Reflector telescopes are mainly open telescopes, meaning that the mirrors are exposed to the air, humidity and dust. With such a geometry, you will never manage to focus perfectly the image of a star with your reflector, because of spherical aberration a delicate situation considering that astronomy requires to observe and photograph faint and diffuse celestial objects. More often, the low-cost manufacturers do not use nor a hyperbolic neither a parabolic mirror but a spherical mirror. However, a parabolic mirror is facing a defect: the coma aberration which deforms and elongates the star around the fields of view. In fact, such a mirror is relatively expensive and telescopes manufacturers choose rather a parabolic mirror instead, far simpler to build. Theoretically, getting a perfect round dot of a star requires having a newtonian reflector made with a hyperbolic primary mirror. However, a big mirror could quickly emphasize the optical aberrations of the telescope. The bigger the mirror is, the brighter the objects appear in the eyepiece. The asset of a reflector is its primary mirror’s very large size. Therefore, a secondary mirror is installed next to the front aperture of the telescope, enabling beams to be deviated on the side of the telescope, and so, to observe an image. Here, it is necessary to find a way to make the light beams going out of the tube. It has to collect and make the light beams converging towards the eyepiece holder, the element where we put our eyes. This primary mirror is the master piece of the reflector. The light coming from a star goes inside the optical tube and is first reflected on the primary mirror, located at the extremity. The newton telescopes are the most widespread reflectors in the market because of their easy building process and their low cost. Reflector telescopes Principle of a reflector They are a lot of differences between both of these categories, in terms of performances, durability and especially optical quality. The reflector telescopes are composed of mirrors whereas the refractor telescopes are only made of lenses. What is a telescope? This question could seem trivial but behind this universal word, we find two main types of instruments to observe the tremendous objects of our starry sky.
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