that was flat and inclined at 45 degrees as a secondary mirror. The purpose of this mirror was to shift the reflected light to an eyepiece at the side of the upper portion of the tube. This arrangement is now known as the Newtonian design and still finds takers.
There is another special design used in reflector type telescopes known as the Cassegrain design. This gives the light from the subject a specific path to reflect it better on the primary mirror. With this design it is possible to position the eyepiece at one’s own convenience. Even modern telescopes like the great Hubble Space Telescope make use of this design, and for the advantages discussed above, astronomers lean towards the reflector, and not the refractor.
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