Since the invention of Optical Telescopes in the early 17 century, inventors have developed many different types of telescopes. Using all the same optical principle of refraction and reflection the different kinds of telescopes for amateur astronomers can be divided into three different categories.
Even the types of telescopes may vary in design and build, they all collect light and focus it for the purpose of creating a magnified image of a distant object. Different kind of telescopes have different observation qualities as light behaves differently when it is refracted through a lens or when it is reflected by a mirror.
The refractor telescope is what comes to peoples mind first when talking about telescopes. It is a long tube with a lens (objective lens) at the front end, which is to be pointed in the direction of the desired object and a small lens (ocular lens) at the other end to look into. Invented in the early 17-century, refractor telescopes where the first kind of telescopes. The first to point it to the night sky was Galileo Galilei one of the founders of modern science.
In a refractor telescope, the incoming light beam is bundled and refracted by the lens. This causes the light to be split into its colors like red, green, blue. Different colors have different wavelengths and therefore a different focus on the optical axis. This is called the chromatic aberration, the effect will be visible as a small rainbowlike color space around the star you observing. To be eliminated it needs an elaborate lens construction and various glass coatings. This is what pushes the price of refractor telescopes. The effect becomes less the longer the focal point of the objective lens is.
Through the unobstructed beam path, the refractor is ideally suited for observation of the sun, moon, twin stars and planets.
Small refractor telescopes with good lenses and an aperture of up to 3 to 4 inches are great tools and are superior to small mirrors in every respect. But anything with lenses of more than 5 inches is out of a rational cost-benefit ratio for amateur astronomers. Better than to invest in:
This type of telescope uses mirrors instead of lenses in order to gather and focus light. The most common here is Isaac Newton’s Newtonian reflector which uses a concave primary mirror at the bottom end of the telescope and a small secondary mirror at the top end that projects the beam to an eyepiece.
The light beam is thus not refracted in comparison to the refractor, as a result, the image does not suffer from color aberration. However, in the beam path, the secondary mirror causes an obstruction which has an effect on the contrast.
Reflection telescopes have a variety of associated advantages including the fact that they do not cause chromatic aberration. These telescopes are also much cheaper to build, especially in large sizes. Furthermore, mounting the mirrors is easy as the entire back surface is free for use as an attachment to the mount.
Reflectors telescope allow high apertures and gather plenty of light. When well-made and well-maintained, the reflector can provide a sharp, contrasting image for all forms of celestial objects for a fraction of the cost of an equal aperture refractor.
A Catadioptric telescope also knows as a Compound telescope uses reflecting and refracting elements to produce an image. This clever combination of mirrors and lenses uses the advantages of reflection and refraction to combine their individual benefits in one instrument.
There is quite a number of different designs of catadioptric telescopes. They all differ slightly in the lights beam path and number of lenses or mirrors used. Each having its respective advantage over the other depending on the purpose they build for.
So even the catadioptric telescopes come with a number of advantages, there are also a couple of disavantages.
Possessing both, the characteristics of the refractor and reflector makes the catadioptric telescope a good choice for the amateur astronomer that want a versatile instrument to observe planets and stars as well as dim objects such as star clusters and galaxies.