Refractor

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Apochromatic refractors have objectives built with special, extra-low dispersion materials. They are designed to bring three wavelengths (typically red, green, and blue) into focus in the same plane. The residual color error (tertiary spectrum) can be up to an order of magnitude less than that of an achromatic lens. Such telescopes contain elements of Fluorite or special, extra-low dispersion (ED) glass in the objective and produce a very crisp image that is virtually free of chromatic aberration. Such telescopes are sold in the high-end amateur telescope market. Apochromatic refractors are available with objectives of up to 553 mm in diameter, but most are between 60 and 152 mm.

APM is known for manufacturing some of the best apochromat telescopes in the world.

The Achromatic refractor was invented in 1733 by an English barrister named Chester Moore Hall although it was independently invented and patented by John Dollond around 1758. The design overcame the need for very long focal lengths in refracting telescopes by using an objective made of two pieces of glass with different dispersion characteristics  (Crown glass and Flint glass) to limit the effects of Chromatic aberration and Spherical aberration. Each side of each piece is ground and polished and then the two pieces are assembled together. Achromatic lenses are corrected to bring two wavelengths (typically red and blue) into focus in the same plane.

APM offers a choice of two 203mm focal length achromat telescopes.

The Petzval Design, most common in Astrographs, consists of two doublets which are spaced apart usch that the rear one will act in  a manner that reduces the inherent  Chromatic aberration and Spherical aberration produced by the front doublet. Invented by Joseph Petzval in 1843, the Astro-Petzval-Objektiv lens is used in astronomy. This objective made a distortion-free illustration of a large part of the sky, as well as permitting photographing of galaxies and star fields which is why this design of scope is popular in astronomical imaging.

APM America is happy to be a dealer for the Orion Optics UK range of reflector telescopes.