An aplanat (from ancient Greek ἀπλάνητος aplánētos “without error”) is an optical system in which the imaging errors, spherical aberration and coma are corrected. However, there is no correction of the astigmatism and the field curvature . The first lens aplanat was invented by CA Steinheil around 1866. The term aplanat was coined by Ernst Abbe . Almost at the same time, John Henry Dallmeyer developed a symmetrical lens based on the same principle, which he called Rapid Rectilinear. Lens construction remained successful under both names for many years. As early as 1857, Thomas Grubb was using the designation Aplanat for his distortion-correcting version of the landscape lenses based on an achromatic lens, which were customary at the time , because it also corrected spherical aberration. Photo-optics historian Rudolf Kingslake suspected that Dallmeyer had simply grouped two Grubb aplanates symmetrically around a common panel, while Dallmeyer's competitor Steinheil suspected plagiarism. Steinheil may have come to his invention through his friendship with the mathematician von Seidel , the theoretician of optical aberrations . According to Kingslake, it is likely that the same invention was made independently in two remote locations, with Steinheil having a lead of a few weeks.
Aplanate in photography
The aplanats used in photography towards the end of the 19th century are lenses made up of two symmetrically arranged achromatic lenses with an aperture diaphragm between them . Aplanates correct the chromatic aberration for two colors, the aperture error (spherical aberration) and the coma. Curvature of field and astigmatism remain effective, while the distortion is insignificant due to the symmetrical construction.
Aplanates are no longer used in photography today, as there are constructions that allow better correction of image errors with roughly the same construction effort. In this respect, the aplanat has historically been replaced by the Cooke triplet and the Zeiss Protar , which as an anastigmat also has no astigmatism and no field curvature.
Aplanatic mirror telescopes
Reflector telescopes , in which the same imaging errors are corrected, are called aplanatic mirror systems. The most important type of aplanatic mirror telescope is the Ritchey-Chrétien-Cassegrain telescope . In this type of telescope, the spherical aberration and the coma are also eliminated. Astigmatism and field curvature are not corrected. If such a telescope is used photographically, a lens system is inserted shortly in front of the sensor to correct the field curvature. Well-known Ritchey-Chrétien telescopes are the Hubble Space Telescope and the VLT telescopes of the Paranal Observatory . The aplanatically corrected Gregory telescope is longer and has a lower light transmission due to the larger secondary mirror.
Aplanate outside of photography
Aplanates made up of two achromatic lenses have also been used in high-quality projection headlights, in which the most flawless image possible is desired. You can also buy Aplanate as Aplanat magnifying glasses . This pays off especially with large magnifications, because here the errors are particularly serious compared to weaker magnifying glasses. Aplanates are expensive because of the high construction costs.
In some older specialist articles, the Mittenzwey eyepiece was also referred to as aplanatic , although this is only approximately true.
- Wilhelm Pape , Max Sengebusch (arrangement): Concise dictionary of the Greek language. 3rd edition, 6th impression, Vieweg & Sohn, Braunschweig 1914. 1914, accessed on November 30, 2016 .
- Rudolf Kingslake: A History of the Photographic Lens , Academic Press 1989