Telescope

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

When Galileo first turned an improved spyglass on the night sky in 1609, the instrument was little more than two ground lenses in a tube, yet it was already good enough to show the moons of Jupiter and the phases of Venus. What separates that first telescope from the giants of modern astronomy is not cleverness so much as one simple physical fact: a telescope's light-gathering power scales with the square of its aperture diameter, so doubling the diameter of the primary lens or mirror collects four times the light and sharpens the finest detail the instrument can resolve. Astronomers have spent four centuries chasing that relationship. The earliest refracting telescopes, built from simple glass lenses, ran into a hard limit quickly: different colors of light bend by different amounts passing through a single lens, so a bright star smears into a small rainbow-fringed blur, an effect called chromatic aberration. Isaac Newton concluded the problem was unsolvable with lenses alone and built the first practical reflecting telescope in 1668, using a curved mirror to gather and focus light instead, since a mirror reflects every color at the same angle. The reflector design that bears his name remained the standard route to a larger aperture for the next two and a half centuries, because a mirror is far easier to support from behind at large sizes than a lens, which can only be held at its edge. Refracting telescopes were not abandoned. In 1733 the invention of the achromatic lens, a compound lens built from two different kinds of glass, partially corrected the color-fringing problem, enabling shorter and more capable refractors for the two centuries that followed. The aperture race that both designs fed continues today in the giant segmented-mirror telescopes now being built on mountaintops around the world, each new generation still chasing the same square-of-the-diameter relationship Newton and his contemporaries were already working against three hundred years ago.

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.