How Telescopes Work: Optical, Radio and Space Observatories Explained

A telescope's basic job is to collect more electromagnetic radiation than the human eye can and bring it into focus. The diameter of a telescope's main mirror, lens or antenna — its aperture — sets two things: how much faint light it can gather, and how finely it can separate detail, according to the US National Radio Astronomy Observatory (NRAO). A bigger aperture means fainter, more distant objects become visible and finer structures come into view.

Optical telescopes: lenses, mirrors and the atmosphere problem

Classic optical telescopes gather visible light with either lenses (refractors) or mirrors (reflectors). The Hubble Space Telescope, for example, uses a reflecting design: light strikes a 2.4-meter concave primary mirror, bounces to a smaller secondary mirror about 0.3 meters across, and returns through a hole in the primary mirror to reach the science instruments, according to NASA's Goddard Space Flight Center. This two-mirror configuration, called a Ritchey-Chrétien design, is built to keep images sharp across a wide field of view.

On the ground, even the best sites suffer from atmospheric turbulence, which makes stars twinkle and blurs fine detail, according to the European Southern Observatory (ESO), which operates the Very Large Telescope in Chile. ESO's fix is adaptive optics: a wavefront sensor measures how a star's light has been distorted, and a computer-controlled deformable mirror corrects the distortion in real time. When no bright natural star sits near the target, ESO fires a powerful laser into the upper atmosphere to create an artificial reference star. ESO says this correction can make ground-based images at visible wavelengths nearly as sharp as space telescope images, at a fraction of the cost of a space telescope.

Why some telescopes have to leave Earth

Earth's atmosphere does not just blur images — it blocks entire wavelengths outright. According to astronomy education site Las Cumbres Observatory, the atmosphere absorbs or reflects most ultraviolet light, X-rays, gamma rays and most infrared radiation, letting through mainly visible light, some infrared, and parts of the radio spectrum. That is why observatories built to study those blocked wavelengths — such as the X-ray-focused Chandra Observatory, the gamma-ray Fermi Space Telescope, and infrared missions like Spitzer and Herschel — have to orbit above the atmosphere rather than sit on a mountaintop.

X-ray telescopes also need a completely different optical design from visible-light ones. According to the Chandra X-ray Center at the Harvard-Smithsonian Center for Astrophysics, X-ray photons are so energetic that they pass straight through a conventional mirror aimed at them head-on, much as a bullet would punch through a wall. Chandra's solution is to angle its mirrors so X-rays strike at a shallow grazing angle and ricochet into focus, similar to a stone skipping across water. Chandra uses four barrel-shaped mirror pairs polished to a smoothness of a few atoms and coated with iridium, aligned to within about 1.3 micrometers — a small fraction of the width of a human hair, according to the Chandra X-ray Center.

Infrared giants: seeing through dust and back in time

The James Webb Space Telescope (JWST) is built to detect light from visible red through mid-infrared wavelengths, covering roughly 0.6 to 28.8 microns, according to NASA. Its primary mirror spans 6.6 meters across 18 hexagonal gold-coated segments, giving it a collecting area about 5.6 times larger than Hubble's. Gold was chosen because it reflects red-to-mid-infrared light especially well. NASA says this combination of size and wavelength range lets Webb detect the faint infrared light of galaxies more than 13.5 billion light-years away, and that infrared light can pass through dense clouds of dust that block most of the visible light Hubble's instruments can detect. Webb orbits the Sun-Earth L2 point, about 1.5 million kilometers from Earth, where it stays clear of the warm, light-scattering effects of both the Sun and Earth.

Radio telescopes: dishes that act as one giant eye

Radio telescopes collect the longest wavelengths of light astronomers use, from about 1 millimeter to more than 10 meters, according to NRAO. Each antenna has a parabolic dish that focuses incoming radio waves onto a receiver, which amplifies the signal for analysis — a process NRAO describes as functioning much like an optical telescope but tuned to far weaker, far longer waves. NRAO notes that the radio signals studied are extraordinarily faint — by its estimate, a typical cell phone signal is roughly a billion billion times stronger than the cosmic radio waves these telescopes detect.

A single dish faces a resolution problem: because angular resolution depends on wavelength divided by antenna diameter, and radio wavelengths are so long, even a large dish like the Green Bank Telescope cannot match the fine detail of an optical telescope on its own, NRAO says. The solution is interferometry — linking many dishes spread over long distances so they act as one much larger telescope. NRAO cites the Very Large Array, whose 27 active dishes (plus one spare) in New Mexico can be spread across configurations using roughly 40 miles of rail track, and the Atacama Large Millimeter/submillimeter Array (ALMA), which links 66 dishes in Chile, as examples of this approach, used to map galaxies, detect black holes and study the earliest cosmic structures.

What different wavelengths reveal

• Visible-light telescopes, ground- or space-based, capture the stars, galaxies and nebulae closest to what the human eye would see.
• Infrared telescopes like Webb see through dust clouds and can reach some of the most distant, oldest galaxies known, according to NASA.
• X-ray telescopes like Chandra are built to study exploded stars, galaxy clusters and matter swirling around black holes.
• Radio telescope arrays such as the VLA and ALMA map cold gas, distant galaxies and black hole environments by combining many dishes into one effective instrument, per NRAO.