How Space Telescopes Work: Mirrors, Wavelengths and Seeing Cosmic History

Every space telescope does the same basic job: collect as much light as possible from a faint, distant source and focus it onto a detector. How that light is collected, and which wavelengths can be captured, depends on the design of the mirrors and where the telescope sits in space, according to NASA's overview of telescope science.

Why put a telescope in space at all

Ground-based telescopes look through Earth's atmosphere, where shifting pockets of air distort incoming light — the same effect that makes stars appear to twinkle. NASA's Hubble program page explains that this motion blurs images taken from the ground and that the atmosphere also absorbs most ultraviolet and infrared radiation before it reaches the surface.

By orbiting roughly 300 miles above Earth, Hubble avoids that atmospheric blurring and can detect objects about ten times fainter than equivalent ground telescopes can, NASA states. Space telescopes can also observe continuously, without weather, daylight or city light pollution interfering.

Mirrors, not just lenses

Early telescopes used glass lenses to bend light, a design called refracting. NASA's Telescopes 101 explainer notes that large lenses become heavy and difficult to launch, so most modern telescopes are reflecting telescopes that use curved mirrors instead, since mirrors can be made thinner and lighter for the same size.

In a typical reflecting design, light strikes a large primary mirror shaped like a paraboloid and bounces to a smaller secondary mirror. On the James Webb Space Telescope specifically, NASA's description of Webb's mirrors adds two more steps: light from the secondary mirror reflects to a concave tertiary mirror and then to a flat fine steering mirror before finally reaching the science instruments.

Different telescopes, different light

Visible, ultraviolet and infrared light can all be focused with similar mirror shapes but different coatings. Webb's 18 hexagonal primary-mirror segments, made of beryllium metal and coated with a 100-nanometre layer of gold, are tuned specifically to reflect infrared light efficiently, according to NASA's description of Webb's mirrors. Hubble, by contrast, is built to collect ultraviolet, visible and near-infrared light, NASA says.

X-rays behave differently: they would simply penetrate or be absorbed by an ordinary mirror, much like bullets hitting a wall, the Chandra X-ray Center at Harvard explains. Chandra's mirrors are instead shaped like nested cylindrical barrels coated with iridium, reflecting X-rays only at a shallow 'grazing' angle rather than head-on, the center says. Chandra uses four nested mirror pairs, eight mirrors in total, polished and aligned to within 1.3 micrometres.

Gamma rays are even more energetic and pass straight through conventional optics altogether. NASA notes that gamma-ray telescopes such as Fermi instead use detectors that track particles produced when gamma rays strike dense material like tungsten, reconstructing the ray's origin from those particle tracks rather than focusing the rays with a mirror.

Why location matters: Webb's distant orbit

Webb does not orbit Earth closely the way Hubble does. NASA's mission page says Webb loops around the second Sun-Earth Lagrange point, L2, about 1.5 million kilometers from Earth, completing one circuit roughly every six months. At that location, the Sun, Earth and Moon stay on one side of the spacecraft, letting Webb's tennis-court-sized sunshield block their heat and light continuously.

That shielding is essential because Webb observes infrared light, which is essentially heat. NASA states the telescope operates at extremely cold temperatures — roughly minus 370 degrees Fahrenheit, according to Webb's orbit page — so that any stray warmth from the Sun or Earth does not flood the detectors with infrared noise and overwhelm the faint signals from distant galaxies.

Seeing the past, not just the distant

Because light travels at a fixed, finite speed, looking farther into space means looking further back in time. NASA's science overview of Webb notes the telescope can detect infrared light from galaxies more than 13.5 billion light-years away — meaning the light being captured today left those galaxies more than 13.5 billion years ago, when the universe was young. Observing the light, in effect, shows the object as it existed at the time the light departed, not as it looks now.

Webb carries four science instruments to analyze that ancient light: NIRCam and NIRSpec for near-infrared imaging and spectroscopy, MIRI for mid-infrared observations, and a Fine Guidance Sensor paired with the NIRISS instrument, according to NASA's telescope overview.

From mirror to image on Earth

Modern space telescopes do not use photographic film. NASA's explainer on how telescopes work states that light focused by the mirrors falls on electronic detectors, which convert it into digital signals. Those signals are stored by onboard computers and transmitted to ground stations on Earth, where scientists reconstruct the data into the images released publicly.