Refracting telescope: Encyclopedia II - Refracting telescope - Technical difficulties

Refracting telescope - 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 -or all- these problems are avoided 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. APO refractors are available with objectives of up to 553mm in diameter, but most are between 80 and 152mm.

Adapted from the Wikipedia article "Technical difficulties", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki