The main distinction when buying a telescope is deciding between reflectors or refractors. Despite the pros and cons of each telescope type, there are some basic facts that you should consider when choosing a telescope. The first question you should ask yourself before buying a telescope is: "What do I actually want to observe?" Or more specifically: "Do I want to mainly observe planets or mainly observe deep sky objects such as faint galaxies?"

If you are clear on this issue, you can proceed directly to step two: the actual choice of telescope. Not every telescope is suitable for all purposes. The ultimate telescope that meets all your observing requirements does not exist, but you can get very close to it.

Light gathering power 

An important property of telescopes is their ability to gather light, as the more light a telescope can collect, the fainter the objects that can be seen in the night sky. And there are a lot of these faint objects out there in the depths of the universe. Ignoring the observing of the Moon, the Sun and the bright planets, the aperture of a telescope is its most important feature. This does not mean that small telescopes cannot have a decent performance, as every telescope has its use.
The larger the aperture of a telescope, the ‘faster’ it is. The following is a comparison of the various apertures:

Light, aperture and faint stars

The smallest telescope is the human eye, which has a maximum aperture of 7mm. With a 7mm aperture we can observe stars down to a limiting magnitude of 6. The magnitude of a star is a unit indicating its brightness: the smaller the number, the brighter the star. Stars of magnitude 1 are among the brightest stars. A 6th mag star is 100 times fainter than a mag 1 star. One can see from this that our eye can see relatively poor light sources. But it is nowhere near sensitive enough to observe fainter astronomical objects.

If we assign a light collecting ability of 1 to the human eye, you can marvel at how this ability increases with increasing aperture. A telescope with a 50mm aperture has a light collecting ability of 51 times, and a lens with a 100mm aperture collects 204 times as much light as the human eye.

Also important and of interest is the magnification of a telescope. The small telescopes sold in supermarkets are often advertised as providing a magnification of 500 times or more. The most amazing astronomical images are also shown on their packaging, giving the impression that you hold a small Hubble telescope in your hands. These are promises that do not hold up and you would be quickly ‘brought back to earth’.

In general, one can say that the maximum useful magnification of a telescope is twice its lens aperture in millimetres. So a telescope with a 150mm aperture should support a maximum magnification of 300 times and a 200mm aperture telescope a maximum of 400 times. If you increase the magnification any further, you risk a dimmer and more blurred image. The magnification of a telescope is not the most important thing, but rather its aperture and the resolution it can achieve thereby.

It is also extremely important to consider the mount when buying a telescope, as even the best instrument is useless if the mount does not do its job in keeping the telescope stable. If the mount is undersized for the weight of the instrument, then it will be very vulnerable to vibration and the result will be that you will have no real fun using your telescope. If you intend to do astrophotography, then the mount you use must be even more capable and be even more stable.

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