Telescope

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Light and Telescopes:

Light and Telescopes Chapter 26.1

Guidepost:

The study of the universe is so challenging, astronomers cannot ignore any source of information; that is why they use the entire spectrum, from gamma rays to radio waves. This chapter shows how critical it is for astronomers to understand the nature of light. Guidepost

Light and Other Forms of Radiation:

Light and Other Forms of Radiation The Electromagnetic Spectrum In astronomy, we cannot perform experiments with our objects (stars, galaxies, …). The only way to investigate them, is by analyzing the light (and other radiation) which we observe from them. 1

Light as a Wave (1):

Light as a Wave (1) Light waves are characterized by a wavelength l and a frequency f. l l 2

Wavelengths and Colors:

Wavelengths and Colors D i f f e r e n t colors of visible light correspond to different wavelengths. 3

Light as Particles:

Light as Particles Light can also appear as particles, called photons (explains, e.g., photoelectric effect). The energy of a photon does not depend on the intensity of the light!!! 4

The Electromagnetic Spectrum:

The Electromagnetic Spectrum Need satellites to observe Wavelength Frequency High flying air planes or satellites 5

Optical Telescopes:

Optical Telescopes Astronomers use telescopes to gather more light from astronomical objects. The larger the telescope, the more light it gathers. 6

Refracting/Reflecting Telescopes:

Refracting/Reflecting Telescopes Refracting Telescope: Lens focuses light onto the focal plane Reflecting Telescope: Concave Mirror focuses light onto the focal plane Almost all modern telescopes are reflecting telescopes. Focal length Focal length 7

Secondary Optics:

Secondary Optics In reflecting telescopes: Secondary mirror , to re-direct light path towards back or side of incoming light path. Eyepiece : To view and enlarge the small image produced in the focal plane of the primary optics. 8

The Powers of a Telescope: Size Does Matter:

The Powers of a Telescope: Size Does Matter Light-gathering power: Depends on the surface area of the primary lens. The larger the lens the greater ability to capture images. D 9

Seeing:

Seeing Weather conditions and turbulence in the atmosphere set further limits to the quality of astronomical images. Bad seeing Good seeing 10

The Powers of a Telescope:

The Powers of a Telescope 3. Magnifying Power = ability of the telescope to make the image appear bigger. 11

The Best Location for a Telescope:

The Best Location for a Telescope Far away from civilization – to avoid light pollution 12

The Best Location for a Telescope:

The Best Location for a Telescope On high mountain- tops – to avoid atmospheric turbulence and other weather effects Paranal Observatory (ESO), Chile 13

Traditional Telescopes:

Traditional Telescopes Traditional primary mirror: sturdy, heavy to avoid distortions. Secondary mirror 14

Traditional Telescopes:

Traditional Telescopes The 4-m Mayall Telescope at Kitt Peak National Observatory (Arizona) 15

Advances in Modern Telescope Design:

Advances in Modern Telescope Design 2. Simpler, stronger mountings to be controlled by computers 1. Lighter mirrors with lighter support structures, to be controlled by computers Floppy mirror Segmented mirror Modern computer technology has made possible significant advances in telescope design: 16

Adaptive Optics:

Adaptive Optics Computer-controlled mirror support adjusts the mirror surface (many times per second) to compensate for distortions by atmospheric turbulence 17

Examples of Modern Telescope Design:

Examples of Modern Telescope Design Design of the Large Binocular Telescope (LBT) The Keck I telescope mirror 18

Examples of Modern Telescope Design:

Examples of Modern Telescope Design 8.1 -m mirror of the Gemini Telescopes The Very Large Telescope (VLT) 19

Interferometry:

Interferometry This holds true even if not the entire surface is filled out. Combine the signals from several smaller telescopes to simulate one big mirror In-ter-fer-om-e-try 20

CCD Imaging:

CCD Imaging CCD = Charge-coupled device More sensitive images than photographs. Negative image to enhance contrasts False-color image to visualize brightness contours 21

The Spectrograph:

The Spectrograph Using a prism (or a grating), light can be split up into different wavelengths (colors!) to produce a spectrum . Spectral lines in a spectrum tell us about the chemical composition and other properties of the observed object 22

Radio Astronomy:

Radio Astronomy Radio penetrate the Earth’s atmosphere and can be observed from the ground. 23

Radio Telescopes:

Radio Telescopes Large dish focuses the energy of radio waves onto a small receiver (antenna) Amplified signals are stored in computers and converted into images, spectra, etc. 24

Radio Interferometry:

Radio Interferometry For radio telescopes, this is a big problem: Radio waves are much longer than visible light  Use interferometry to improve resolution! 25

Radio Interferometry:

Radio Interferometry The Very Large Array (VLA): 27 dishes are combined to simulate a large dish of 36 km in diameter. Even larger arrays consist of dishes spread out over the entire U.S. (VLBA = Very Long Baseline Array) or even the whole Earth (VLBI = Very Long Baseline Interferometry)! 26

The Largest Radio Telescopes:

The Largest Radio Telescopes The 100-m Green Bank Telescope in Green Bank, WVa. The 300-m telescope in Arecibo, Puerto Rico 27

Science of Radio Astronomy:

Science of Radio Astronomy Radio astronomy reveals several features, not visible at other wavelengths: Neutral hydrogen clouds (which don’t emit any visible light), containing ~ 90 % of all the atoms in the Universe. Molecules (often located in dense clouds, where visible light is completely absorbed). Radio waves penetrate gas and dust clouds, so we can observe regions from which visible light is heavily absorbed. 28

Infrared Astronomy:

Infrared Astronomy However, from high mountain tops or high-flying air planes, some infrared radiation can still be observed. NASA infrared telescope on Mauna Kea, Hawaii Most i nfrared radiation is absorbed in the lower atmosphere. 29

Space Astronomy:

Space Astronomy 30

NASA’s Space Infrared Telescope Facility (SIRTF):

NASA’s Space Infrared Telescope Facility (SIRTF) 31

Ultraviolet Astronomy:

Ultraviolet Astronomy Much of the sun’s ultraviolet radiation is completely absorbed in the ozone layer of the atmosphere. Ultraviolet astronomy has to be done from satellites. Several successful ultraviolet astronomy satellites: IRAS, IUE, EUVE, FUSE 32

X-Ray Astronomy:

X-Ray Astronomy X-rays are completely absorbed in the atmosphere. X-ray astronomy has to be done from satellites. NASA’s Chandra X-ray Observatory X-rays trace hot (million degrees), highly ionized gas in the Universe. 33

Gamma-Ray Astronomy:

Gamma-Ray Astronomy Gamma-rays : most energetic electromagnetic radiation; traces the most violent processes in the Universe The Compton Gamma-Ray Observatory 34

The Hubble Space Telescope:

The Hubble Space Telescope Avoids turbulence in the Earth’s atmosphere Extends imaging and spectroscopy to (invisible) infrared and ultraviolet Launched in 1990; maintained and upgraded by several space shuttle service missions throughout the 1990s and early 2000’s 35

Discussion Questions:

1. Why does the wavelength response of the human eye match so well the visual window of Earth’s atmosphere? 2. Most people like beautiful sunsets with brightly glowing clouds, bright moonlit nights, and twinkling stars. Most astronomers don’t. Why? Discussion Questions

Quiz Questions:

Quiz Questions 1. The visible part of the electromagnetic spectrum can be divided into seven color bands of Red, Orange, Yellow, Green, Blue, Indigo, and Violet (from long to short wavelength). A single photon of which of these colors has the greatest amount of energy? a. Red b. Orange c. Green d. Blue e. Violet

Quiz Questions:

2. The entire electromagnetic spectrum can be divided into the seven bands of Radio, Microwave, Infrared, Visible, Ultraviolet, X-ray, and Gamma-ray (from longest to shortest wavelength). To which of these two bands is Earth's atmosphere the most transparent? a. X-ray & Gamma-ray b. Ultraviolet & Infrared c. Visible & Ultraviolet d. Microwave & Radio e. Visible & Radio Quiz Questions

Quiz Questions:

3. Why do the pupils of a cat's eyes open wider at night? a. To reduce the buildup of cat eye wax. b. Cats are the only animals besides humans to observe the stars. c. The cat sleeps all day and is wide awake at night. d. To increase light gathering power. e. To attract a mate. Quiz Questions

Quiz Questions:

4. Astronomers are both hindered and assisted by chromatic aberration. In which device is chromatic aberration a big problem for astronomers? a. The primary mirrors of reflecting telescopes. b. The primary lenses of refracting telescopes. c. The prism. d. Both a and b above. e. All of the above. Quiz Questions

Quiz Questions:

5. Why have no large refracting telescopes been built in the years since 1900? a. Refracting telescopes suffer from chromatic aberration. b. Making large glass lenses without interior defects is difficult. c. Refracting telescopes have several surfaces to shape and polish. d. Large glass lenses are more difficult to support than large mirrors. e. All of the above. Quiz Questions

Quiz Questions:

6. What do large-diameter gently curved convex (thicker in the middle) lenses and large-diameter gently curved concave (thinner in the middle) mirrors have in common? a. They both have short focal lengths. b. They both have long focal lengths. c. They can be used as primary light collectors for a telescope. d. Both a and c above. e. Both b and c above. Quiz Questions

Quiz Questions:

7. Which power of a telescope might be expressed as "0.5 seconds of arc"? a. Light gathering power. b. Resolving power. c. Magnifying power. d. Both a and b above. e. Both a and c above. Quiz Questions

Quiz Questions:

8. Which power of a telescope is the least important? a. Light gathering power. b. Resolving power. c. Magnifying power. d. Both a and b above. e. Both a and c above. Quiz Questions

Quiz Questions:

9. Which power of an optical telescope is determined by the diameter of the primary mirror or lens? a. Light gathering power. b. Resolving power. c. Magnifying power. d. Both a and b above. e. Both a and c above. Quiz Questions

Quiz Questions:

10. What advantage do the builders of large telescopes today have over the previous generation of telescope builders? a. Large mirrors can now be made thinner and lighter than before. b. Tracking celestial objects today is computer controlled and can take advantage of simpler, stronger mounts. c. High-speed computing today can be used to reduce the effect of Earth's atmosphere. d. Both b and c above. e. All of the above. Quiz Questions

Quiz Questions:

11. In which device do astronomers take advantage of chromatic aberration? a. The primary mirrors of reflecting telescopes. b. The primary lenses of refracting telescopes. c. The prism. d. Both a and b above. e. All of the above. Quiz Questions

Quiz Questions:

12. Which power of a large ground-based optical telescope is severely limited by Earth's atmosphere on a cloudless night? a. Light gathering power. b. Resolving power. c. Magnifying power. d. Both a and b above. e. Both a and c above. Quiz Questions

Quiz Questions:

13. The primary mirror of telescope A has a diameter of 20 cm, and the one in telescope B has a diameter of 100 cm. How do the light gathering powers of these two telescopes compare? a. Telescope A has 5 times the light gathering power of telescope B. b. Telescope B has 5 times the light gathering power of telescope A. c. Telescope A has 25 times the light gathering power of telescope B. d. Telescope B has 25 times the light gathering power of telescope A. e. The light gathering power depends on the focal length of the eyepiece also. Quiz Questions

Quiz Questions:

14. What do the newer light-sensitive electronic CCD chips do better than the older photographic plates coated with light-sensitive chemicals? a. They have a greater sensitivity to light. b. They can detect both bright and dim objects in a single exposure. c. Photometry can be done with the CCD images. d. The CCD images are easier to manipulate. e. All of the above. Quiz Questions

Quiz Questions:

15. What can radio telescopes do that optical telescopes cannot? a. Find the location of cool hydrogen gas. b. See through dust clouds. c. Detect high temperature objects. d. Both a and b above. e. All of the above. Quiz Questions

Quiz Questions:

16. What is a disadvantage of radio telescopes compared to optical telescopes? a. Radio photons have lower energy, thus radio waves have low intensity. b. Interference from nearby sources of radio waves. c. Poor resolving power. d. Both a and b above. e. All of the above. Quiz Questions

Quiz Questions:

17. Radio telescopes are often connected together to do interferometry. What is the primary problem overcome by radio interferometry? a. Poor light gathering power. b. Poor resolving power. c. Poor magnifying power. d. Interference from nearby sources of radio waves. e. The low energy of radio photons. Quiz Questions

Quiz Questions:

18. Why are near-infrared telescopes located on mountaintops and ultraviolet telescopes in Earth orbit? a. The primary infrared blocker, water vapor, is mostly in the lower atmosphere. b. The primary ultraviolet blocker, ozone, is located high in the atmosphere, far above mountaintops. c. Ultraviolet telescopes require the low temperature of space to operate. d. Both a and b above. e. Both a and c above. Quiz Questions

Quiz Questions:

19. Why must far-infrared telescopes be cooled to a low temperature? a. To reduce interfering heat radiation emitted by the telescope. b. To protect the sensitive electronic amplifiers from overheating by sunlight. c. To improve their poor resolving power. d. To improve their poor magnifying power. e. To make use of the vast supplies of helium stockpiled by the United States. Quiz Questions

Quiz Questions:

20. Why are the sources of cosmic rays difficult to locate? a. Cosmic rays are high-energy photons that penetrate the surfaces of telescope mirrors rather than reflecting to a focal point. b. Cosmic rays are charged particles, thus their paths are curved by magnetic fields, which masks the location of their source. c. Cosmic rays are neutral particles that weakly interact with matter and are difficult to detect. d. Cosmic rays are positively and negatively charged particles, which masks the location of their source. e. Cosmic rays are theoretical and have never been detected. Quiz Questions

Answers:

Answers 1. e 2. e 3. d 4. b 5. e 6. e 7. b 8. c 9. d 10. e 11. c 12. b 13. d 14. e 15. d 16. e 17. b 18. d 19. a 20. b

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