Light and Telescopes

1. 0 Note that the following lectures include animations and PowerPoint effects such as fly-ins and transitions that require you to be in PowerPoint's Slide Show mode (presentation mode).

2. 0 Light and Telescopes Chapter 6

3. 0 Guidepost In earlier chapters of this book, you looked at the sky the way ancient astronomers did, with the unaided eye. In the previous chapter, you got a glimpse through Galileo’s telescope that revealed amazing things about the moon, Jupiter, and Venus. Now you can consider the telescopes, instruments, and techniques of modern astronomers. Telescopes gather and focus light, so you need to study what light is, and how it behaves, on your way to understanding how telescopes work. You will learn about telescopes that capture invisible types of light such as radio waves and X-rays. These enable astronomers to put together a more complete view of celestial objects.

4. 0 Guidepost (continued) This chapter will help you answer these five important questions: • What is light? • How do telescopes work? • What are the powers and limitations of telescopes? • What kind of instruments do astronomers use to record and analyze light gathered by telescopes? • Why must some telescopes be located in space? The science of astronomy is based on observations. Astronomers cannot visit distant galaxies and far-off worlds, so they have to study them using telescopes. Twenty chapters remain in your exploration, and every one will present information gained by astronomers using telescopes.

5. 0 Outline I. Radiation: Information from Space A. Light as Waves and as Particles B. The Electromagnetic Spectrum II. Telescopes A. Two Ways to Do It: Refracting and Reflecting Telescopes B. The Powers and Limitations III. Observatories on Earth: Optical and Radio A. Modern Optical Telescopes B. Modern Radio Telescopes

6. 0 Outline (continued) IV. Airborne and Space Observatories A. Airborne Telescopes B. Space Telescopes C. High Energy Astronomy V. Astronomical Instruments and Techniques A. Imaging Systems and Photometers B. Spectrographs C. Adaptive Optics D. Interferometry VI. Nonelectromagnetic Astronomy A. Particle Astronomy B. Gravity Wave Astronomy

7. 0 Radiation: Information from Space 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.

8. 0 Light as a Wave (1) l c = 300,000 km/s = 3*108 m/s • Light waves are characterized by a wavelength l and a frequency f. f = c/l • f and l are related through

9. 0 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)

10. 0 Wavelengths and Colors Differentcolors of visible light correspond to different wavelengths.

11. 0 Light as Particles • 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*sis the Planck constant The energy of a photon does notdepend on the intensity of the light!!!

12. 0 The Electromagnetic Spectrum Wavelength Frequency High flying air planes or satellites Need satellites to observe

13. 0 Optical Telescopes Astronomers use telescopes to gather more light from astronomical objects. The larger the telescope, the more light it gathers.

14. 0 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.

15. 0 Secondary Optics In reflecting telescopes: Secondary mirror, to re-direct the light path towards the back or side of the incoming light path. Eyepiece: To view and enlarge the small image produced in the focal plane of the primary optics.

16. 0 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

17. 0 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

18. 0 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]

19. 0 Seeing Weather conditions and turbulence in the atmosphere set further limits to the quality of astronomical images. Bad seeing Good seeing

20. 0 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 (Fp) and the eyepiece (Fe): M = Fp/Fe A larger magnification does not improve the resolving power of the telescope!

21. 0 The Best Location for a Telescope Far away from civilization – to avoid light pollution

22. 0 The Best Location for a Telescope (2) Paranal Observatory (ESO), Chile On high mountain-tops – to avoid atmospheric turbulence (seeing) and other weather effects

23. 0 Traditional Telescopes (1) Secondary mirror Traditional primary mirror: sturdy, heavy to avoid distortions

24. 0 Traditional Telescopes (2) The 4-m Mayall Telescope at Kitt Peak National Observatory (Arizona)

25. 0 Advances in Modern Telescope Design (1) Modern computer technology has made significant advances in telescope design possible: Segmented mirror 1. Lighter mirrors with lighter support structures, to be controlled dynamically by computers Floppy mirror

26. 0 Advances in Modern Telescope Design (2) 2. Simpler, stronger mountings (“Alt-azimuth mountings”) to be controlled by computers

27. 0 Examples of Modern Telescope Design (1)

28. 0 Examples of Modern Telescope Design (2)

29. 0 Radio Astronomy Recall: Radio waves of l ~ 1 cm – 1 m also penetrate the Earth’s atmosphere and can be observed from the ground.

30. 0 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.

31. 0 The Largest Radio Telescopes The 300-m telescope in Arecibo, Puerto Rico The 100-m Green Bank Telescope in Green Bank, WVa

32. 0 Observing Beyond the Ends of the Visible Spectrum Most infrared radiation is absorbed in the lower atmosphere. NASA infrared telescope on Mauna Kea, Hawaii However, from high mountain tops or high-flying air planes, some infrared radiation can still be observed. Infrared cameras need to be cooled to very low temperatures, usually using liquid nitrogen.

33. 0 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

34. 0 Infrared Astronomy from Orbit: NASA’s Spitzer Space Telescope Infrared light with wavelengths much longer than visible light (“Far Infrared”) can only be observed from space.

35. 0 CCD Imaging CCD = Charge-coupled device • More sensitive than photographic plates • Data can be read directly into computer memory, allowing easy electronic manipulations Visible light (top) and infrared (bottom) image of the Sombrero Galaxy

36. 0 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 .

37. 0 Adaptive Optics Computer-controlled mirror support adjusts the mirror surface (many times per second) to compensate for distortions by atmospheric turbulence A laser beam produces an artificial star, which is used for the computer-based seeing correction.

38. 0 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

39. 0 Radio Maps Radio maps are often color coded: Like different colors in a weather map may indicate different weather patterns, … … colors in a radio map can indicate different intensities of the radio emission from different locations on the sky.

40. 0 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!

41. 0 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)!

42. 0 Cosmic Rays • Radiation from space does not only come in the form of electromagnetic radiation (radio, …, gamma-rays) • Earth is also constantly bombarded by highly energetic subatomic particles from space: Cosmic Rays • The source if cosmic rays is still not well understood.