Apochromat

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Beam paths of different colored light through an apochromat

An optical system is called an apochromat ( Greek for colorless, colorless ), e.g. B. a lens in which the color error is largely corrected, so that there is only a very small variation of the focal length with the wavelength. In addition, the Gaussian error must be small.

According to the idea, an apochromat is a system in which those aberrations that are most disruptive to visual observation are corrected for more than two wavelengths. The apochromats are a further development of the achromats . As a rule, these are optics with lenses made of three different types of glass, at least one of which has a dispersion curve that deviates from the norm .

Definition according to Abbe

The term apochromasy was first introduced by Ernst Abbe . According to Abbe, an apochromat is an optical system whose longitudinal chromatic aberration has been corrected for three wavelengths and the color dependency of spherical aberration , the Gaussian error, has been corrected for two wavelengths that are far apart.

functionality

With a simple lens , the light rays passing through - depending on their wavelength - are refracted to different degrees and therefore do not hit exactly the same point on the image plane. Blurs and color fringes arise (see chromatic aberration ).

This error can be reduced by combining two different lenses. The construction of such achromatic lens systems is based on the fact that the ratio of refractive index and dispersion is different for different types of glass , which is expressed in different Abbe numbers . If the ratio were the same, there would be no way of compensating for the color aberration of lens systems. The remaining color error of an achromatic lens is described by a number that relates the back focal length at three wavelengths, the so-called secondary spectrum .

By using at least three types of optical glass , the secondary spectrum can be reduced, with the real apochromat it can be eliminated entirely. For this you have to use at least one lens made of glass (or other material) with special dispersion properties, such as fluorite , long crown glass (fluorocron glass) and short flint glass. Long-crown glass has a high partial dispersion in the short-wave (blue) range of the spectrum, that is, the refractive index changes here strongly with the wavelength compared to its partial dispersion in the long-wave (red) range. Short flint glass, however, has a relatively low partial dispersion here. Such special types of glass are necessary to influence the secondary spectrum. In the case of common types of glass, the partial dispersion is closely linked to the general dispersion (Abbe number). If you only use such glasses, you cannot significantly reduce the secondary spectrum.

Since the dispersion of the available materials differs only slightly, strongly curved surfaces are required to completely eliminate the secondary spectrum, which is at the expense of other imaging errors. That is why one is often satisfied with a considerable reduction in the secondary spectrum instead of completely eliminating it. These lens systems are sometimes referred to as semi-apochromatic lenses , and sometimes also referred to as improved achromatic lenses .

astronomy

Telescope by Johannes Hevelius . 46 m long to reduce the aberrations of the individual objective lens

The classic way to reduce the residual chromatic aberration of lens telescopes, e.g. B. in astronomy , the choice was always longer focal lengths (relative to the aperture). Only the desire for more compact and more powerful telescopes (f: 8 or shorter) led to the demand for the much more expensive apochromats. These usually consist of three lenses , which can be cemented on one or two contact surfaces or joined with oil.

microscopy

Since microscope - lenses for higher magnifications always with a large opening ( numerical aperture ) are working to achieve the necessary resolution, color mistake here is particularly troublesome and development apochromatic lenses by Zeiss was considered a major step forward. For microscope photography there are additional requirements such as the leveling of the image field also in the edge areas; Lenses that do this are called planapochromats ; they were developed by Hans Boegehold at Carl Zeiss in 1938 .

Photography and spotting scopes

In photography , lenses with a (partially) corrected secondary spectrum are often identified with the abbreviation “APO”. These are mainly higher quality, high-light telephoto lenses . Particularly when taking photos with an open aperture , a noticeably increased image quality is achieved. However, these camera lenses are rarely (if ever) true apochromats. The complete correction of the secondary spectrum only makes sense if the other imaging errors are also corrected with similar effectiveness. However, this would require an extremely high effort. Such a lens would be very expensive and its image quality could hardly be used in practice.

Manufacturers such as Zeiss, Leica, Swarovski, Nikon, Kowa and the like a. have ranges of spotting scopes in their range that are also equipped with APO technology. These spotting scopes are a little heavier than the identical devices without APO and cost significantly more. The better color quality and the higher contrast are almost essential for astronomical observations.

Footnotes

  1. ^ Siegfried Czapski : Theory of optical instruments according to Abbe, Leipzig 1904.
  2. ↑ Uwe Laux: Astrooptik, Weimar 1999.

Literature and web links