Apochromat
As Apochromat (Greek for free from colors, colorless ) refers to an optical system such as a lens in which the color error is corrected as far as possible, so there is very little variation in the average length of the wavelength.
The idea according to a Apochromat is thus a system in which those aberrations which interfere with visual observation most are corrected for more than two wavelengths.
Definition according to Abbe
The concept of Apochromasie was first introduced by Abbe. After an Abbe Apochromat is an optical system whose longitudinal chromatic aberration is corrected for three wavelengths and also the color dependence of the spherical aberration, the Gaussian error is corrected for two widely separated wavelengths (see ).
Operation
The continuous light beams are - to varying degrees refracted by a lens, and thus not meet exactly at the same point on the image plane - depending on their wavelength. It caused blurring and color bleeding (see chromatic aberration).
The construction of the achromatic lens systems based on the fact that the ratio of the refractive index and dispersion of different types of glass is different, which is reflected in different Abbe numbers. If this ratio is the same, there would be no way to compensate for the chromatic aberrations of the lens systems. The remaining color error of achromat is described by a measure that relates the average width at three wavelengths in relationship, the so-called secondary spectrum.
Through the use of at least three optical glass types the secondary spectrum can be reduced, eliminated altogether the true apochromat. Therefore you have to use with special dispersion properties, such as fluorite, Langkronglas ( Fluorkronglas ) and short flint glass at least one lens of glass ( or other material ). Langkronglas has a high partial dispersion in the short wavelength ( blue ) region of the spectrum, ie the refractive index changes here greatly with wavelength, compared with its partial dispersion in the long wavelength ( red ) area. Short flint glass has here, however, a relatively small partial dispersion. Such specific types of glass are needed to influence the secondary spectrum. At ordinary places the partial dispersion glass is closely related with the general dispersion ( Abbe number ). If used, such glasses can not substantially reduce the secondary spectrum.
Because different not strong enough, the refractive index profiles of the available materials, strongly curved surfaces to the total elimination of the secondary spectrum necessary, to the detriment of other aberrations or the usable light intensity limits. That's why you often satisfied with a significant reduction of the secondary spectrum, rather than to eliminate it completely. This lens systems are sometimes referred to as Halbapochromate, sometimes also referred to as improved achromats.
Astronomy
The classical way to reduce the residual chromatic aberration of lens telescopes, such as in astronomy, the choice was always longer focal lengths ( relative to the opening ), only the desire for more compact and higher aperture telescopes ( f: 8 or less) led to the demand by the much more expensive apochromatic lenses. These usually consist of three lenses, which can be joined with cemented oil at one or two contact surfaces or.
However for large telescopes it is cheaper, instead of a Apochromaten move on mirror optics that have no color aberration.
Microscopy
Since microscope lenses for higher magnifications always with large aperture ( NA ) work to achieve the required resolution, the color error is particularly disturbing and the development of apochromatic lenses by Zeiss was considered a great step forward. For the photomicrograph further requirements such as the flattening of the image field in the edge regions are added; Lenses that can achieve this, hot Planapochromaten, they were developed in 1938 by Hans Boegehold at Carl Zeiss.
Photography and Spotting Scopes
In Camera Lenses with (partially) corrected secondary spectrum are often referred to by the abbreviation " APO ". This is mainly to higher quality, fast telephoto lenses. Especially when shooting with open aperture so that a markedly increased image quality is achieved. These camera lenses are rarely (if ever) true Apochromats. The complete correction of the secondary spectrum is only useful if the other aberrations are well corrected similar. However, this would require an extremely high effort. Such a lens would be very expensive, and its image quality could be hardly used in practice.
Manufacturers such as Zeiss, Leica, Swarovski, Nikon, Kowa, inter alia, have Spektivreihen in the program, which are also equipped with the APO technology. These scopes are a little heavier than the identical devices without APO and cost significantly more. The better color quality and higher contrast are almost indispensable in astronomical observations.