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Choosing a telescope, especially the first one, is always a very important question. What will you be able to see, almost all your emotions and comfort depend on the instrument, so you have to choose it carefully.

The most important thing, I suppose, is the quality of optics. You will not get any pleasure from using a scope with wide-diameter, but not very good mirror or lens. So always try to get the best thing you can afford, and there's some reason in buying less smaller, but more qualitative telescope.

 

Optical system

First of all, you shold know that all telescopes are divided into three main groups: refractors (those use lens as a primary optical element), reflectors (mirror), and catadioptric (combining lens and mirror). Reflectors are easiest to produce (there's only one surface to be made), and, consequently, the cheapest. Almost all large telescopes are reflectors.

Refractor

Reflector

Catadioptric

There are hot discussions of choosing a type of scope. I'm not going to cite all the things, just name some most important facts:

• Reflectors are cheaper => you can get bigger diameter and see more objects (see below)
• Reflectors have less weight
• Reflectors provide upturned image. There's practically no difference for vieving celestial objects, but this may be inconvinient for ground observations.
• Refractors are less sensitive to humidity, temperature, there's less air turbulence in the tube
• Refractors have practically no reflected light inside their tube, so the sky seems darker and you can distinguish more faint objects
• Refractors do not have central mirror that screens a part of the objective, so they utilise almost 100% of following light (except percents of light reflected and absorbed in lens)
• Catadioptrics provide the best image
• Catadioptrics are the most compact scopes, and the most expensive

So, for a beginner, I'd recommend a following rule:

• If you are going to buy a telescope with a diameter less than 100mm, take refractor, greate than 100 mm - take reflector.

As well, if you are going to buy a telescope with aperture greater than 150mm, you may take a catadioptric system (or a reflector, if you cannot afford catadioptric).

 

Objective diameter (aperture)

This is a diameter of your telescope primary optical surface (a lens or a mirror). The bigger is it, the more light collects your telescope, and the more faint objects become available to distinguish.

For a beginner, optimal diameter is 110-150 mm. However, if you are eager for watching deep-sky objects, like nebulaes, galaxies etc, take as greater aperture as you can.

 

Telescope mount

A mount is a system that support your telescope and allows its pointing into different direction. It consists of a stationary part (usually a tripod or a pole) and a moving one.

There are two main types of mounts: equatorial and azimuthal. The last one is a simple structure that allows you to rotate the scope around vertical axis - to different corners of earth, and point it higher or lower. It's very easy to use and understand. However, it requires constant adoption of telescope direction because of rotation of the sky.

Equatorial mount is free of this weakness. It has an axis pointed to the celestial pole (e.g., nearby a Polar Star in Nothern hemisphere). As you know, all the sky rotates around this pole, so rotating the scope around polar axis with the speed of Earth rotation (~7.29  × 10⁻⁵ radians per second) will keep the scope pointed to the same point of the sky all the time.

Look here for more info: Equatorial coordinate system, Celestial coordinates, Equatorial mount.

 

I'd strongly recommend to purchase an equatorial mount. An azimuthal mount may be suitable only if you are plannning to do ground observations as well.

There are several typical mounts on the market. For amateur scopes there are EQ1, EQ2 and EQ3. EQ1 is the cheapest, but it's sensitive to vibrations. For most scopes an EQ2 will be optimal.

 

 

These three parts are constituitive for a scope. As well as there are another things, such as a set of oculars, aceessoires, finder, hour drive, authomatization etc. But those are a theme for a separate article.

 

© S.Tushev, 2010. No copying allowed without the permission of the author.