Telescopes in Astronomy

Jennifer Boyer

SCI/151

June 12, 2015
Robert Austin

Telescopes in Astronomy

What people currently know about the universe, along with all of its contents, is due in large part to the invention of telescopes. This paper discusses the science of sunlight and stars by explaining how the telescope has changed people’s view of the universe, as well as their place in it. This essay also discusses the major designs of telescopes, provides a list of each design’s strengths and weaknesses, describes the best places to build ground-based telescopes and why astronomers choose those places, and contrasts the strengths and weaknesses between building telescopes on Earth, in orbit, or even on the moon. Additionally, this paper explains how different frequencies of light tell more about the birth, life, and death in the nature and properties of the Sun, stars, and the universe. Lastly, this essay explains how telescopes operate in wavelengths of light that range from radio waves to gamma rays.
How Telescopes Changed People’s View The invention of the telescope significantly impacts the way people in the past and present view the Earth, other planets and solar systems, as well as the universe as a whole (Bennett, J., Donahue, M., & Schneider, N., & Voit, M., 2015). Until the invention of Galileo Galilei's (1564-1642) simple telescope, many people thought that the earth was the center of our solar system (Bennett, J., Donahue, M., & Schneider, N., & Voit, M., 2015). By showing that Venus underwent phases, Galileo successfully “proved that Venus must revolve around the Sun” and “thus supports the heliocentric solar system” (Lecture notes, n.d. Galileo: A Crisis in Conscience, para. 1). Earth is just a planet “orbiting a rather ordinary star, in a galaxy of more than a hundred billion stars, in an incredibly vast universe” (Bennett, J., Donahue, M., & Schneider, N., & Voit, M., 2015, p. 54). To gain a better understanding of this concept, telescopes provide people with a better perspective of their location in the universe and their galactic surroundings. Telescopes enable astronomers to see and study far away objects by using different forms of light, along with its varying wavelengths, to analyze these objects and determine their size, location, composition, movement, and temperature (Bennett, J., Donahue, M., & Schneider, N., & Voit, M., 2015). Depending on the type of telescope, astronomers can learn different things about celestial objects (Bennett, J., Donahue, M., & Schneider, N., & Voit, M., 2015).
Major Designs of Telescope Telescopes capture light that are invisible to the naked eye (Bennett, J., Donahue, M., & Schneider, N., & Voit, M., 2015). This light tells astronomers much information about celestial objects. “Telescopes come in two basic designs: refracting and reflecting” (Bennett, J., Donahue, M., & Schneider, N., & Voit, M., 2015, p. 122). All telescopes collect light and focus it with either a lens or a curved mirror. Another design of the telescope is the catadioptric telescope. “Catadioptric telescopes use a combination of mirrors and lenses to fold the optics and form an image” (Sharma, 2015, Catadioptric Telescope, para. 1). Both refracting and reflecting telescopes have their strengths and weaknesses.
Refracting Telescopes A refracting telescope “uses lenses to focus light” (Bennett, J., Donahue, M., & Schneider, N., & Voit, M., 2015, p. G-11). “A refracting telescope operates similar to an eye, using transparent lenses to collect and focus light (Bennett, J., Donahue, M., & Schneider, N., & Voit, M., 2015, p. 122). Galileo’s first simple telescopes were refracting telescopes that used lenses made of glass (Bennett, J., Donahue, M., & Schneider, N., & Voit, M., 2015). Refracting telescopes, or refractors, are the most commonly used telescopes by amateurs and “the most common form of the telescope - a long, thin tube where light passes in a straight line from the front objective lens directly to the eyepiece at the opposite end of the tube” (Sharma, 2015, The Refracting Telescope or Refractor, para. 1). Strengths of refracting telescopes. According to Gary Sharma (2015), some strengths of refracting telescopes are that they are “easy to use and consistent due to the simplicity of design, good for distant terrestrial viewing, and excellent for lunar, planetary and binary stargazing especially with larger apertures” (The Refracting Telescope or Refractor, para. 1). Refractors also possess other qualities, such as the “sealed tube protects optics and reduces image-degrading air currents” (Sharma, 2015, The Refracting Telescope or Refractor, para. 1). Furthermore, refracting telescopes are “rugged” and “need little or no maintenance” (Sharma, 2015, The Refracting Telescope or Refractor, para. 1). Weaknesses of refracting telescopes. “For most research purposes the refracting telescope has been superseded by the reflecting telescope which allows larger apertures” (Refracting telescope, 2015, para. 1). Refractors come with several other disadvantages, such as they “generally have small apertures, typically 3 to 5 inches” (Sharma, 2015, The Refracting Telescope or Refractor, para. 1). Refracting telescopes are also “less suited for viewing small and faint deep sky objects such as distant galaxies and nebulae” (Sharma, 2015, The Refracting Telescope or Refractor, para. 1). Furthermore, refractors are “heavier, longer and bulkier than equivalent aperture reflectors and catadioptrics” (Sharma, 2015, The Refracting Telescope or Refractor, para. 1). Another weakness of refracting telescopes is their “limited practical usefulness” (Sharma, 2015, The Refracting Telescope or Refractor, para. 1). Lastly, “good-quality refractors cost more per inch of aperture than any other kind of telescope” (Sharma, 2015, The Refracting Telescope or Refractor, para. 1).

Reflecting Telescopes Reflecting telescopes use curved mirrors to gather and reflect light (Bennett, J., Donahue, M., & Schneider, N., & Voit, M., 2015). The reflecting telescope consists of a primary and secondary mirror (Bennett, J., Donahue, M., & Schneider, N., & Voit, M., 2015). Most telescopes used for astronomical research are reflecting telescopes (Bennett, J., Donahue, M., & Schneider, N., & Voit, M., 2015). Sir Isaac Newton (1642–1727) built the first reflecting telescopes (Bennett, J., Donahue, M., & Schneider, N., & Voit, M., 2015). Strengths of reflectors. Reflecting telescopes possess numerous advantages when comparing them to refractors. Some of these strengths are that they are “easy to use and even construct,” and “excellent for faint deep sky objects such as remote galaxies, nebulae and star clusters because of their larger apertures for light gathering” (Sharma, 2015, The Reflecting Telescope or Reflector, para. 1). Other strengths include: reflectors are “low in optical irregularities and deliver very bright images” and are “reasonably compact and portable” (Sharma, 2015, The Reflecting Telescope or Reflector, para. 1). Additionally, “a reflector costs the least per inch of aperture compared to refractors and catadioptrics since mirrors can be produced at less cost than lenses” (Sharma, 2015, The Reflecting Telescope or Reflector, para. 1). Weaknesses of reflecting telescopes. Reflecting telescopes, or reflectors, are not without their weaknesses. Sharma (2015) lists several disadvantages of reflecting telescopes, such as reflectors are “generally, not suited for terrestrial applications” (The Reflecting Telescope or Reflector, para. 1). Sharma (2015) also states that reflectors offer a “slight light loss due to secondary obstruction when compared with refractors” (The Reflecting Telescope or Reflector, para. 1). Additionally, “the tube is open to the air, which means dust on the optics even if the tube is kept under wraps” (The Reflecting Telescope or Reflector, para. 1). Another disadvantage is that “reflectors may require a little more care and maintenance” (The Reflecting Telescope or Reflector, para. 1). Best Places to Build Ground-Based Telescopes The best places to put ground-based telescopes are locations that are “far from big city lights, high in altitude, and located in an area where the air tends to be calm and dry” (Bennett, J., Donahue, M., & Schneider, N., & Voit, M., 2015, p. 127).
Why Astronomers Choose These Places Astronomers choose locations that are “far from big city lights” because the light from these cities cause light pollution (Bennett, J., Donahue, M., & Schneider, N., & Voit, M., 2015, p. 127). Light pollution is “human-made light that hinders astronomical observations. (Bennett, J., Donahue, M., & Schneider, N., & Voit, M., 2015, p. G-8). Areas that are higher in altitude offer a more turbulence-free atmosphere. "Turbulence" from the wind distorts light from images and alters viewing (Bennett, J., Donahue, M., & Schneider, N., & Voit, M., 2015). Lastly, astronomers choose to place telescopes in locations that are mainly “calm and dry” because both rain and wind turbulence interferes with viewing and possesses the ability to cause image blurring (Bennett, J., Donahue, M., & Schneider, N., & Voit, M., 2015, p. 127).
Strengths and Weaknesses of Building Telescopes on Earth in Orbit or on Moon This section contrasts the strengths and weaknesses between building telescopes on Earth, in orbit, or even on the moon. Where the best place is to build telescopes depends greatly on the type of light astronomers wish to capture and study (Bennett, J., Donahue, M., & Schneider, N., & Voit, M., 2015). Building telescopes on Earth. Telescopes built on Earth are only able to collect certain types of spectral light. These types of light include visible light and radio light waves (Bennett, J., Donahue, M., & Schneider, N., & Voit, M., 2015). Some infrared light penetrates the Earth's atmosphere but is only detected at high altitudes. Earth-based telescopes possess many disadvantages, as this paper previously discusses. These weaknesses include atmospheric and weather disturbances, as well as light pollution (Bennett, J., Donahue, M., & Schneider, N., & Voit, M., 2015 ). Building telescopes in orbit. Space is the best location for building telescopes for many reasons. First, space telescopes are able to detect and collect all types of spectral light, including x-ray, gamma ray, and ultraviolet light (Bennett, J., Donahue, M., & Schneider, N., & Voit, M., 2015). Furthermore, atmospheric disturbances and light pollution do not affect telescopes in space (Bennett, J., Donahue, M., & Schneider, N., & Voit, M., 2015). The only disadvantage to space telescopes is that they are expensive to launch (Bennett, J., Donahue, M., & Schneider, N., & Voit, M., 2015). However, the quality of images that a space telescope captures far exceeds those of ground-based telescopes (Bennett, J., Donahue, M., & Schneider, N., & Voit, M., 2015). Building telescopes on the moon. “The first, and so far only, lunar astronomical observatory was deployed by the Apollo 16 crew in 1972” (Lunar Exploration Science, 2009, para. 1). During this mission, 178 images were taken by a “Far Ultraviolet Camera/Spectrograph” and returned to Earth for viewing (Lunar Exploration Science, 2009, para. 1). “The observatory still stands on the Moon today” (Lunar Exploration Science, 2009, para. 1). The building of telescopes on the moon offers many advantages, such as no atmospheric interference because “the moon has no atmosphere” (Lunar Exploration Science, 2009, Telescopes, para. 2). Another advantage of lunar telescopes is that all types of spectral light is observable. Furthermore, "night time on the Moon lasts about 350 hours” (Lunar Exploration Science, 2009, Telescopes, para. 3). “This would permit scientists to watch deep space objects for very long periods, or to accumulate signals on very faint sources such as dim stars, galaxies, or planets around other stars” (Lunar Exploration Science, 2009, Telescopes, para. 3).
Different Frequencies of Light This section explains how different frequencies of light tell more about the birth, life, and death in the nature and properties of the Sun, stars, and the universe. Different object emits varying types of spectral light. Astronomers use various types of spectra to learn information about objects (Bennett, J., Donahue, M., & Schneider, N., & Voit, M., 2015). “There are three basic types of spectra: continuous, emission line, and absorption line (Bennett, J., Donahue, M., & Schneider, N., & Voit, M., 2015, p. 112).
What Celestial Objects Are Made of “Every kind of atom, ion, and molecule produces a unique spectral “fingerprint.” (Bennett, J., Donahue, M., & Schneider, N., & Voit, M., 2015, p. 115). Therefore, each object in space consists of molecules, atoms, and ions that possess particular emission and absorption lines that only occur “at specific wavelengths that correspond to particular energy level transitions in atoms or molecules” (Bennett, J., Donahue, M., & Schneider, N., & Voit, M., 2015, p. 129). Thus, these lines tell astronomers what the composition of that object is (Bennett, J., Donahue, M., & Schneider, N., & Voit, M., 2015).
Temperature of Planets and Stars Planets and stars make thermal radiation spectra (Bennett, J., Donahue, M., & Schneider, N., & Voit, M., 2015). Thermal radiation is “the spectrum of radiation produced by an opaque object that depends only on the object’s temperature; sometimes called blackbody radiation. (Bennett, J., Donahue, M., & Schneider, N., & Voit, M., 2015, p. G-13). Astronomers are able to estimate temperatures of planets and stars because “hotter objects emit more total radiation per unit area and emit photons with a higher average energy (Bennett, J., Donahue, M., & Schneider, N., & Voit, M., 2015, p. 129). Not only does light tell astronomers what objects consist of and the temperatures of these objects, but light also tells them how fast an object is moving toward or away from them. The Doppler effect, “the effect that shifts the wavelengths of spectral features in objects”, tells astronomers which direction the object is moving (Bennett, J., Donahue, M., & Schneider, N., & Voit, M., 2015, p. G-3). Spectral lines shift toward the blue end of the spectra when an object is moving closer to someone and towards the red end of the spectra when an object is moving farther away (Bennett, J., Donahue, M., & Schneider, N., & Voit, M., 2015). Astronomers can tell the birth, life, and death in the nature and properties of the Sun, stars, and other celestial bodies by the composition, temperature, and motion of them. This is possible because each celestial object consists of certain components and varying temperatures at differing times (Bennett, J., Donahue, M., & Schneider, N., & Voit, M., 2015). Thus, light tells astronomers a great deal about planets, stars, and other celestial bodies. How Telescopes Operate in Varying Wavelengths Individual telescopes are designed to detect different types of light and their accompanying wavelengths, such as radio, X-ray, gamma ray, ultraviolet, and infrared (Bennett, J., Donahue, M., & Schneider, N., & Voit, M., 2015). The distinct wavelengths of the various forms of light tell astronomers different things about the universe (Bennett, J., Donahue, M., & Schneider, N., & Voit, M., 2015).

Radio Telescopes Radio telescopes pick up long wavelengths of light. “The long wavelengths of radio waves therefore mean that very large telescopes are necessary to achieve reasonable angular resolution” (Bennett, J., Donahue, M., & Schneider, N., & Voit, M., 2015, p. 124).
X-ray Telescopes X-ray telescopes collect short wavelengths of light, or light in the x-ray portion of the spectrum. “X-ray telescopes are used to study mainly the Sun, stars and supernovas” (Telescope Types, 2015, X-RAY, para. 1). X-ray telescopes only operate in space “because the Earth's atmosphere interferes with the x-ray signals they receive” (Telescope Types, 2015, X-RAY, para. 1). X-ray telescopes are equipped with mirrors that are angled to where X-ray light does not penetrate and destroy the telescope, but bounces off of it (Bennett, J., Donahue, M., & Schneider, N., & Voit, M., 2015)
Gamma-Ray Telescopes Gamma ray telescopes also detect and collect short wavelengths of light that the Earth’s atmosphere blocks (Bennett, J., Donahue, M., & Schneider, N., & Voit, M., 2015). “Gamma ray telescopes detect bursts of gamma rays” (Telescope Types, 2015, GAMMA RAY, para. 1). “They help astronomers confirm events in outer space like supernovas, pulsars and black holes” (Telescope Types, 2015, GAMMA RAY, para. 1). Gamma rays are so powerful that they cannot be captured by mirrors (Bennett, J., Donahue, M., & Schneider, N., & Voit, M., 2015). So, gamma ray telescopes “typically use massive detectors to capture photons and determine the direction they came from” (Bennett, J., Donahue, M., & Schneider, N., & Voit, M., 2015, pp. 124-125).

Conclusion Visible and other forms of light tell astronomers many important things about the universe and its contents. Telescopes enable scientists to learn more about objects than ever before because they pick up light that is unseen by the naked eye. Different types of telescopes collect light that ranges from one end of the spectrum to the other. These varying forms of spectral light reveal information that is vital to understanding the nature and workings of the universe and its contents.

References
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Lunar Exploration Science. (2009, December 8). Retrieved June 16, 2015, from https://web.archive.org/web/20100713114412/http://aerospacescholars.jsc.nasa.gov/HAS/Modules/Earth-to-Mars/6/4.cfm
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