The refractor
The refractor |
1. The Galilean telescope 2. The Kepler Fernrohr |
Both of which, as regarding their construction, are in principle very simple systems. The Galilean system consists of a converging lens at the front and a diverging lens at the back. This type of system is used mainly in opera glasses. However, since the exit pupil lies inside the tube (i.e. in front of the diverging lens), it produces a small field of view which is rather diffuse at the edge. It is intended only for low magnifications. But the advantage is that it provides an upright image.
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Achromat: Consists of two lens e lements - of flint and crown glass. These lenses are generally arranged so that there is an air gap, but there are also cemented systems. ED apochromat: This system is basically an achromat, with one lens e lement made of ED glass. These also have an air gap. The colour aberration is almost entirely corrected for. Fluorite-apo: This consists of two lens e lements cemented together, one of which is made of fluorite. This produces a similar effect to using an ED glass e lement. Apochromat: This system is a full apochromat, whereas the two preceding systems are both referred to as semi-apos. It typically consists of three lens e lements. Chromatic aberration is virtually entirely corrected for. |
For some time now, so-called 'superapochromats' have also been available. This type of refractor consists of five lens e lements, generally arranged in two groups. The first group of three lens e lements perform the same function as a triplet apochromat. The second group of two lens e lements correct for field curvature with the aim of being able to producing perfect astronomy photographs.
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