Refractors

Refractors are optical instruments that form images by refraction of light through lenses. In astronomy, a refracting telescope uses a convex objective lens to gather light and form an image at the eyepiece’s focal plane, which is then magnified by the eyepiece. Refractors rely on transparent lenses rather than mirrors to bend light.

Design and components: The primary element is the objective lens, housed in a tube that directs light to the eyepiece. Common designs include achromatic refractors, which combine two glasses to reduce color fringing, and apochromatic refractors, which use three elements for greater correction across a wider color range. Aperture governs light gathering and resolving power, while the focal length governs magnification in combination with an eyepiece. Longer focal lengths produce higher magnification with a given eyepiece but result in longer, heavier tubes.

Advantages and limitations: Refractors offer high-contrast, crisp images with no central obstruction and generally good mechanical stability and weather sealing. They are often ready to use with minimal alignment. Limitations include chromatic aberration in simple designs, which is mitigated in more advanced lenses at higher cost. Large-aperture refractors are expensive and heavy to manufacture, and very long tubes can be cumbersome. Compared with reflectors, refractors typically have smaller maximum apertures for a given price but deliver steady performance with excellent color correction in premium designs.

History and use: Early practical refractors appeared in the 17th century, with refinements by 19th-century makers improving achromatic lenses. They were dominant in astronomy until reflectors became more economical for large apertures. Today, apochromatic refractors remain popular among amateurs for sharp, wide-field viewing of the Moon, planets, and bright deep-sky objects. In photography and microscopy, refracting optics remain fundamental to image formation.