3.3: Telescopes

Measuring Light

Since light travels at incredibly high speed, astronomers must collect and record it before they can analyze it. To collect light, astronomers can take advantage of the fact that light behaves like a particle, called a photon. So, the first step to measuring light is building a tool that can collect photons. The best tool for this job is the telescope. Just as a garbage can collects more rain than a coffee cup, large telescopes gather much more light than smaller telescopes, and significantly more than the human eye. Each type of light in the electromagnetic spectrum requires a different type of tool to collect it. Sometimes, astronomers can use the same telescope to observe different wavelengths of light by using a filter. In other cases, such as radio wavelengths, the design of the telescope is completely different.

The Telescope

Optics is the science that deals with all aspects of visible light. A lens is a transparent piece of material that bends the rays of light passing through it. If the light rays are parallel as they enter, the lens brings them together in one place to form an image, Figure \(\PageIndex{1}\). Refraction is when the path of light bends as it passes from one medium to another, such as from air into a transparent lens. If the curvatures of the lens surfaces are just right, all parallel rays of light are refracted, in such a way that they converge toward a point, called the focus of the lens. At the focus, an image of the light source appears. In the case of parallel light rays, the distance from the lens to the location where the light rays focus, or image, behind the lens is called the focal length of the lens.

Figure \(\PageIndex{1}\): Lens Focusing Light. When the path of parallel rays of light is bent by a convex lens, the light will all come together at the focus to form an image of the light source. The focal length is the distance from the lens to the focus. (CC BY; Fraknoi, et al. via Openstax Astronomy 2nd ed.). Alternative description of Figure \(\PageIndex{1}\).

Consider a lens bringing the light of a distant star into focus. If a star radiates light in all directions, how are two rays of light from the same star parallel to each other? Remember that the stars, and other astronomical objects, are all extremely far away. By the time the few rays of light pointed toward us actually arrive at Earth, they are mostly parallel to each other.

In a telescope, the primary objective refers to the part of the telescope that collects and focuses the light. The first telescopes used a lens as the primary objective, but as we will see in the next section, the lens can be replaced with a mirror. The aperture of a telescope is the size of the main opening of the telescope and also the size of the primary objective lens or mirror. The amount of light a telescope can collect increases with the size of the aperture. A telescope with an aperture that is 4 meters in diameter can collect 16 times as much light as a telescope that is 1 meter in diameter. The diameter is squared because the area of a circle is \(\frac{\pi d^2}{4} \nonumber \), where d is the diameter of the circle.

To view the image formed by the lens in a telescope, we use an additional lens called an eyepiece. Using different eyepieces, we can change the magnification (or size) of the image and also redirect the light to a more accessible location. Stars look like points of light, and magnifying them makes little difference, but the image of a planet or a galaxy, which has structure, can often benefit from being magnified.

Types of Telescopes

Since a lens refracts light, a telescope that has a primary lens is called a refracting telescope. Refractors are usually built using a long tube with a large glass lens at one end. Eventually, astronomers discovered a problem with refractors called chromatic aberration. Glass refracts light by varying amounts depending on wavelength, so each wavelength of light is focused at a slightly different spot, creating a blurry image. As astronomers attempted to build larger and larger refractors to see fainter objects they discovered multiple challenges with building large refractors. Light must pass through the lens, so the glass must be perfect all the way through and have a perfect shape on both sides, which is very hard to make on a large scale. Finally, the lens can only be supported around its edges, just like the frames of eyeglasses. Finally, a very large lens will sag under its own weight and distort the path of the light rays as they pass through it.

Reflection is when light bounces off of a surface. Any telescope that has a primary mirror is called a reflecting telescope. Reflectors use a concave, or bowl shaped, mirror as the telescope’s primary objective, rather than a lens or lenses. The mirror is curved like the inner surface of a sphere, and it reflects light in order to form an image, Figure \(\PageIndex{3}\). Telescope mirrors are coated with a shiny metal, usually silver, aluminum, or, occasionally, gold, to make them highly reflective, and avoiding the challenges with lenses. If the mirror has the correct shape, all parallel rays are reflected back to the same point, the focus of the mirror. Thus, images are produced by a mirror exactly as they are by a lens. Note that telescope mirrors have reflective coating on the surface, while mirrors found at home are coated on the back of the glass.

Figure \(\PageIndex{3}\): A refractor telescope uses a lens to focus light into an image, while a reflector telescope uses a mirror. (CC BY 4.0; Fraknoi, et al. via Openstax Astronomy 2nd ed.). Alternative description of Figure \(\PageIndex{3}\).

Reflector telescope types differ based on what happens to the light after it is reflected from the primary mirror (Figure \(\PageIndex{4}\)). In any reflecting telescope, the concave mirror is placed at the bottom of a tube or open framework. The mirror reflects the light back up the tube to form an image near the front end at a location called the prime focus. A prime focus telescope collects the image data directly at the prime focus, which means that either an eyepiece or a camera is located at the prime focus. As long as the method of collecting data is very small relative to the aperture, the majority of light entering the telescope will reach the primary mirror.

As an alternative, the use of a small secondary mirror at the prime focus allows more light to get through the system. A telescope with a Newtonian focus works first by light entering the telescope and traveling to a concave mirror. The light is then reflected back to a smaller, secondary mirror and reflected out of the side of the telescope to the eyepiece or camera. A telescope with a Cassegrain focus has a primary mirror with a hole in the center. When the light reaches the prime focus, it is reflected through the hole in the primary mirror to the eyepiece or camera at the bottom of the telescope. Most large telescopes have a Cassegrain focus.

Figure \(\PageIndex{4}\): Focus Arrangements for Reflecting Telescopes. Reflecting telescopes have different options for where the light is viewed by the observer. The light can be observed at the prime focus, redirected to the side of the telescope (Newtonian focus), or reflected back through a hole in the primary mirror (Cassegrain focus). (CC BY 4.0; Fraknoi, et al. via Openstax Astronomy 2nd ed.). Alternative description of Figure \(\PageIndex{4}\).

Resolution

In addition to gathering as much light as they can, astronomers also want to have the sharpest images possible. Resolution refers to the precision of detail present in an image, or the smallest features that can be distinguished. One factor that determines how good the resolution will be is the size of the telescope. Larger apertures produce sharper images. As astronomers built larger telescopes, they realized that the new challenge in improving image resolution was Earth's atmosphere. The constant motion of the air causes the twinkling of stars seen from Earth. In space, the light of the stars is steady which is why astronomers build space telescopes like the James Webb Space Telescope (JWST). For ground-based telescopes, astronomers use a technique called adaptive optics that can reduce the blurring effect of Earth's atmosphere. It uses a flexible mirror to constantly correct for atmospheric distortion. The light is thus brought back to an almost perfectly sharp focus at the detector.

For spacecraft mapping objects in the Solar System, the quality of the image is judged by its spatial resolution. This refers to the size of each pixel on the surface of an object. For example, a spatial resolution of 1 km per pixel means that a single pixel in the image collected the light of a region that is roughly 1 km by 1 km. If this was a visible image collected by a satellite orbiting the Earth it means that an image of a city might have a single pixel covering a dozen city blocks. The image might help you understand the size of the city but not much about the buildings. A resolution of 10 meters per pixel would resolve buildings, but not cars or people. When mapping other planets, higher resolution gives astronomers more information about the geological structures on the surface. However, higher resolution requires more powerful detectors that are very expensive to build.

Modern Telescopes

Astronomers and engineers are working on the technologies that will allow us to explore even more distant parts of the universe and to see them more clearly. The Vera Rubin Observatory produced its first images of the sky, also known as first light, in 2025. The observatory is named after the American astronomer whose work led us to the understanding that much of the universe is made of a mysterious substance that scientists call dark matter. This 8.4-meter telescope has a significantly larger field of view than any existing telescopes. It has begun its 10-year program to rapidly scan the sky to find transients, phenomena that change quickly, such as exploding stars and chunks of rock that orbit near Earth.

Figure \(\PageIndex{3}\): Located on Cerro Pachon in Chile, the Vera Rubin Observatory has an 8.4 meter primary mirror and has been operating since 2025. (CC BY 4.0; Rubin Observatory/NOIRLab/NSF/AURA/B. Quint via Wikimedia Commons). Alternative description of Figure \(\PageIndex{3}\).

Several groups of astronomers around the globe interested in studying visible and infrared light are exploring the feasibility of building ground-based telescopes with mirrors larger than 30 meters across, one-third the length of a football field. We do not have the technology to build and transport a single astronomical mirror that is 30 meters or larger in diameter. The primary mirror of these giant telescopes will consist of smaller mirrors, all aligned so that they act as a very large mirror in combination.

The most ambitious of these projects is the European Extremely Large Telescope (ELT). The design of the European ELT calls for a 39.3-meter primary mirror, which will follow the Keck Telescope design and be made up of 798 hexagonal mirrors, each 1.4 meters in diameter and all held precisely in position so that they form a continuous surface. Construction on the site in the Atacama Desert in Northern Chile started in 2014, and the telescope structure is nearing completion, Figure \(\PageIndex{4}\). Scientific first light is planned for 2030.

Figure \(\PageIndex{4}\): The Heart of the ELT. ESO's Picture of the Day for February 2, 2026, construction is ongoing on the mirror support system of the Extremely Large Telescope (ELT). (CC BY 4.0; ESO/J. C. Muñoz-Mateos via ESO). Alternative description of Figure \(\PageIndex{4}\).

International consortia with major contributions from U.S. astronomers have developed plans for the construction of two new large telescopes. One is a Thirty-Meter Telescope (TMT). The design of this telescope is similar to that of the European ELT and will make use of 492 hexagonal elements. Each segment is about 1.44 meters (56.6 inches) across corners. The segments are closely spaced, with gaps between the segments only 2.5 mm (0.1 inch) wide.

The Giant Magellan Telescope (GMT) is the second ELT project with major participation by U.S. astronomers. The GMT is also a segmented mirror telescope that employs seven stiff monolith 8.4-meter mirrors as segments. Construction has started at the selected site, which is near the Las Campanas Observatory on the southern edge of the Atacama Desert.

These giant telescopes will combine light-gathering power with high-resolution imaging. These powerful new instruments will enable astronomers to tackle many important astronomical problems. As just one example, they provide us images and spectra of planets around other stars and thus, perhaps, give us the first real evidence from the chemistry of these planets’ atmospheres that life exists elsewhere.

Radio Astronomy

Some objects in the universe give off light in the form of radio waves. Radio astronomy has been used to discover many amazing objects such as radio galaxies, quasars, and pulsars, all of which will not be discussed in this textbook but are worth an internet search. Radio astronomy is also used to study Solar System objects such as the Sun and Jupiter. Radio astronomy does not replace astronomy at different wavelengths, it adds to the data by giving astronomers new and different information. We will briefly review how radio telescopes are different from the ones discussed previously.

Radio Telescopes

Radio waves can produce a current in conductors of electricity such as metals. An antenna is such a conductor: it intercepts radio waves, which create a feeble current in it. The current is then amplified in a radio receiver until it is strong enough to measure or record. Radio astronomers use sophisticated data-processing techniques that allow thousands of separate frequency bands to be detected simultaneously. Radio waves are reflected by conducting surfaces, just as light is reflected from a shiny metallic surface. A radio-reflecting telescope consists of a concave metal reflector, or dish, similar to a telescope mirror. The radio waves collected by the dish are reflected to a focus, where they can then be directed to a receiver and analyzed.

Because radio waves have such long wavelengths, they present challenges for astronomers who need good resolution. In fact, even the largest radio dishes on Earth, cannot make out as much detail as the small reflecting telescope. To overcome this difficulty, radio astronomers have learned to sharpen their images by linking two or more radio telescopes together electronically, called an interferometer. Interference is the way that multiple waves interact with each other when they arrive in an instrument. This interaction allows astronomers to find more detail in the observations. The resolution of an interferometer depends upon the separation of the telescopes, not upon their individual apertures. Two telescopes separated by 1 kilometer provide the same resolution as would a single dish 1 kilometer across.

To get even better resolution, astronomers combine a large number of radio dishes into an interferometer array. Computer processing of the results permits the reconstruction of a high-resolution radio image. The largest interferometer in the United States is the National Radio Astronomy Observatory’s Jansky Very Large Array (VLA) near Socorro, New Mexico. It consists of 27 movable radio telescopes, each having an aperture of 25 meters, spread over a total span of about 36 kilometers, achieving a resolution of about 1 arcsecond. The Atacama Large Millimeter/submillimeter array (ALMA) in the Atacama Desert of Northern Chile, at an altitude of 16,400 feet, consists of 12 7-meter and 54 12-meter telescopes, and can achieve baselines up to 16 kilometers. Since it became operational in 2013, it has made observations at resolutions down to 6 milliarcseconds (0.006 arcseconds)

Initially, the size of interferometer arrays was limited by the requirement that all of the dishes be physically wired together. Astronomers, with the use of current technology and computing power, have learned to time the arrival of electromagnetic waves coming from space very precisely at each telescope and combine the data later. The United States operates the Very Long Baseline Array (VLBA), made up of 10 individual telescopes stretching from the Virgin Islands to Hawaii, Figure \(\PageIndex{3}\). The VLBA, completed in 1993, can form astronomical images with a resolution of 0.0001 arcseconds, permitting features as small as 10 astronomical units to be distinguished at the center of our Galaxy.

Radar Astronomy

Radar is the technique of transmitting radio waves to an object in our Solar System and then detecting the light that the object reflects back. The time required for the round trip can be measured electronically with great precision. Because we know that radio waves travel at the speed of light, we can determine the distance to the object or a particular feature on its surface. In addition, radar observations have determined the rotation periods of Venus and Mercury, probed tiny Earth-approaching asteroids, and allowed us to investigate the mountains and valleys on the surfaces of Mercury, Venus, Mars, and the large moons of Jupiter. Any radio dish can be used as a radar telescope if it is equipped with a powerful transmitter as well as a receiver. Currently, the only operating radar sources is Goldstone Solar System Radar which is part of the Deep Space Network.

Further Exploration: Telescopes

• Explore the Palomar Observatory website to learn about the historic Hale 5-meter telescope and its role in advancing twentieth-century astronomical research.
• Visit the ESO Very Large Telescope website to investigate how one of the world's premier observatories uses large mirrors and advanced instruments to study the universe.
• Explore the Gemini Observatory website to learn how modern optical and infrared telescopes collect high-resolution observations of stars, planets, and galaxies.
• Watch the Modern Marvels: Keck Observatory video from The History Channel to discover how segmented mirrors and innovative engineering made some of the world's largest telescopes possible.
• Explore the Vera C. Rubin Observatory website to learn about large-scale sky surveys and how astronomers search for transient phenomena, near-Earth objects, and evidence of dark matter.
• Visit the ESO Extremely Large Telescope website to explore the design and scientific goals of one of the largest optical and infrared telescopes ever constructed.
• Explore the Thirty Meter Telescope (TMT) website to learn about the technologies and scientific objectives of next-generation ground-based astronomy.
• Visit the Giant Magellan Telescope website to investigate how giant segmented-mirror telescopes will provide unprecedented views of distant planets, stars, and galaxies.
• Explore the International Dark-Sky Association website to learn about light pollution and efforts to preserve dark skies for astronomy, wildlife, and human health.
• Participate in the Globe at Night citizen-science project to measure local light pollution and contribute observations to a global database.
• Read Dark Skies and Light Pollution by Andrew Fraknoi to explore the causes and consequences of light pollution and ways to reduce its impact.
• Explore the National Radio Astronomy Observatory website to learn how radio telescopes detect radio waves from space and support astronomical research across the electromagnetic spectrum.
• Learn about the history and scientific discoveries of the Arecibo Observatory, once the world's largest single-dish radio telescope and a leading center for radar astronomy.
• Explore the Very Long Baseline Array to discover how widely separated radio telescopes work together as an interferometer to produce extremely high-resolution images.
• Watch ALMA: In Search of Our Cosmic Origins to learn how ALMA studies cold gas and dust to investigate the formation of stars, planets, and galaxies.
• Explore the Five-hundred-meter Aperture Spherical Telescope (FAST) to learn how the world's largest filled-aperture radio telescope is used to study pulsars, galaxies, and the radio universe
• Learn more about the Deep Space Network which is used to communicate wit spacecraft and includes Gladstone Solar System Radar.