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All about telescopes: refractors, reflectors and more

A telescope is an optical instrument used to magnify and enhance the view of faraway objects, be they astronomical or terrestrial. Most telescopes fall into one of two main categories: refractor or reflector. Also available are Schmidt-Cassegrain telescopes - a combination of reflector and refractor.

If you are considering buying a telescope you would be well advised to read our Telescope Buyers' FAQ.

Refractor

A refractor telescope uses convex (bending outwards) lenses to collect, focus and magnify light. Rays of light travel through the objective (main) lens where they are focused at the focal length of the eyepiece.

Diagram of a reflector telescope

Reflector

A reflector telescope, on the other hand, uses concave (bending inwards - like a cave) mirrors. As the diagram illustrates, light travels down the tube where it is reflected (hence the name reflector) up to a secondary mirror near the top of the tube, which directs the light into the eyepiece. This exact system is known as a Newtonian Reflector. There are quite a few variations on this including the Georgian and Cassegrain reflectors.

Diagram of a Schmidt Cassegrain telescope

Schmidt Cassegrain

A third type of telescope, which has gained immense popularity over the last 30 years, is the Schmidt Cassegrain telescope (SCT). This type of telescope uses both lenses and mirrors in a compound system as can be seen in the third diagram.

Advantages and Disadvantages

Each telescope has its own advantage, for instance the refractor is better for observing the planets and the moon and the reflector for deep-sky objects (e.g. galaxies). However, the refractor suffers from what is known as chromatic aberration. This is when the different wavelengths of light are brought to focus at different points due to the glass in the objective lens refracting (bending) the different wavelengths to different degrees. This, however, is only noticeable in refractors with an aperture (diameter of objective lens) of 10cm (4") or more. It can be corrected using achromatic and apochromatic (modified) lenses.

This can be very expensive, so very large apertures reflectors are more popular. The third type that I mentioned was the Schmidt Cassegrain telescope. It has grown in popularity because it combines the best of a reflector and refractor. They are very portable as the tube does not have to be as long as that of a reflector - this is because of the light being 'folded over'. They have an ease of use not available with the other telescopes and there are plenty of readily available accessories, e.g. computer controllability. They are most popular in their 20cm (8") guise - I own a 20cm SCT myself. SCTs, with their stable and sturdy tripods, are very useful for astrophotography.

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Author: Marc Delehanty

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