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.

The refracting telescope is a unique device that allows people to see very far away objects up close. The telescope is used to see distant objects such as stars, or anything else a distance away. Refracting telescopes also have 3 main parts to them, the tube which is usually made out of metal, plastic or wood, and 2 glass lenses. Unlike the reflecting telescope, the refracting telescope uses convex glass lens, called the objective lens, instead of mirrors. Convex lenses are lenses that are thicker in the middle compared to the edges. They’re also used in cameras to converge light to one point forming a real image. The other lens that this telescope uses is the eyepiece lens which is also a convex lens and is used to increase the size of the image like a magnifying glass. When light travels through the objective lens, the light converges and creates a real image in the middle of the tube, close to the eyepiece lens. The eyepiece lens at the bottom then magnifies the real image of the object making the image seem larger than it really is.

Technical difficulties

Refractors have been 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.

Notable refracting telescopes

• Yerkes Observatory 100cm
• Swedish Solar Telescope (100cm)
• Lick Observatory (91cm)
• Paris Observatory (83cm + 62cm)
• Nice Observatory (76cm)
• Lowell Observatory (24 in)

See also

• List of largest optical refracting telescopes, from which it is clear that their heyday was in the 19th century
• Maksutov telescope

Telescopes

Linsenfernrohr | Telescopio refractor | Refraktoro | Lunette astronomique | Telescopio rifrattore | Refraktors | Teleskop soczewkowy | Рефрактор (телескоп)