Achromatic versus Apochromatic Lenses in Telescopes

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Achromatic versus Apochromatic Lenses in Telescopes

In 1733, an Englishman, Chester Moore Hall, created the Achromatic refracting lens. His design limited the color aberrations by using two pieces of glass that were both ground and polished. These lenses usually were employed to see red and blue light. However, the design does not block out all of the rainbow of color around the images.
Achromatic lenses are made from a convex and a concave lens. The concave lens is usually made out of Flint glass, and the convex from Crown glass. They form a weak lens together and can bring two wavelengths of light into a single focus.
Apochromatic refracting lenses are made to view red, green, and blue light wavelengths. The first ones were designed by the German physicist, Abbe. Some type of fluorite or extra low dispersion glass is also used and the ending result is a crisp clear image free of the rainbow of color around it. These types of refracting lenses are more expensive than the achromatic lenses.
Apochromatic lenses require lenses that can handle three color crossings. The are usually made from expensive fluoro-crown glass, abnormal flint glass, or transparent liquids that are used in the space between the glass. These newer designs allowed for the objects to be free of color around the edges, and they produced way less aberrations than achromatic lenses.
Prices on these types of refracting lenses can run high depending on the size aperture you need. Two to three inch apertures for Achromatic refractors can run between $250 to $1000. Three to five inch apertures for Apochromatic refractors can run between $2000 to $10,000. Decide which kind you will be using before you buy one. You may just want to look at the night sky or you may want to gaze into the next galaxy.

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