Performance Criteria of a Telescope

When looking at telescopes, it can be very easy to become lost in all the arcane terminology, random figures and flashy sales pitches. Telescopes are more than just simply tubes that make things look closer – they are precision scientific instruments designed for one main function, be it lunar astronomy, studying star clusters, or looking in to deep space to find new stars and planets. When looking for a telescope, there are certain performance criteria that you should take in to account, and use these to help finalize your purchasing decision.

Firstly, look at the light-gathering power of the objective lens, or the mirror in a reflector scope. In a refractor or a catadioptric, this is usually the largest lens, found at the front of the scope, and determines the amount of light allowed into the scope. The larger the lens, the more light is allowed in and the clearer the image will be. The drawback of this is that as the lens size increases, so does the size of the telescope needed to house it.

The second criterion is magnification power. The theory behind this is simple - the higher the magnification power of the scope in question, the further the distance visible. However, as you zoom progressively closer, the field of view becomes more restricted, and the image may not be quite as clear as a lower magnification could provide. A high magnification is therefore not necessarily a better option, particularly for near-space stargazing and lunar astronomy. Finding a happy medium is therefore a key part of choosing a good telescope. Many cheaper factory telescopes will make bold claims to 400x magnification and other such improbable numbers – while this may be strictly true, what they DON’T tell you is that at such magnifications even the most distant star appears as a hazy smudge because the optics are invariably built to a very low quality, and cannot process the magnified images correctly.

Lastly, consider resolution. This is the ability of the scope to define and discern markings and other visual aspects of interest at varying distances. As mentioned above, a 400x scope will probably have minimal resolution at that power. What is needed is a scope that matches clear, crisp imaging with enough magnification to do the job required of it. Remember that in just about every case, bigger does not mean better when it comes to buying a telescope.

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