refractor

A refractor, in optics, is an instrument that forms images primarily by refraction of light through lenses or prisms. The term is most commonly used for a refracting telescope, but any optical device that relies on refraction, such as camera lenses and magnifiers, can be described as refractors.

In a refracting telescope, light from a distant object passes through a large objective lens, which refracts (bends) the light to form an image at a focal plane near the eyepiece. The eyepiece then magnifies the image for the observer. The quality of the image depends on lens quality, glass, coatings, and proper alignment; refractors are prized for high-contrast, unobstructed views since there is no secondary mirror in the light path.

Chromatic aberration is a key historical challenge: different wavelengths of light bend by different amounts, causing colored fringes. This was mitigated by using combining glass types (achromatic doublets) and later by apochromatic designs, which bring multiple wavelengths to the same focus. Refractors can be compact and rugged but scale poorly to very large apertures due to lens fabrication, weight, and cost. They typically have longer tube lengths for higher focal ratios and are popular among amateur astronomers for their portability and minimal maintenance.

Applications include astronomy, terrestrial viewing, photography, and surveying. Notable examples of large refractors include astronomical observatories such as Yerkes and Lick, which house some of the historically largest refracting telescopes. Refractors continue to be valued for their sharp images, stable mounting, and long-term optical performance.