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Refractor

This telescope uses lenses for astronomical observations and is also called a "lens telescope" or "refractor". The refractor is the first optical instrument that was used for astronomical observations by Galileo around 1600.

The refractor uses a lens that is placed in the front of the tube. The light passes through the lens and is deflected to a focal point where the image is formed.
The lens telescope is a simple, sound and optical (almost) perfect instrument. A well-built lens telescope has, in fact, only 1 true optical error, which is indicated by the term "Chromatic aberration".

Chromatic aberration, also known as "purple fringing", is caused because a single lens is not capable to focus all colours in the optical spectrum at the same time. In short, this means that for example one will see a separate blue, a red and a green colour of the moon. Practically this means that if one wants to focus on the blue colour then the green and red will look a bit blurred. If one looks at the moon then this will result in a rainbow halo around the borders of the moon.

To minimize the effects of chromatic aberration two measures were taken about 10 years ago. The first measure was to give the telescope a long focal length to force the light as little as possible "around the corner". The second measure was to utilize two lenses of different kinds of glass. These "Achromatic lens telescopes" make use of a so-called 'Fraunhofer lens'.


Advantages of refractors

- Provides good images of planets
- Good for details of planets and binary stars.
- Affordable when the lens is smaller than 10cm (100mm)

Disadvantages of refractors

- Suffers from rainbow halo effects (chromatic aberration)
- Very expensive for lenses larger than 10cm (100mm)
- Refractor is less suitable to view nebulae, as this requires larger apertures because of the need for greater light collecting.

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