While initially the most common type of telescope, these are today used primarily by amateurastronomers, and have been supplanted in professional research work by reflecting telescopes. However, some relatively small instruments with 100-150mm objective lenses regularly produce astrophotography that rivals images created by professionals as recently as 20 years ago using what were then the largest telescopes on Earth.
Refractors are criticized for their relatively high-degree of chromatic and spherical aberration. There is also the problem of lens sagging, a result of gravity affecting glass. There is a further problem of mis-refraction; caused by airbubbles trapped within the lenses. In addition, glass is opaque to certain wavelengths, and even visible light is dimmed when it passes through glass. Many of these problems are avoided by using reflecting telescopes.
In fact, all telescopes were refractors until Isaac Newton invented the first reflecting telescope in 1669, six decades after Galileo first pointed his telescope to the heavens.
Refractors gather light by using an objective lens located at the front (skyward) end of the telescope.
This makes refractors well-suited as multi-purpose telescopes for both terrestrial observing and stargazing, unlike Newtonian reflectors which produce inverted images and are only suited for astronomical observing.
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