The instrument of choice for many first-time astronomers, refractors offer fantastic views of the night sky.
The magnifying abilities of lenses have been known for centuries. In the late 16th and early 17th Centuries this knowledge was refined and in the hands of a few talented opticians, lenses were combined and the telescope was born. This instrument was then turned on the sky, most famously by Galileo Galilei who observed Jupiter and its moons, the lunar surface and spots on the Sun.
It was well understood that glass could bend (refract) light and that it had a magnifying effect. As optical technology improved so did the telescope, although it remained fundamentally the same; using an objective lens to gather and focus the light and a series of smaller lenses near this focal point to magnify the image. Nowadays, the lenses have become bigger and developments in optics introduced doublet or even triplet lenses; in other words the placement of two or even three lenses close together as the main or objective lens to reduce and correct problems noticed when using a single piece of glass. Primarily, these compound lenses help to reduce ‘chromatic aberration’ (see Jargon Buster boxout). A single lens doesn’t focus all the colours of the spectrum at the same point, but this can be corrected considerably, by using two lenses of different shape and type of glass put close together. This type of telescope lens is called an ‘achromatic lens’, or just an achromat. These are found in just about every type of refracting telescope made today, from the cheapest to the more expensive. The effect of chromatic aberration is to make bright objects appear to have a coloured halo around them. This can be completely eradicated by using a triplet lens, but due to high costs these are only ever used in the more expensive instruments.
Because refractors are particularly good at giving highly magnified and high contrast images, they are ideal for observing the Moon and planets.
If you are thinking of buying one, then there are a couple of things you need to look out for: very cheap refractors have poor quality lenses which manufacturers try to improve by introducing a masking ring a short distance behind the main lens that helps to reduce the false colour effect. It also reduces the effective aperture, so don’t be tempted to buy one of these. Make sure that all the lenses are ‘fully multi-coated’ in the technical specification. This helps to make sure that all the light is passed through the lens system and reduces flares and other unwanted artefacts. Also ensure the focuser is smooth and that it is supplied with a diagonal mirror which makes viewing more comfortable. If eyepieces are supplied, check they are of decent quality. If you are hoping to see stars and nebulas as well as planets, then go for an instrument of a moderate focal ratio. Finally, avoid purchasing a telescope which is too big, making it unwieldy. You’ll see more with a telescope that you can handle. Remember, quality nearly always costs a little more.