A telescope is mainly a light bucket
People often assume a telescope’s job is to magnify. Magnification matters, but the main job is to collect light. Your eye’s pupil is only a few millimeters wide, so it gathers very little light from a faint star or galaxy. A telescope’s large opening, called the aperture, collects far more and focuses it to a point, making faint objects visible.
The collecting area grows with the square of the aperture’s diameter. A telescope with an aperture twice as wide collects four times as much light (twice the width, so two times two). This is why astronomers keep building bigger telescopes, and why aperture is usually a better guide to performance than magnification claims on a box.
Refractors: lenses
The earliest telescopes used lenses. A refracting telescope has a large objective lens at the front that bends incoming light to a focus. A smaller lens at the eyepiece then magnifies the focused image for your eye. Telescopes of this type appeared in the Netherlands in the early 1600s, and Galileo famously improved on the design and used it to look at the Moon, Jupiter’s moons and the phases of Venus around 1609 and 1610.
Lenses have drawbacks. A lens bends different colors by slightly different amounts, which can produce colored fringes called chromatic aberration. Large lenses are also heavy and can only be supported at their edges, so they sag. For these reasons, the biggest research telescopes are not refractors.
Reflectors: mirrors
A reflecting telescope uses a curved mirror instead of a lens to gather light. Isaac Newton is credited with building an early practical reflector in the late 1600s. In a Newtonian design, a concave primary mirror at the bottom of the tube sends light back toward a small flat mirror, which directs it out to the side to an eyepiece.
Mirrors have several advantages. They reflect all colors equally, avoiding color fringing; they can be supported from behind, so they can be made much larger; and only one surface needs to be shaped precisely. Almost all large modern telescopes are reflectors. Many use more than two mirrors, and some large research mirrors are made of many segments that work together as a single surface.
Some backyard telescopes combine mirrors and a lens. These compound designs fold the light path, making a short, portable tube with a long focal length.
Magnification and resolution
Magnification equals the telescope’s focal length divided by the eyepiece’s focal length. Swapping eyepieces changes it. But there is a limit: past a certain point, enlarging the image just makes it blurry, because the telescope’s resolving power depends on aperture and the wavelength of light. A larger aperture can distinguish finer detail. Very high magnification on a small telescope is generally not useful.
Why the atmosphere gets in the way
Ground-based telescopes look through the air, which causes two problems.
First, the atmosphere is turbulent. Pockets of air at different temperatures bend light unevenly, so a star’s image shimmers and blurs. This is the same effect that makes stars twinkle. Astronomers call the blurring “seeing.” Observatories are usually built on high, dry mountains to reduce it, and many large telescopes use adaptive optics, in which a flexible mirror adjusts many times per second to cancel out some of the distortion.
Second, the atmosphere absorbs or blocks some kinds of light. Much of the ultraviolet, X-ray and infrared range cannot reach the ground, and what does reach the ground is partly obscured. Layers of the atmosphere that protect life from harmful radiation also stop astronomers from observing in those wavelengths. Visible light and radio waves pass through relatively well.
Why go to space
Telescopes in space avoid both problems. The Hubble Space Telescope, launched in 1990, has a mirror about 2.4 meters across, modest by the standard of today’s largest ground-based mirrors. It is nonetheless famous for sharp images because it sits above the turbulent air. Its early images were blurred by a flaw in the main mirror, which was corrected by astronauts in 1993.
The James Webb Space Telescope, launched in December 2021, has a segmented mirror about 6.5 meters across and observes mainly in the infrared. Infrared light lets astronomers see through dust clouds and observe very distant objects whose light has been stretched to longer wavelengths by cosmic expansion. Because warm objects glow in the infrared, Webb is shielded from the Sun and kept very cold, and it operates far from Earth near a location called the second Lagrange point, around 1.5 million kilometers away.
Space telescopes are costly and cannot easily be repaired or upgraded, so ground-based and space-based instruments are usually seen as complementary rather than competing.
Beyond visible light
Optical telescopes are only one family. The visible spectrum is just a narrow slice of the electromagnetic spectrum, and astronomers also build radio dishes, infrared, X-ray and gamma-ray instruments. Each reveals different physical processes, and combining them gives a fuller picture of an object.
If you want to try this yourself, simple binoculars are a good start. They show the Moon’s craters and many star clusters. Many science museums and planetarium shows also offer chances to look through larger instruments or learn the night sky. As a starting guide, astronomy basics can help you decide what to look for first.