For most of human history, the sky could only be studied with the naked eye. Then, in the early seventeenth century, a simple tube with two lenses changed everything. When Galileo Galilei pointed his small telescope at the sky in 1609 and 1610, he saw mountains on the Moon, four moons orbiting Jupiter and countless stars in the Milky Way. Since then, telescopes have grown from handheld tubes to giant mirrors on mountaintops, radio dishes the size of valleys and observatories floating in space. They have revealed galaxies billions of light-years away and the faint afterglow of the Big Bang. This explainer describes how telescopes work and why there are so many different kinds.
The main job of a telescope: collecting light
Many people think a telescope's main purpose is to magnify. In fact, its most important job is to collect light. Most objects in the sky are extremely faint. The human eye's pupil is only about a quarter of an inch (7 millimeters) across at most in the dark, so it can collect only a tiny amount of light. A telescope acts like a giant light bucket, gathering far more light and focusing it into a bright image.
The size of the main lens or mirror, called the aperture, determines how much light a telescope collects. Light-gathering power grows with the area of the aperture, so doubling the diameter collects four times as much light. A telescope with a 6-inch (150 mm) aperture collects roughly 460 times as much light as a dark-adapted human eye.
Resolution: seeing fine detail
The second important job of a telescope is resolution, the ability to see fine details and separate objects that are close together, such as two stars in a double star system. Larger apertures also give better resolution. Because of the wave nature of light, there is a fundamental limit to how sharp an image can be, and a bigger aperture lowers that limit.
Magnification: the least important number
Magnification depends on the eyepiece used with the telescope. It is calculated by dividing the telescope's focal length by the eyepiece's focal length. A telescope with a focal length of 1,000 mm used with a 10 mm eyepiece magnifies 100 times.
However, more magnification is not always better. Magnifying a dim, blurry image only produces a larger dim, blurry image. A useful rule of thumb is that the maximum practical magnification is about 50 times the aperture in inches, or about 2 times the aperture in millimeters, and atmospheric conditions often limit it further. Telescopes advertised with "600× magnification" on the box are often a warning sign of low quality.
Refracting telescopes
A refractor uses lenses to bend, or refract, light to a focus. Light enters through a large lens at the front, called the objective lens, which bends the light so it converges at a focal point near the back of the tube. An eyepiece then magnifies the image.
Advantages:
- Sealed tube, so little maintenance.
- Sharp, high-contrast views, excellent for the Moon, planets and double stars.
- Rugged and quick to set up.
Disadvantages:
- Large lenses are heavy, expensive and difficult to make.
- Simple lenses suffer from chromatic aberration: different colors of light bend by different amounts, producing colored fringes around bright objects. High-quality "apochromatic" refractors use special glass to reduce this, but they are expensive.
The largest refracting telescope ever used for research, at Yerkes Observatory in Wisconsin, has a lens 40 inches (1 meter) across. Larger lenses would sag under their own weight.
Reflecting telescopes
A reflector uses a curved mirror instead of a lens to collect and focus light. Isaac Newton built one of the first practical reflecting telescopes in 1668. In a Newtonian reflector, light enters the open tube and hits a concave primary mirror at the bottom, which reflects it back up to a small flat secondary mirror. The secondary mirror directs the light out the side of the tube to the eyepiece.
Advantages:
- Mirrors do not suffer from chromatic aberration, because all colors reflect the same way.
- Large mirrors are easier and cheaper to make than large lenses, and they can be supported from behind.
- Reflectors offer the most aperture for the money.
Disadvantages:
- The open tube can collect dust, and the mirrors occasionally need alignment, called collimation.
- The secondary mirror blocks a small part of the incoming light.
All of the world's largest optical telescopes are reflectors. Some, such as the two Keck telescopes in Hawaii, use mirrors made of 36 hexagonal segments working together as one, giving each telescope a mirror 10 meters (33 feet) across.
Catadioptric telescopes
Catadioptric telescopes, such as Schmidt-Cassegrain and Maksutov-Cassegrain designs, combine lenses and mirrors. They fold the light path, so a long focal length fits into a short, compact tube. They are popular with amateur astronomers who want portability and versatility.
Telescopes beyond visible light
Visible light is only a small part of the electromagnetic spectrum. Objects in space also emit radio waves, infrared light, ultraviolet light, X-rays and gamma rays. Each type of light reveals different things.
Radio telescopes
Radio telescopes use large dish-shaped antennas to collect radio waves. Because radio waves are much longer than visible light waves, radio telescopes must be very large to achieve good resolution. Astronomers solve this by linking many dishes together, a technique called interferometry. Arrays like the Very Large Array in New Mexico and ALMA in Chile combine signals from many antennas. The Event Horizon Telescope linked radio observatories across the globe to create a virtual telescope the size of Earth and captured the first image of a black hole's shadow in 2019.
Radio telescopes have revealed the cosmic microwave background, pulsars and the structure of cold gas clouds where stars form.
Infrared telescopes
Infrared light lets astronomers see through dust clouds that block visible light, revealing newborn stars and the centers of galaxies. It also allows them to see very distant galaxies whose light has been stretched to longer wavelengths by the expansion of the universe. The James Webb Space Telescope, launched in 2021, is the most powerful infrared telescope ever built, with a 6.5-meter (21-foot) segmented gold-coated mirror.
X-ray and gamma-ray telescopes
X-rays and gamma rays come from extremely hot and energetic objects, such as gas falling into black holes, exploding stars and neutron stars. These high-energy rays pass through ordinary mirrors, so X-ray telescopes use nested mirrors arranged at very shallow angles, like skipping stones, to focus them. Because Earth's atmosphere absorbs X-rays and gamma rays, these telescopes must operate in space.
Why put telescopes in space?
Earth's atmosphere causes several problems for astronomers:
- Turbulence: Moving air makes stars twinkle and blurs images. Astronomers call this "seeing."
- Absorption: The atmosphere blocks most ultraviolet, X-ray, gamma-ray and much infrared light.
- Light pollution and weather: City lights and clouds interfere with observations.
Space telescopes, such as the Hubble Space Telescope, launched in 1990, avoid these problems and produce extremely sharp images. Ground-based telescopes fight back with adaptive optics, a technology that uses deformable mirrors adjusting hundreds of times per second to cancel out atmospheric distortion. Large telescopes are also built on high, dry mountaintops in places like Chile, Hawaii and the Canary Islands, above much of the atmosphere.
Choosing a first telescope
For beginners, the best telescope is the one you will actually use. Some practical tips:
- Start with binoculars. A good pair, such as 7×50 or 10×50, shows craters on the Moon, Jupiter's moons and many star clusters, and teaches you to find your way around the sky.
- Prioritize aperture and a stable mount. A wobbly mount makes even a good telescope frustrating to use.
- Consider a Dobsonian reflector. These simple, sturdy reflectors on a rocking base offer large apertures for a modest price and are popular first telescopes.
- Ignore magnification claims. Focus on aperture and optical quality.
- Join an astronomy club. Many clubs hold public observing nights where you can try different telescopes before buying.
Questions readers ask
What is the most important feature of a telescope? Aperture, the diameter of the main lens or mirror. It determines how much light the telescope collects and how much detail it can show.
Why are big telescopes reflectors? Large mirrors are easier to make, support and keep free of color distortion than large lenses.
Why do space telescopes give better images? They avoid the blurring and absorption caused by Earth's atmosphere.
Can I see galaxies with a backyard telescope? Yes, especially bright ones like the Andromeda Galaxy, but they usually appear as faint smudges. Photos show far more detail because cameras collect light over long exposures.
What does the James Webb Space Telescope see? Mainly infrared light, allowing it to see the earliest galaxies, peer through dust into star-forming regions and study the atmospheres of planets around other stars.
In short
Telescopes are light collectors. Their aperture determines how much light they gather and how much detail they reveal, while magnification is set by the eyepiece. Refractors use lenses, reflectors use mirrors and catadioptric designs combine both. Radio, infrared, X-ray and gamma-ray telescopes reveal parts of the universe invisible to our eyes, and space telescopes escape the blurring atmosphere. From Galileo's small tube to the James Webb Space Telescope, these instruments have transformed our understanding of the cosmos, and a pair of binoculars is enough to begin exploring it yourself.