5.1: Telescopes

Measuring Light

If someone needs to know the size of a car, they can measure its dimensions with a meter stick or measuring tape. If they also wanted to know the weight of the car, they would just need to find a large enough scale. How do astronomers measure light? Because 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.

There are many different types of light, including visible, infrared, and more, and each one requires a different type of tool to collect it. Sometimes, astronomers can use the same telescope to observe different wavelengths of light by including an instrument attached to the telescope that sorts the incoming light by wavelength. For example, the astronomers might want to separate blue light from red light so that they can determine the temperature of a star. This can be accomplished by using a filter. In other cases, such as radio wavelengths, the design of the telescope is completely different. For now, we will focus on visible light telescopes, and we will discuss other wavelengths by the end of this chapter.

Optics and Spyglasses

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.

The creation of lenses similar to the one in Figure \(\PageIndex{1}\) eventually led to the invention of the telescope. Jan Lipperhey of Holland, a spectacle maker, is given credit for inventing the first telescope in 1608. He called his telescope Kijker, meaning looker in Dutch. Lipperhey thought the telescope’s best use was military. Some historians argue that Lipperhey stole the idea from Hans and Zacharias Janssen, other spectacle makers, who supposedly built a telescope in 1595. Jacob Metius also applied for a patent for the telescope around the same time as Lipperhey and the Janssens. We may never know who made the first telescope, but it was Galileo Galilei's use of a telescope, Figure \(\PageIndex{2}\), to observe the night sky that revolutionized ideas about the nature of the planets and the position of Earth.

Figure \(\PageIndex{2}\): Galileo's Telescope. Galileo used this telescope to make observations of the Moon, Venus, and the moons of Jupiter that fundamentally changed astronomy. (CC BY 4.0; Fraknoi, et al. via Openstax Astronomy 2nd ed.).

How Telescopes Work

These first telescopes changed astronomy because they could collect and magnify visible light so that the observer could see greater detail. The most important functions of a telescope are to collect the faint light from an astronomical source and to focus all the light into a point or an image. Most objects of interest to astronomers are extremely faint. The more light a telescope can collect, the easier it is for astronomers to study distant objects.

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.

In addition to holding the mirror, the steel structure of a telescope is designed so that the entire telescope can be pointed quickly toward any object in the sky. Since Earth is rotating, the telescope must have a motorized drive system that moves it very smoothly from east to west at exactly the same rate that Earth is rotating from west to east, so it can continue to point at the object being observed. A telescope mount is the telescope’s support structure. The two main types of telescope mounts are alt-azimuth and equatorial mounts. The Alt-azimuth mounts move in two directions: left to right, or up and down. The word alt-azimuth is a combination of altitude and azimuth. Equatorial mounts track the apparent motion of the stars by aligning one of its axes parallel to Earth’s axis. All this machinery must be housed in a dome to protect the telescope from weather and other debris. The dome has an opening in it that can be positioned in front of the telescope and moved along with it, so that the light from the objects being observed is not blocked.

Types of Telescopes

As mentioned previously, the primary objective of a telescope can be a lens or a mirror. 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. Galileo’s telescopes were refractors, as are today’s binoculars and field glasses. 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.

After Galileo, astronomers attempted to build larger and larger refractors to see fainter objects. However, they discovered multiple challenges with building large refractors. One problem is that light must pass through the lens, so the glass must be perfect all the way through. Over time, lens makers learned that it is very difficult to make large pieces of glass without flaws and bubbles in them. Also, both sides of the lens must be manufactured to precisely the right shape in order to produce a sharp image, adding a great deal of time and cost to craft larger lenses. Finally, the lens can only be supported around its edges, just like the frames of eyeglasses. The force of gravity causes a large lens to sag and distort the path of the light rays as they pass through it. Currently, the largest refracting telescope is the 40-inch refractor at Yerkes Observatory in Wisconsin, one of many telescope projects led by George Ellery Hale.

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 \(\PageIndex{3}\).

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. 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.

Telescopes designed with mirrors avoid the problems of refracting telescopes. Because the light is reflected from the front surface only, flaws and bubbles within the glass do not affect the path of the light. In a telescope designed with mirrors, only the front surface has to be manufactured to a precise shape, and the mirror can be supported from the back. For these reasons, most astronomical telescopes today use a mirror rather than a lens to form an image. The first successful reflecting telescope was built by Isaac Newton in 1668.

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.

Earth's atmosphere is in constant motion. It contains many small-scale blobs of gas that range in size from inches to several feet. Each cell has a slightly different temperature from its neighbor, and each cell acts like a lens, refracting the path of the light by a small amount. This bending slightly changes the position where each light ray finally reaches the detector in a telescope. The blobs of air are in motion, often in different directions at different altitudes. As a result, the path followed by the light is constantly changing. The result is a blurred image, and because the cells are being blown by the wind, the nature of the blur will change many times each second. You have probably noticed this effect as the twinkling of stars seen from Earth. The light beams are bent enough that part of the time they reach your eye, and part of the time some of them miss, thereby making the star seem to vary in brightness or even color. In space, the light of the stars is steady.

The resolution of an image is measured in units of angle on the sky, typically in units of arcseconds. One arcsecond is 1/3600 degree, and there are 360 degrees in a full circle. So we are talking about tiny angles on the sky. The best images obtained from the ground with traditional techniques reveal details as small as several tenths of an arcsecond across. One of the main reasons for launching the Hubble Space Telescope was to escape Earth’s atmosphere and obtain even sharper images.

Since we can’t put every telescope into space, astronomers have devised a technique called adaptive optics that can reduce the blurring effect of Earth's atmosphere. This technique makes use of a small flexible mirror placed in the beam of a telescope. A sensor measures how much the atmosphere has distorted the image, and as often as 500 times per second, it sends instructions to the flexible mirror on how to change shape in order to compensate for distortions produced by the atmosphere. The light is thus brought back to an almost perfectly sharp focus at the detector. With adaptive optics, ground-based telescopes can achieve resolutions of 0.1 arcsecond or a little better in infrared wavelengths, Figure \(\PageIndex{5}\). This impressive image is the equivalent of the resolution that the Hubble Space Telescope achieves in the visible light.

Figure \(\PageIndex{5}\): One of the clearest pictures of Jupiter ever taken from the surface of Earth, this image was produced with adaptive optics using an 8-meter-diameter telescope at the Very Large Telescope in Chile. Adaptive optics uses infrared wavelengths to remove atmospheric blurring, resulting in a much clearer image. (CC BY 4.0; Fraknoi, et al. via Openstax Astronomy 2nd ed.).

Exercise \(\PageIndex{1}\) Calculating the Light-Collecting Area of a Telescope

Calculate the area of a 1-m diameter telescope, then calculate the area of a 4-m diameter telescope. 

Answer

Using the equation for the area of a circle,

$$ A = \frac{\pi d^2}{4} \nonumber$$

the area of a 1-m telescope is

$$ \frac{\pi d^2}{4} = \frac{\pi (1\text{ m})^2}{4} = 0.79\text{ m}^2 \nonumber$$

and the area of a 4-m telescope is

$$ \frac{\pi d^2}{4} = \frac{\pi (4\text{ m})^2}{4} = 12.6\text{ m}^2 \nonumber$$

Exercise \(\PageIndex{1}\) Calculating Ratios of Areas

Show that the ratio of the areas of the 1-m and 4-m telescopes is 16:1.

Answer

\[ \frac{12.6\text{ m}^2}{0.79\text{ m}^2} = 16. \nonumber \]

Therefore, with 16 times the area, a 4-m telescope collects 16 times the light of a 1-m telescope.

Further Exploration: Interactive Activity

• Explore the Yerkes Observatory website to learn about the history of astronomical observation, the development of large refracting telescopes, and the role of observatories in advancing our understanding of the universe.
• Explore the Galileo Project History of the Telescope resource to learn how telescopes evolved from Galileo’s early observations to the powerful instruments used in modern astronomy.