Achromatic Lens

Achromatic Lens – getting the colours under control

An achromatic lens is is corrected for chromatic aberration. That reduces the false colour around a star. This type of lens comprises either two or three different elements which together, correct the aberration.

An achromatic lens is made from a crown glass and a flint glass element. The crown glass is a glass type that has a positive low refractive index. The flint glass has a negative high refractive index. In that way, the two refractive indices cancel out and the ultimate result is that there is little or no scattering of the colours.

In comparison to a singlet lens (consists of only one piece of glass) the additional design freedom provided by using a doublet (consisting of two pieces of glass) design allows for further optimisation, for example, spherical aberration can also be better controlled. An achromatic lens will have noticeable advantages over a comparable diameter and focal length singlet lens.

The different elements of an achromatic lens are either cemented together or held in place by a frame that allows an air space between the two.

However, it isn’t possible to construct a perfect achromatic lens as the two glass combination will only bring the red and blue ends of the spectrum to the exact same focal point. But well made achromatic lenses make a very good job of things. When looking through a good quality doublet or triplet, to all but the most discerning and well trained eye, you will not see the coloured rings of chromatic aberration around stars.

Achromatic lenses can be used as either the objective lens ot as part of the eyepiece. The first achromatic eyepiece was devised in 1849 by Kellner. He simply replaced the single lens element closest to the eye with a doublet. The Kellner eyepiece is a good type to have for low to medium power work.

Building your own telescope is described here – http://www.funsci.com/fun3_en/tele/tele.htm

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