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Highlands Astronomical Society


The refracting telescope uses a lens to focus the light. This lens is placed at one end of a tube with the focuser and eyepiece at the other end of the tube.

Most refracting telescopes use a cemented ‘doublet’, a lens consisting of two components, to reduce chromatic aberration which is prominent in this type of telescope. Such scopes are commonly referred to as achromatic refractors. To completely get rid of chromatic aberration, however, you need more than two lenses, or you may want to use lenses of different materials such as fluorite. Such telescopes are called apochromatic refractors. The latter type is not very common yet, due to the fact that they can be extremely costly.

Most refractors use a small diagonal mirror between the focusser and the eyepiece to make it easier to look through the scope and avoid neck strain. The disadvantage is that it mirrors the image, making it more difficult to recognise features on the Moon, for instance. Because a refractor turns images upside down, if you want to use the scope for terrestrial work, you will require additional lenses. Some scopes have these lenses built into the tube assembly and are usually less suitable for astronomical viewing. These additional lenses often reduce the field of view and cause some additional loss of light.

Refractors are very robust in the sense that they do not normally require adjustment of the optics in order to put them back into alignment. Another advantage is that they have a closed construction, so dust cannot gather inside the tube. Refractors also don't take as long to cool down to ambient temperatures before you can use them effectively, and suffer less from tube currents.

Achromatic and apochromatics lenses are quite expensive to produce, and refractors are therefore the most expensive type of telescope in relation to their aperture. They do, however, give the sharpest image possible per mm of aperture. Some apochromatics telescopes can be used with magnifications of up to 4 times the aperture in mm, which is twice as much as ‘normal’ telescopes. To reduce aberrations even further, refractors tend to have slower focal ratios, making them quite long and more awkward to handle, particularly as aperture increases. Slower focal ratios and chromatic aberration make them less suitable for photography and their field of view can also be somewhat restricted.

Nevertheless, smaller aperture refractors can be good value for money and are therefore excellent for the beginner. They are also very useful as guide scopes for astrophotography. Small aperture, short focus (= wide angle) refractors are normally used as finder scopes on most telescopes. Larger aperture refractors are great for observing planetary detail.

Refractors are the most robust telescopes, very easy to use, and therefore very suitable for novice astronomers.

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