I remember learning a long time ago, that there were just two types of astronomy telescope, refractors and reflectors.
In recent years however, various hybrid types have become available, which attempt to combine the best features of the two traditional types. But we will discuss more about hybrid designs in a moment.
Refracting telescopes use a main lens made from glass and called the objective lens. This lens bends (or refracts) the incoming rays of light as they pass through it, so they come to a focus. This was the type of telescope that Gallileo famously used back in the early 1600′s to make the first astronomical observations of the Moon and Jupiter.
Reflecting telescopes on the other hand, do not use an objective lens, instead they use a main or primary mirror. The surface of this mirror is specially shaped into a concave curve called a parabola, which has the handy property of reflecting all incoming light rays to a focus.
Let’s look at these two traditional types in more detail.
Refractors
Refracting astronomy telescopes have a main lens ranging in size from about 60mm (2.5″) in diameter and then on upwards. The cost increases quickly, as it is expensive to make good quality lenses, so it is rare to see an amateur instrument with a size greater that 150mm (6″).
Refractors have the advantage of allowing all the light collected by the objective lens, to pass unobstructed through the telescope to the eyepiece at the other end of the tube. Consequently, even small diameter instruments can work well. A decent quality 70-80mm refractor is still an excellent telescope for amateur use.
The problem with refractors however, is so-called chromatic abberation. This simply means that false colour can be introduced to the image you are observing. This happens because the different colours of light within white light, are bent by the lens through slightly different angles, resulting in slightly different focal points.
This used to be a big problem until it was discovered that by sandwiching two (and more recently three) lenses into a doublet or triplet lens, and also by using special glass, these false colour effects in the image could be removed.
Refractor telescopes with colour correction are called achromatic and the best ones, using the multi-element lenses are called apochromatic.
However, the manufacture of these clever lenses is expensive, although the prices of achromatic refractors has come down greatly in the last few years because of Far Eastern factory production.
Reflectors
The reflecting telescope was invented by Sir Isaac Newton in the early 1700′s. As we have said, it uses a mirror to focus incoming light.
This has two big advantages – firstly mirrors are cheaper to make than lenses and you can make them in large sizes, and secondly, mirrors do not have the problem of false colour.
As a result, you can get a much bigger reflector for your money and they have always been very popular amongst astronomers. The largest telescopes in the World all use mirrors.
The disadvantage however, is that you need a bigger reflector to get the same light-gathering power as a refractor because it is necessary to use a second mirror (the secondary) to divert the focussed light beam into the eyepiece. This second mirror within the telescope tube, blocks out some of the light.
Hybrid Telescope Designs
Recently, we have seen the emergence of hybrid designs which combine mirrors and lenses. These catadioptric reflectors are compact telescopes that are capable of producing high quality images, while being available at reasonable cost (typically a few hundred £ or $).
These hybrid telescopes use a lens called a corrector plate at the top of the tube, together with a primary mirror at the bottom. Mounted inside the tube is a secondary mirror. This reflects the light a second time back down the tube and out through a central hole in the primary mirror, and into the eyepiece (or other imaging device).
There are a few disadvantages, but they usually work very well and have consequently become very popular amongst amateur astronomers.