Astronomers use telescopes to focus and collect electromagnetic radiation from cosmic sources.
Most people are only familiar with optical telescopes, that is telescopes that detect radiation in the visible spectrum. BUT....
Since gamma-rays, x-rays, radio waves, infrared waves and ultraviolet waves are also forms of electromagnetic radiation, telescopes can also be designed to detect those waves.
Optical telescopes make use of either lenses or mirrors to focus the incoming optical light waves.
A refracting telescope uses lenses. Because big lenses are expensive and difficult to build and then support as part of a telescope, the refractor design is not commonly used for large telescopes today. Refractors also suffer from chromatic aberration, because different wavelengths do not all focus to the same point.
A reflecting telescope uses mirrors to bend and focus the light rays.
Radio telescopes reflect and focus radio waves similarly; they are all reflecting telescopes of one design or another.
The Green Bank Telescope will be a fully steerable paraboloid, with an offset feed allowing an unblocked aperture.
The Arecibo telescope is a 305m fixed spherical antenna with a movable feed suspended by cables above the dish.
There are lots of other telescopes!
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