With his reflective telescope, William Herschel discovered infrared radiation.
Not long after Galileo was the first to build a telescope capable of imaging the heavens, his compatriot Niccoló Zucchi invented the reflecting telescope. A few decades later, James Gregory and Isaac Newton improved on Zucchi's work with their own designs for a reflective telescope. Newton was especially motivated by the desire to avoid chromatic aberration, a property of refractive telescopes that blurs light. Almost all significant astronomical telescopes built in the last hundred years have been reflective telescopes.
Visible Light
Many serious amateur astronomers use reflective telescopes.
Telescopes look at electromagnetic radiation, whose energy is dependent on its wavelength. If there's electromagnetic radiation to be seen, there's a reflective telescope looking at it. The most obvious form of energy detected by reflecting telescopes is what we see every time we open our eyes: visible light. A common practice is to grind and polish a glass blank to the right curvature, then coat it with aluminum. The aluminum reflects a little bit of ultraviolet light, all visible light and infrared light far out into the spectrum. Amateur astronomers' reflective telescopes usually have glass eyepieces, which will transmit visible light quite well, and even a little bit of near infrared light, up to a wavelength of around 2 micrometers (millionths of a meter).
Into the Infrared
Infrared wavelengths reveal features not visible in normal light.
Most large astronomical telescopes have the capability to extend their imaging into the longer wavelength, lower energy infrared wavelengths. The 10-meter diameter Keck Telescopes in Hawaii, for example, can image wavelengths greater than 5 micrometers. There's even a proposal for a terahertz reflecting telescope. Terahertz radiation is extremely low energy, with extremely long wavelengths, up to 540 micrometers and beyond.
The Ultraviolet
Infrared is on the low energy side of visible light; ultraviolet is on the high energy side. Mirrors that reflect ultraviolet use special materials and coatings. The Extreme Ultraviolet Imaging Telescope flown on the Solar and Heliospheric Observatory satellite, for example, can image wavelengths as short as 17 nanometers (billionths of a meter). That's more than twenty times as energetic as visible light.
Going Higher
X-ray images, like this one from the Chandra Observatory, reveal highly energetic events.
X-rays are created when there's lots of energy flying around in such places as supernovas and black holes. These phenomena can reveal new information about the laws that govern our universe. X-ray telescopes, like NASA's Chandra X-Ray Observatory, image these interesting places. Chandra detects energy 4,000 times higher than the light we see. Even this telescope cannot detect gamma rays, which have even higher energy. These rays are so powerful they fly right through materials, instead of reflecting off of them.
Radio
This dish antenna is a giant radio mirror, creating a reflective telescope that detects low energy radiation.
Although we don't often think of them as telescopes, radio antennas can also be reflecting telescopes. Radio waves can have wavelengths longer than 10 meters. Radio waves have about one ten billionth of the energy of visible light. Reflecting telescopes are used from low energy radio all the way up to high energy x-rays, covering an energy range of 40 trillion times.
Tags: visible light, reflective telescope, reflective telescopes, astronomers reflective, astronomers reflective telescopes, astronomical telescopes
