Light and Telescopes

I. Radiation: Information from Space A. Light as a Wave and a Particle B. The Electromagnetic Spectrum II. Optical Telescopes A. Two Kinds of Telescopes B. The Powers of a Telescope C. New-Generation Telescopes D. Interferometry III. Special Instruments A. Imaging Systems B. The Spectrograph

Outline (continued) IV. Radio Telescopes A. Operation of a Radio Telescope B. Limitations of the Radio Telescope C. Advantages of Radio Telescopes V. Space Astronomy A. Infrared Astronomy B. Ultraviolet Astronomy C. X-Ray Astronomy D. Gamma-Ray Telescopes E. Cosmic Rays F. The Hubble Space Telescope

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.

Light as a Wave (1) f = c/l l c = 300,000 km/s = 3*108 m/s Light waves are characterized by a wavelength l and a frequency f. f and l are related through f = c/l

Light as a Wave (2) Wavelengths of light are measured in units of nanometers (nm) or Ångström (Å): 1 nm = 10-9 m 1 Å = 10-10 m = 0.1 nm Visible light has wavelengths between 4000 Å and 7000 Å (= 400 – 700 nm).

Wavelengths and Colors Different colors of visible light correspond to different wavelengths.

Light as Particles E = h*f Light can also appear as particles, called photons (explains, e.g., photoelectric effect). A photon has a specific energy E, proportional to the frequency f: E = h*f h = 6.626x10-34 J*s is the Planck constant. The energy of a photon does not depend on the intensity of the light!!!

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

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

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

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.

Disadvantages of Refracting Telescopes Chromatic aberration: Different wavelengths are focused at different focal lengths (prism effect). Can be corrected, but not eliminated by second lens out of different material. Difficult and expensive to produce: All surfaces must be perfectly shaped; glass must be flawless; lens can only be supported at the edges

The Powers of a Telescope: Size Does Matter 1. Light-gathering power: Depends on the surface area A of the primary lens / mirror, proportional to diameter squared: D A = p (D/2)2

The Powers of a Telescope (2) 2. Resolving power: Wave nature of light => The telescope aperture produces fringe rings that set a limit to the resolution of the telescope. Resolving power = minimum angular distance amin between two objects that can be separated. amin = 1.22 (l/D) amin For optical wavelengths, this gives amin = 11.6 arcsec / D[cm]

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

The Powers of a Telescope (3) 3. Magnifying Power = ability of the telescope to make the image appear bigger. The magnification depends on the ratio of focal lengths of the primary mirror/lens (Fo) and the eyepiece (Fe): M = Fo/Fe A larger magnification does not improve the resolving power of the telescope!

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

The Best Location for a Telescope (2) Paranal Observatory (ESO), Chile On high mountain-tops – to avoid atmospheric turbulence ( seeing) and other weather effects

Traditional Telescopes (1) Secondary mirror Traditional primary mirror: sturdy, heavy to avoid distortions.

Traditional Telescopes (2) The 4-m Mayall Telescope at Kitt Peak National Observatory (Arizona)

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

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

Examples of Modern Telescope Design (1) Design of the Large Binocular Telescope (LBT) The Keck I telescope mirror

Examples of Modern Telescope Design (2) The Very Large Telescope (VLT) 8.1-m mirror of the Gemini Telescopes

Interferometry Recall: Resolving power of a telescope depends on diameter D: amin = 1.22 l/D. This holds true even if not the entire surface is filled out. Combine the signals from several smaller telescopes to simulate one big mirror  Interferometry

CCD Imaging CCD = Charge-coupled device More sensitive than photographic plates Data can be read directly into computer memory, allowing easy electronic manipulations Negative image to enhance contrasts False-color image to visualize brightness contours

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

Radio Astronomy Recall: Radio waves of l ~ 1 cm – 1 m also penetrate the Earth’s atmosphere and can be observed from the ground.

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.

Radio Interferometry Just as for optical telescopes, the resolving power of a radio telescope is amin = 1.22 l/D. For radio telescopes, this is a big problem: Radio waves are much longer than visible light  Use interferometry to improve resolution!

Radio Interferometry (2) 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)!

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

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.

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

Space Astronomy

NASA’s Space Infrared Telescope Facility (SIRTF)

Ultraviolet Astronomy Ultraviolet radiation with l < 290 nm 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 Ultraviolet radiation traces hot (tens of thousands of degrees), moderately ionized gas in the Universe.

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

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

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

New Terms electromagnetic radiation wavelength frequency Nanometer (nm) Angstrom (Å) photon infrared radiation ultraviolet radiation atmospheric window focal length refracting telescope reflecting telescope primary lens, mirror objective lens, mirror eyepiece chromatic aberration achromatic lens light-gathering power resolving power diffraction fringe seeing magnifying power light pollution prime focus secondary mirror Cassegrain focus Newtonian focus Schmidt-Cassegrain focus sidereal drive equatorial mounting polar axis alt-azimuth mounting active optics adaptive optics

New Terms (continued) interferometry charge-coupled device (CCD) false-color image spectrograph grating comparison spectrum radio interferometer cosmic ray

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?

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

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