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. The reflector telescopes are composed of mirrors whereas the refractor telescopes are only made of lenses. They are a lot of differences between both of these categories, in terms of performances, durability and especially optical quality.
Principle of a reflector
The newton telescopes are the most widespread reflectors in the market because of their easy building process and their low cost. The light coming from a star goes inside the optical tube and is first reflected on the primary mirror, located at the extremity. This primary mirror is the master piece of the reflector. It has to collect and make the light beams converging towards the eyepiece holder, the element where we put our eyes. Here, it is necessary to find a way to make the light beams going out of the tube. 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.
The asset of a reflector is its primary mirror’s very large size. The bigger the mirror is, the brighter the objects appear in the eyepiece. However, a big mirror could quickly emphasize the optical aberrations of the telescope.
Optical quality of reflectors
Theoretically, getting a perfect round dot of a star requires having a newtonian reflector made with a hyperbolic primary mirror. In fact, such a mirror is relatively expensive and telescopes manufacturers choose rather a parabolic mirror instead, far simpler to build. However, a parabolic mirror is facing a defect: the coma aberration which deforms and elongates the star around the fields of view.
More often, the low-cost manufacturers do not use nor a hyperbolic neither a parabolic mirror but a spherical mirror. 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.
Reflectors in practice
Reflector telescopes are mainly open telescopes, meaning that the mirrors are exposed to the air, humidity and dust. This is why they require to be manipulated with precision and attention. For example, a mirror frequently exposed to this harsh environment could be less reflective within years, or put it in a different way, its ability to reflect light decreases. To this point, the cleaning of the mirror is highly recommended, paying extreme attention to the fragile optical parts while dismounting the telescope.
A key element not to be forgotten with a newtonian-like telescope is the necessity to collimate it. This procedure allows you to perfectly adjust the alignment of the telescope and must be done before each observation or astrophotography session. This should be ideally carried out each time you observe or start an astrophotography session.
Finally, reflector telescopes are first choice instruments when you want to collect the most light as possible. But the side effects are that you will need to know in details the optical system and should not be shy when you will have to modify or clean the mirrors, as mentioned above.
Pros & Cons
- Large mirror = better light collecting capacity
- No chromatic aberrations (colored fringes around stars)
- Relatively low cost
- Optical quality often disappointing
- Collimation and mirrors cleaning processes
- Open tube = high vulnerability to dust, topdeblogs.com
- Bulky and heavy
The principle of keplerian telescope is very similar to a monocular. The light goes through the front lens, key element making the light beams
converging to the eyepiece holder, where we install an eyepiece or a camera. Because of their compactness and ligthness, refractor telescopes do not collect as much light as reflector but have a more stable optical quality and do not need any adjustement from the user/observer.
Optical Quality of the reflectors
They have the specificity to let us observing the starry sky with an amazing sharpness and contrast. These features are really appreciated in astronomical observation and astrophotography. Nonetheless, keep attention on what type of refractor you choose. The cheapest are made of a single lens which undergoes the light dispersion. Consequently, a star will not be a single color point anymore but surrounded by colored rings. This is what we call chromatic aberrration.
Today, there are different manners to get rid of this optical defect such as adding a second lens to obtain a Doublet telescope.
Stellina : a refractor inspired by astrophotographers
Most of amateur astrophotographers prefer a telescope whose durability, compactness and simplicity to use are better than the amount of light collected. Therefore they choose refractor telescopes rather than reflectors.
Vaonis has responded to this demand and built a refractor telescope from scratch, helped by one of the most reknown metrology laboratory in France : AiryLab.
The optical design is a Lanthane ED Doublet, permitting to decrease drastically chromatic aberration. A special treatment on the lenses has been applied in order to select only the wavelength of interest and reject all ultraviolet and infrared lights.
All these optimizations brought into this telescope are just contained in a small, transportable and entirely automated refractor. Refractors are thus a reliable choice for stargazers who want a dependable, pratical and easy-to-use astronomical instrument.
Pros & Cons
- Impressive contrast and sharpness
- Light and transportable
- Closed tube = protection against humidity and dust
- Maintenance and cleaning almost nonexistent
- Small diameter = less light collected
- Chromatic aberrations
- Higher price