Telescope

• 1. Introduction

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  The word telescope did not exist until 1611. That year, the Greek mathematician Giovanni Demisiani coined it for one of Galileo Galilei's instruments, presented at a banquet at the Accademia dei Lincei. Galileo himself had used a different word. In his Starry Messenger, he called his device the Latin perspicillum. The new name fused two pieces of Ancient Greek: tele, meaning far, and skopein, meaning to look or see. Put together, teleskopos meant far-seeing.

  A telescope, at its core, is a device for observing distant objects by the radiation they emit, absorb, or reflect. The first ones used glass lenses and curved mirrors to gather visible light. But the modern definition has stretched far beyond glass. Today the word covers a wide range of instruments built to catch different regions of the electromagnetic spectrum, and in some cases other kinds of detectors entirely. How did an instrument invented to look across a Dutch landscape grow into something that listens to radio waves and traps gamma rays? Who built the first ones, and why did the simple lens prove to be a dead end at the largest sizes? And what forces the most powerful of these instruments off the ground entirely?

• 2. Lenses And Mirrors

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  The earliest existing record of a telescope is a 1608 patent. A Middelburg spectacle maker named Hans Lipperhey submitted it to the government in the Netherlands for a refracting telescope. The true inventor is unknown, but word spread through Europe. Galileo heard about it in 1609, built his own version, and turned it toward the sky. These first practical telescopes were refractors, using glass lenses, and they served both terrestrial uses and astronomy.

  The idea of swapping the lens for a mirror appeared soon after. A mirror used as the light-gathering element promised real advantages: reduced spherical aberration and no chromatic aberration at all. Parabolic mirrors became the goal of many proposed designs and several attempts to build a working instrument. In 1668, Isaac Newton built the first practical reflecting telescope. Its design still carries his name, the Newtonian reflector.

  The achromatic lens, invented in 1733, partly fixed the color aberrations of a simple lens. It allowed shorter, more functional refractors. Reflectors avoided the color problem but carried their own curse: speculum metal mirrors that tarnished fast through the 18th and early 19th century. Silver-coated glass mirrors in 1857 eased that, and aluminized mirrors followed in 1932. In 1956, John Dobson invented the Dobsonian telescope, a name now familiar to amateur astronomers.

• 3. The One-Meter Ceiling

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  Roughly one meter is the hard limit for a refracting telescope. Past that size, the lens simply stops working as a serious research tool. That ceiling reshaped the entire field. Since the turn of the 20th century, the vast majority of large optical research telescopes have been reflectors instead.

  The scale of modern reflectors dwarfs anything a lens could manage. The largest in use today carry objectives larger than 10 meters. Work is already underway on several designs in the 30 to 40-meter range. A larger aperture does more than gather more light. It also sharpens the angular resolution, letting astronomers separate finer detail in the sky.

  The optical family splits into three main types. The refracting telescope uses lenses to form an image. The reflecting telescope uses an arrangement of mirrors. The catadioptric telescope combines mirrors with lenses. Beyond these basics lie many sub-types named for their job, including astrographs, comet seekers, and solar telescopes. There is even a proposed ultra-lightweight space design, the Fresnel imager, which would focus light with a Fresnel lens.

• 4. Beyond Visible Light

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  Radio telescopes arrived in the 1930s, the first purpose-built one going into operation in 1937. Infrared telescopes followed in the 1960s. Across the 20th century, the instruments spread to cover wavelengths from radio all the way to gamma rays. The single word telescope now stretches across that whole span, even though astronomers must collect light very differently in each band.

  Longer wavelengths make antenna technology easier to use, though very tiny antennas are possible too. Near-infrared can be gathered much like visible light. In the far-infrared and submillimetre range, a telescope starts to behave like a radio instrument. The James Clerk Maxwell Telescope shows the blur between categories. It observes from 3 micrometers out to 2000 micrometers, yet it uses a parabolic aluminum antenna.

  Mirrors still rule at shorter wavelengths. The Spitzer Space Telescope, observing from about 3 to 180 micrometers, uses reflecting optics. The Hubble Space Telescope, fitted with Wide Field Camera 3, can see from about 0.2 to 1.7 micrometers, spanning ultraviolet to infrared. For even shorter wavelengths and higher frequencies, instruments turn to glancing-incident optics. TRACE and SOHO use special mirrors to reflect extreme ultraviolet, producing higher resolution and brighter images than fully reflecting optics could.

• 5. Listening To The Sky

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  A radio telescope is a directional radio antenna, usually a large dish built to collect radio waves. Some dishes are made of conductive wire mesh, with openings smaller than the wavelength being observed. Unlike an optical telescope, which produces a magnified image of a patch of sky, a traditional radio dish holds a single receiver. It records one time-varying signal from the observed region, sampled at various frequencies. Newer designs pack an array of receivers into one dish, a setup called a focal-plane array.

  The real power comes from combining dishes. By collecting and correlating signals received simultaneously across several dishes, astronomers can compute high-resolution images. These multi-dish arrays are astronomical interferometers, and the technique is aperture synthesis. The virtual aperture grows to roughly the distance between the telescopes. As of 2005, the record array size ran many times the diameter of the Earth, achieved with space-based very-long-baseline interferometry. The Japanese HALCA satellite, part of the VLBI Space Observatory Program, made that possible.

  The same idea now reaches optical instruments through optical interferometers and aperture masking interferometry at single reflecting telescopes. Radio telescopes also collect microwave radiation, which can slip through the atmosphere and through interstellar gas and dust clouds. Some, like the Allen Telescope Array, serve programs such as SETI, and the Arecibo Observatory has joined the search for extraterrestrial life.

• 6. Off The Ground

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  The atmosphere is opaque to most of the electromagnetic spectrum. Only a few bands reach the ground: visible light, near-infrared, and a portion of the radio-wave range. That is why no X-ray or far-infrared telescope works from the Earth's surface. Those wavelengths must be observed from orbit. Even an observable wavelength can favor a satellite, escaping clouds, astronomical seeing, and light pollution.

  Going to space carries a price. The disadvantages of a space telescope include cost, size, maintainability, and upgradability. NASA's instruments show the trade-off in action. The Hubble Space Telescope detects visible, ultraviolet, and near-infrared light. The Spitzer Space Telescope detects infrared radiation. The Kepler Space Telescope discovered thousands of exoplanets.

  The most recent launch was the James Webb Space Telescope, on the 25th of December 2021. It lifted off from Kourou, in French Guiana, atop an Ariane 5 rocket. Webb detects infrared light. Rather than circling close to Earth, it orbits the L2 Lagrange Point of the earth-sun system.

• 7. Hardest To Catch

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  X-rays resist collection far more than longer wavelengths do. They will not bounce off an ordinary mirror, so X-ray telescopes use Wolter telescopes, built from ring-shaped glancing mirrors of heavy metals that deflect the rays by just a few degrees. The mirrors are sections of a rotated parabola paired with a hyperbola or ellipse. In 1952, Hans Wolter outlined three ways to build a telescope from this kind of mirror alone. The Einstein Observatory, ROSAT, and the Chandra X-ray Observatory all rely on the design. In 2012, the NuSTAR X-ray Telescope launched, placing Wolter optics at the end of a long deployable mast to reach photon energies of 79 keV.

  Gamma rays push the problem further. The highest-energy X-ray and gamma-ray telescopes give up on focusing and use coded aperture masks. The shadow pattern the mask casts is reconstructed into an image. Because the atmosphere is opaque here, these instruments ride high-flying balloons or Earth-orbiting satellites. The Fermi Gamma-ray Space Telescope, launched in June 2008, is one example.

  Very high energy gamma rays demand still more specialization. They can be detected with Imaging Atmospheric Cherenkov Telescopes, such as H.E.S.S. and VERITAS, with the Cherenkov Telescope Array under construction. Water Cherenkov Detectors offer another path, used by HAWC and LHAASO. A discovery in 2012 hinted at a future where gamma rays might finally be focused. Above 700 keV, the index of refraction starts to increase again.