Reflecting Telescopes

The reflecting telescope is a type of telescope which uses a mirror rather than a lens to collect and focus light. They became popular after the 17th century, when optic specialists and astronomers required a need for the replacement of the standard refractor, which suffered from a optical impairment known a chromatic aberration, or color distortion. The refractors were the earliest telescopes, popularized by Galileo.

Several advantages became apparent with the introduction of the reflecting telescope. The chief advantage was that larger mirror diameters could be made, resulting in better light-gathering capability in detecting fainter objects. In comparison, refractor lens at larger diameters were hard to manufacture and were heavy. Telescopes by now were used almost exclusively by the astronomical community, with the reflector being the most preferred. The Newtonian reflecting telescope design, made in 1669, became the most popular mirror telescope. It consisted of a curved primary mirror collecting light and focusing onto a secondary mirror, in which the image was magnified by an eyepiece lens.

The original mirror objectives were a speculum metal alloy consisting of copper and tin, grinded and polished into a spherical or parabolic shape. One of the largest telescopes in the 1700s and 1800s could now be made reaching objective diameters up to nearly 50 inches. After refinements in some negative optical aspects of a mirror objective, such as spherical aberration, the glass mirror coated with an reflective metal oxide became popular. Spherical aberration, although can be corrected, still plagues modern telescope design, the most famous being the retrofits of the Hubble Space Telescope over the past decade.

Today’s research telescopes use various glass and lens designs in producing specialty telescopes, such as catadioptric telescopes. Examples are the Schmidt Cassegrain and Schmidt camera. The Newtonian design is still popular with amateur astronomers.