This telescope uses mirrors for astronomical observations and is also called a "mirror telescope" a "Newtonian telescope" or "reflector". The first practical application for astronomical observations was developed by Sir Isaac Newton in 1668.
The reflector uses a concave mirror that is placed in the back of the tube. The concave mirror reflects the light back to a much smaller mirror that is placed in the middle or front of the telescope. The little mirror then reflects the light to an eyepiece which in turn allows you to observe the universe. As a reflector utilizes a small mirror that is placed in the middle of the telescope it is said that the reflector performs not as well as the refractor because a small part of the light is blocked.
The real reason why reflectors sometimes have a lesser reputation than the refractor is because the building quality. The reflector has only 1 optical active element present (the mirror), which means that this mirror has to be of excellent quality. A refractor (lens telescope) has multiple lenses which means that if the first lens does not perform perfectly this can be corrected by the other lenses. With a reflector (mirror telescope) this is not possible because all the light is reflected by just one mirror. More expensive telescopes will have better and bigger mirrors than cheaper telescopes.
Advantages of reflectors
- Reflectors are cheaper than refractors (if they have the same aperture)
- Reflectors do not have chromatic aberration (colour shifting) while refractors do.
- Good for deep sky observations
Disadvantages of reflectors
- The telescopes need to be collimated once a while (line up the focal distances of the mirrors)
- Because the reflector uses a very small mirror in front of the telescope (secondary mirror) this causes a little bit of light loss which makes it refractors (lens telescopes) better for planetary observation