WWGD? What would Galileo do (if he could see the things we can see through a telescope)?
If you think oceans are inhospitable, try space! Humans have been to our Moon and many have orbited Earth in spacecraft, even staying for months at a time in a space station. Much of what has been learned about space since Galileo has been through a telescope. Although astronomers use very large telescopes, many of which pick up wavelengths of energy other than visible light, there is still much to be gained from looking at the planets and stars on a clear night. If you haven't ever looked at the night sky through a telescope, you should try to soon!
Electromagnetic (EM) radiation is energy that is transmitted through space as a wave. Light is one type of EM wave. An EM wave has two components: an electric field and a magnetic field. Each of these components oscillates between positive and negative values. The distance between two adjacent oscillations is called a wavelength. Frequency measures the number of wavelengths that pass a given point every second. Wavelength and frequency are reciprocal, which means that as one increases, the other decreases.
Visible light—the light that human eyes can see—comes in a variety of colors. The color of visible light is determined by its wavelength. Visible light ranges from wavelengths of 400 nm to 700 nm, corresponding to the colors violet through red. EM radiation with wavelengths shorter than 400 nm or longer than 700 nm exists all around you—you just can’t see it. The full range of electromagnetic radiation, or the electromagnetic spectrum, is shown in Figure below.
(a) Visible light is part of the electromagnetic spectrum, which ranges from gamma rays with very short wavelengths, to radio waves with very long wavelengths. (b) These are images of the same scene. In the top, only the wavelengths of visible light show. In the bottom, a layer of thick clouds appears in the infrared wavelengths.
Like our Sun, every star emits light at a wide range of wavelengths, all across the visible spectrum and even outside the visible spectrum. Astronomers can learn a lot from studying the details of the spectrum of light from a star.
Types of Telescopes
The term "telescope" was coined by the Italian scientist and mathematician Galileo Galilei (1564–1642). Galileo built the first telescope in 1608 and subsequently made many improvements to telescope design.
Telescopes that rely on the refraction, or bending, of light by lenses are called refracting telescopes, or simply "refractors." Galileo’s and other early telescopes were all refractors. Many of the small telescopes used by amateur astronomers today are refractors. Refractors are particularly good for viewing details within our solar system, such as the surface of Earth’s Moon or the rings around Saturn.
Around 1670, Sir Isaac Newton created the first reflecting telescopes, or "reflectors." The mirrors in a reflecting telescope are much lighter than the heavy glass lenses in a refractor. This is significant, because:
- To support the thick glass lenses, a refractor must be strong and heavy.
- Mirrors are easier to make precisely than glass lenses.
- Because they do not need to be as heavy to support the same size lens, reflectors can be made larger than refractors.
Larger telescopes can collect more light and so they can study dimmer, or more distant, objects. The largest optical telescopes in the world today are reflectors. Several large reflecting telescopes are located at the summit of Mauna Loa volcano in Hawaii, shown in Figure below.
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Telescopes on top of a volcano in Hawaii.
Even larger telescopes are built to collect light at longer wavelengths—radio waves. Radio telescopes collect and focus radio waves, or microwaves, the waves with the shortest wavelength, from space.
The largest single telescope in the world is at the Arecibo Observatory in Puerto Rico (Figure below). This telescope is located in a naturally occurring hole so that it does not collapse under its own weight. Since the telescope is set into the ground, it cannot be aimed to different parts of the sky and so can only observe the part of the sky that happens to be overhead at a given time.
The radio telescope at the Arecibo Observatory has a diameter of 305 m.
A group of radio telescopes can be linked together with a computer so that they are all observing the same object (Figure below). The computer combines the data, making the group function like one single telescope.
Radio telescopes at the Very Large Array, the National Radio Observatory in New Mexico.
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Earth’s atmosphere not only blocks radiation in some parts of the EM spectrum, but also distorts light. Observatories built on high mountains lessen these problems, but space telescopes avoid such problems completely because they orbit outside Earth’s atmosphere. Space telescopes can carry instruments to observe objects emitting various types of electromagnetic radiation, such as visible, infrared, or ultraviolet light, gamma rays, and x-rays.
The Hubble Space Telescope (HST), shown in Figure below, has orbited Earth for more than 20 years, sending back amazing images and helping to answer many of the biggest questions in astronomy. The James Webb Space Telescope, designed to replace the aging Hubble, is targeted for launch in 2018.
(a) The Hubble Space Telescope orbits Earth at an altitude of 589 km (366 mi). It collects data in visible, infrared, and ultraviolet wavelengths. (b) This starburst cluster is one of the many fantastic images taken by the HST over the past two decades.
- Electromagnetic radiation is energy transmitted as waves with different wavelengths, which appear in the electromagnetic spectrum.
- Refracting and reflecting telescopes are optical telescopes that use lenses to gather light.
- Radio telescopes collect radio waves and are sometimes used in large arrays.
- Space telescopes can see much more than Earth-bound telescopes, since the atmosphere doesn’t affect their information.
- Describe each of the types of telescopes discussed here: reflecting, refracting, radio, and space.
- What are the limitations of each type of telescope discussed here?
- Look at the electromagnetic spectrum. Do you think other types of telescopes could get other types of information if they gathered different wavelengths?
Use this resource to answer the questions that follow.
- Why do we need telescopes?
- What type of telescopes do modern astronomers use and where do they view the information?
- Describe the three powers of a telescope.
- What is refraction? What does the light eventually reach? Why does this happen?
- What is caused by dispersion?
- Why do stars twinkle?
- How does a refractor work?