A refracting telescope or refractor is a telescope that uses lenses to magnify rather than mirrors. It was the first type of telescope developed, likely through accidental discovery of the magnification provided by two lenses. Refractors have fallen out of favor for the largest research telescopes due the challenge of creating very large lenses, due to the additional aberrations (especially chromatic aberration) inherent in lenses, and probably due to the length of the telescopes. They are still very common for small telescopes including those for astronomy, some for research.
In the nineteenth and early twentieth century, the best and largest research telescopes were refractors: practical highly-reflective mirrors were still in development, thus limiting the light-capturing ability of reflector telescopes and nineteenth-century lens development had mitigated refractors' aberration issues considerably. The creation of the very large lenses was extremely challenging. Only a couple of telescopes had apertures larger than about 26" (68 cm), the largest practical one being 40" (102 cm, at Yerkes Observatory). Today, many observatories dating from that age still have their refractors, used for education and research. There is one modern research solar telescope, the Swedish Solar Telescope that is a 1-meter refractor.
A refractor's primary lens, corresponding to a reflector's primary mirror, is termed the objective lens.