Refracting Telescopes

The Refracting Telescope (also known as the refractor) precedes the Reflecting Telescope and was invented in 1608 by three Dutch spectacle makers. It was first used for astronomical purposes by Galileo a few years later, and it is still one of the most telescope designs due to its simple construction and ease of use.

The Refracting Telescope consists of a long hollow tube with a large lens, known as the objective lens, situated at the front end and an eyepiece at the opposite end. Starlight enters the telescope through the objective lens which gathers it and focuses it into the eyepiece. The eyepiece magnifies the image thereby making distant objects appear closer.

The Refracting Telescope has several advantages:

• its simple construction makes the refractor more sturdy and is therefore less liable to go out of alignment than a Reflecting Telescope; indeed a refractor can go many years without needing any realignment
• small refractors are extremely portable and very easy to set up, an important consideration if you need to travel long distances to reach a dark-sky site
• the telescope tube assembly is sealed at both ends (by the lenses), therefore providing protection against dust and moisture
  

Some disadvantages of Refracting Telescopes:

• all Refracting Telescopes suffer from chromatic aberration, which leads to false colour appearing around images, due to the fact that they use lenses to focus light
  
• small errors in the grinding of the lens can result in distorted images
• larger telescopes are less portable

Chromatic aberration (false colour) occurs because the objective lens focuses different frequencies of light at different distances. In the achromatic refractors the lenses are specially designed to reduce this effect; most refractors available to buy are achromatic refractors. Apochromatic refractors go even further and using exotic glasses they nearly eliminate chromatic aberration altogether; they are thought by many to provide the best image quality of all telescope designs, however they are generally very expensive (eg upwards of $500/£250 for a small 80mm refractor) and for that reason are not recommended for a first time buyer.

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