Are reflecting telescopes popular

Answer 1

The vast majority of astronomical telescopes are reflectors.

• Mirrors can be made that are much larger than lenses. Of presently operating astro scopes, the largest refractor is the 40" one at the Yerkes Observatory. The largest operating single-mirror reflector is the 238" BTA-6 in the Caucasus Mountains of Russia, which is almost six times larger than the Yerkes unit.

• Unlike lenses, telescope mirrors can be segmented. The Keck telescopes in Hawaii each have 36 hexagonal segments equivalent to single mirrors 400" in diameter. There is no theoretical limit to how big such telescopes can be.

• The mass of a lens is much higher than a comparable mirror, and the difficulty of manufacturing a huge chunk of glass to the exacting specifications needed for astronomy is close to insurmountable.

• With a lens, all incoming light must pass though it, requiring that both surfaces be machined as precisely as possible and the glass must be free of optical imperfections. Glass also attenuates the light. With a mirror, incoming light is reflected from its polished surface. Light does not go through it, so aside from the curved reflecting surface, there are no tight specifications for the rest of the mirror and its mount. And mirrors don't absorb light.

• Glass bends light differently depending on wavelength. A single lens cannot be made that will precisely focus all colors of light at one plane -- blue light focuses closer to the lens than red light. The effect can be minimized by using a long focal length so the light isn't bent as drastically. The Yerkes scope uses a 60-foot tube for that reason. Compound or "doublet" lenses made of different kinds of glass -- typically crown and flint glass -- can account for most of the chromatic aberration, but that adds a lot of mass to the lens.

• Because of that problem with refractors, reflector telescopes can be much shorter than equivalent refractors, and can fit into smaller observatory domes.

• In a refracting telescope, the objective lens is out at the front end of the telescope, meaning that the heavier it is, the harder it is to design a telescope that will keep it rigid and aligned, and move it easily and exactly.

• Very large lenses tend to sag from the weight of the glass, introducing distortion. Large mirrors can also sag. The difference is that lenses can be supported only around their edges, whereas mirrors can be supported by their entire back surfaces, and thus are far less susceptible to sag. Lenses much larger than the Yerkes 40" would experience unacceptable sag for astronomical purposes.

• For astronomical telescopes, lenses are considerably more expensive than equivalently-sized mirrors.

For terrestrial uses such as spotting scopes and binoculars, lenses will not be replaced any time soon.

Answer 2

more duradle than a refracting telescope