Refracting telescope

From Free net encyclopedia

(Redirected from Refractor)

A refracting or refractor telescope is a type of optical telescope that refracts or bends light at each end using lenses. This refraction causes parallel light rays to converge at a focal point; while those which were not parallel converge upon a focal plane. This can enable a user to view the image of a distant object as if it were brighter, clearer, and/or larger. These are similar to microscopes. The monocular is a type of refractor. A typical refractor has two lenses, an objective lens and an eyepiece lens. The objective lens has two pieces of glass (with different dispersion), "crown" and "flint glass". Each side of each piece is ground and polished, and then the two pieces are -sometimes- glued together. The curvatures are designed to limit the effects of chromatic and spherical aberration.

The other major type of telescope is the reflecting telescope, which uses curved mirrors, or a combination of mirrors and lenses, to gather light and form an image. The earliest telescopes were all refracting telescopes, including the famous and very simple telescope used by Galileo Galilei in the early 1600s.

[edit]

Technical difficulties

Refractors are criticized for their relatively high-degree of residual chromatic and spherical aberration. This affects shorter focal lengths more than longer ones. A 4" F6 achromatic refractor is likely to show considerable color fringing. (generally a purple halo around bright objects) A 4" F16 will have little color fringing.

In very large apertures, there is also a problem of lens sagging, a result of gravity deforming glass. There is a further problem of glass defects, striae or small air bubbles trapped within the glass. In addition, glass is opaque to certain wavelengths, and even visible light is dimmed by reflection and absorption when it crosses the air-glass interfaces and passes through the glass itself. Most of these problems are avoided or diminished by using reflecting telescopes, that can be made in far larger apertures.

However, modern designs using apochromatic optics built with special, extra low-dispersion materials essentially eliminate these problems. Such telescopes contain elements of fluorite or special, extra low-dispersion (ED) glass in the objective and produce a very crisp image which 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 553mm in diameter, but most are between 80 and 152mm.

[edit]