There are so many types of telescopes that it is not possible to list them on one page and avoid forcing you to scroll down for ages. That’s why we’ll stick to the most popular ones.
There are two main telescope classes: Refractors and Reflector. After we cover the basic characteristics of each type, we’ll go over their main specifications, what’s important and what’s just a sales pitch.
All refractors have very simple design. One large lens in front of a tube (the objective lens) and one at the back (the eyepiece). The reality is a bit more complicated. The objective lens is usually a composite of two or three pieces to help reduce/remove the distortions of the image, which we call aberrations (mainly the chromatic one). The eyepiece is constructed in similar manner. Nevertheless, the general concept remains the same.
Reflectors don’t have lenses. They have mirrors. No matter what kind of refractor we’re talking about, they all have a concave main mirror. Let’s start with a simple refractor.
Of all Refractors the Newtonian type has the most basic design. Aside from the main mirror there is a secondary flat one that redirects the focused light into the eyepiece. A disadvantage of the Newtonian system is that one side of the tube is always open and dust could easily reach the mirrors. Also, because of its design, large focal lengths are usually avoided for the tube could become too long. Although depending what you’re going to use it for, it might not necessarily be a big drawback. On the plus side the simple design makes the Newtonian one of the cheapest telescope types.
A subclass of the reflectors is the catadioptric. Such telescopes include a refractive element in their scheme. The most famous example is the Cassegrain telescope. Here are the two main variations.
The Cassegrain telescope consists of a concave main mirror and a convex secondary. In the case of Maksutov-Cassegrain system there is a correction plate (a carefully designed “lens”, which two surfaces have different curvature radii) to reduce the aberrations.
The design is similar to the Maksutov-Cassegrain. The only difference is in the shape of the correction plate. In the case of the Schmidt-Cassegrain it has more complex design. The correction plate weights much less than the one in the Maksutov systems, which is the reason most Cassegrains with large aperture have Schmidt correction plate.
Because both Cassegrain systems have a convex secondary mirror in their designs, the tube is fairly compact even with large focal lengths. It’s sturdier than the Newtonian systems and requires less (or maybe none if handled with care) collimation. However, the Cassegrain systems are much more expensive than the simple Newtonians.
There are a few specs you’ll definitely need to know about.
Aperture. That’s the diameter of your mirror. The aperture determines the light gathering power of your system. The more light the telescope can collect the better.
Focal Length. The focal length controls the size of your target in the focal plane (where you place a camera or an eyepiece) of the telescope. Do not, I repeat, do not be fooled by large focal lengths seen in telescope ads. The larger the focal length the less bright your image would be. You’ll need to balance it with the aperture. See the next of the specifications.
Focal Ratio. Divide the focal length by the aperture and you get the focal ratio, a.k.a f-number or f-stop (e.g. f/10). The best f-number would depend on what you want to do with your system. If you would hunt comets you’d need a light bucket with f/2-f/4, for planets you’d want something like f/15 (give or take several f-stops).
Magnification. This is another thing to avoid when you are buying a telescope. Whenever you see an ad boosting the system’s magnification, run away. The magnification does not play any role in the quality of the telescope. None whatsoever. The magnification of the system is simply the focal length of the mirror/lens divided by the focal length of the eyepiece. Nothing more.
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