Seeing Past Visible Light: The Telescopes That Are Not Telescopes

This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.

A telescope is usually pictured as a tube pointed at the night sky, gathering visible light with a lens or a mirror. But the word covers a far wider family of instruments today, because visible light is only a narrow slice of the electromagnetic spectrum objects in space actually emit. Optical and infrared telescopes are generally built to work across roughly 0.2 to 2000 micrometers of wavelength, but dedicated instruments now reach for radio waves at one end of the spectrum and gamma rays at the other, each requiring a fundamentally different kind of hardware to focus and detect. Radio telescopes look nothing like an optical instrument: rather than glass lenses, they typically use a large parabolic dish of metal or wire mesh to reflect long radio wavelengths to a single receiving antenna, and because radio waves pass through the atmosphere largely unimpeded, radio telescopes work perfectly well from the ground, day or night, in almost any weather. At the opposite end of the spectrum, X-ray and gamma-ray telescopes face the opposite problem: Earth's atmosphere absorbs these high-energy photons almost completely, which is fortunate for life on the surface but means an X-ray or gamma-ray telescope has to fly above the atmosphere on a satellite or a high-altitude balloon to see anything at all. X-rays also cannot simply be reflected by an ordinary mirror pointed straight at the source, since they pass through most materials rather than bouncing off them; X-ray telescopes instead use mirrors set at a shallow grazing angle, so the X-rays skim off the surface the way a stone skips across water. The result is that modern astronomy is really conducted through an array of specialized instruments, each opening a different window on the universe, rather than through one all-purpose telescope design.