06 lecture outline
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Telescopes allow astronomers to observe the universe in different wavelengths of light. Ground-based telescopes are limited by Earth's atmosphere, so many telescopes have been placed in space. There are two main types of telescopes - refracting telescopes which use lenses and reflecting telescopes which use mirrors. Multiple telescopes can work together using interferometry to achieve the resolution of a larger single telescope. Astronomers use telescopes to take images of celestial objects, perform spectroscopy to analyze light, and monitor changes over time.
06 lecture outline
- 1. Chapter 6 Lecture © 2014 Pearson Education, Inc. The Cosmic Perspective Seventh Edition Telescopes Portals of Discovery
- 2. Telescopes Portals of Discovery
- 3. 6.1 Eyes and Cameras: Everyday Light Sensors • Our goals for learning: – How do eyes and cameras work?
- 4. The Eye
- 5. Refraction • Refraction is the bending of light when it passes from one substance into another. • Your eye uses refraction to focus light.
- 6. Example: Refraction at Sunset • Sun appears distorted at sunset because of how light bends in Earth's atmosphere.
- 7. Focusing Light • Refraction can cause parallel light rays to converge to a focus.
- 8. Image Formation • The focal plane is where light from different directions comes into focus. • The image behind a single (convex) lens is actually upside-down!
- 9. Digital cameras detect light with charge-coupled devices (CCDs). Recording Images • A camera focuses light like an eye and captures the image with a detector. • The CCD detectors in digital cameras are similar to those used in modern telescopes.
- 10. • Astronomers often use computer software to combine, sharpen, or refine images. • This image of Saturn's moon Enceladus has been processed to highlight the plume of water ice coming from its surface. Image Processing
- 11. What have we learned? • How do eyes and cameras work? – Eyes use refraction to bend parallel light rays so that they form an image. – The image is in focus if the focal plane is at the retina. – Cameras focus light like your eye and record the image with a detector.
- 12. 6.2 Telescopes: Giant Eyes • Our goals for learning: – What are the two most important properties of a telescope? – What are the two basic designs of telescopes? – What do astronomers do with telescopes?
- 13. What are the two most important properties of a telescope? 1. Light-collecting area: Telescopes with a larger collecting area can gather a greater amount of light in a shorter time. 2. Angular resolution: Telescopes that are larger are capable of taking images with greater detail.
- 14. • A telescope's diameter tells us its light-collecting area: • The largest telescopes currently in use have a diameter of about 10 meters. A = π(d / 2)2 Light-Collecting Area
- 15. Thought Question How does the collecting area of a 10-meter telescope compare with that of a 2-meter telescope? a) It's 5 times greater. b) It's 10 times greater. c) It's 25 times greater.
- 16. Thought Question How does the collecting area of a 10-meter telescope compare with that of a 2-meter telescope? a) It's 5 times greater. b) It's 10 times greater. c) It's 25 times greater.
- 17. Angular Resolution • The minimum angular separation that the telescope can distinguish
- 18. Angular Resolution • Ultimate limit to resolution comes from interference of light waves within a telescope. • Larger telescopes are capable of greater resolution because there's less interference.
- 19. Close-up of a star from the Hubble Space Telescope Angular Resolution • The rings in this image of a star come from interference of light wave. • This limit on angular resolution is known as the diffraction limit.
- 20. What are the two basic designs of telescopes? • Refracting telescope: focuses light with lenses • Reflecting telescope: focuses light with mirrors
- 21. Refracting Telescope • Refracting telescopes need to be very long, with large, heavy lenses.
- 22. Reflecting Telescope • Reflecting telescopes can have much greater diameters. • Most modern telescopes are reflectors.
- 23. Designs for Reflecting Telescopes
- 24. Twin Keck telescopes on Mauna Kea in Hawaii Segmented 10-meter mirror of a Keck telescope Pick-up images. Pick-up images. Mirrors in Reflecting Telescopes
- 25. What do astronomers do with telescopes? • Imaging: taking pictures of the sky • Spectroscopy: breaking light into spectra • Time Monitoring: measuring how light output varies with time
- 26. Imaging • Astronomical detectors generally record only one color of light at a time. • Several images must be combined to make full-color pictures.
- 27. Imaging • Astronomical detectors can record forms of light our eyes can't see. • Color is sometimes used to represent different energies of non-visible light.
- 28. Spectroscopy • A spectrograph separates the different wavelengths of light before they hit the detector.
- 29. Spectroscopy • Graphing relative brightness of light at each wavelength shows the details in a spectrum.
- 30. Time Monitoring • A light curve represents a series of brightness measurements made over a period of time.
- 31. Want to buy your own telescope? • Buy binoculars first (e.g., 7×35)—you get much more for the same money. • Ignore magnification (sales pitch!). • Notice: aperture size, optical quality, portability. • Consumer research: Astronomy, Sky & Telescope, Mercury, astronomy clubs
- 32. What have we learned? • What are the two most important properties of a telescope? – Collecting area determines how much light a telescope can gather. – Angular resolution is the minimum angular separation a telescope can distinguish. • What are the two basic designs of telescopes? – Refracting telescopes focus light with lenses. – Reflecting telescopes focus light with mirrors. – The vast majority of professional telescopes are reflectors.
- 33. What have we learned? • What do astronomers do with telescopes? – Imaging – Spectroscopy – Time Monitoring
- 34. 6.3 Telescopes and the Atmosphere • Our goals for learning: – How does Earth's atmosphere affect ground-based observations? – Why do we put telescopes into space?
- 35. How does Earth's atmosphere affect ground-based observations? • The best ground-based sites for astronomical observing are: – calm (not too windy) – high (less atmosphere to see through) – dark (far from city lights) – dry (few cloudy nights)
- 36. Light Pollution • Scattering of human-made light in the atmosphere is a growing problem for astronomy.
- 37. Bright star viewed with ground-based telescope Same star viewed with Hubble Space Telescope Twinkling and Turbulence • Turbulent air flow in Earth's atmosphere distorts our view, causing stars to appear to twinkle.
- 38. Without adaptive optics With adaptive optics Adaptive Optics • Rapidly changing the shape of a telescope's mirror compensates for some of the effects of turbulence.
- 39. Summit of Mauna Kea, Hawaii Calm, High, Dark, Dry • The best observing sites are atop remote mountains.
- 40. Why do we put telescopes into space?
- 41. Transmission in Atmosphere • Only radio and visible light pass easily through Earth's atmosphere. • We need telescopes in space to observe other forms.
- 42. What have learned? • How does Earth's atmosphere affect ground- based observations? – Telescope sites are chosen to minimize the problems of light pollution, atmospheric turbulence, and bad weather. • Why do we put telescopes into space? – Forms of light other than radio and visible do not pass through Earth's atmosphere. – Also, much sharper images are possible because there is no turbulence.
- 43. 6.4 Telescopes and Technology • Our goals for learning: – How can we observe invisible light? – How can multiple telescopes work together?
- 44. How can we observe invisible light? • A standard satellite dish is essentially a telescope for observing radio waves.
- 45. Radio Telescopes • A radio telescope is like a giant mirror that reflects radio waves to a focus.
- 46. SOFIA Spitzer Infrared and Ultraviolet Telescopes • Infrared and ultraviolet light telescopes operate like visible-light telescopes but need to be above atmosphere to see all wavelengths.
- 47. Chandra X-Ray Observatory X-Ray Telescopes • X-ray telescopes also need to be above the atmosphere.
- 48. X-Ray Telescopes • Focusing of X-rays requires special mirrors. • Mirrors are arranged to focus X-ray photons through grazing bounces off the surface.
- 49. Fermi Gamma-Ray Observatory Gamma-Ray Telescopes • Gamma-ray telescopes also need to be in space. • Focusing gamma rays is extremely difficult.
- 50. How can multiple telescopes work together?
- 51. Interferometry • Interferometery is a technique for linking two or more telescopes so that they have the angular resolution of a single large one.
- 52. Very Large Array (VLA) Insert 7e, figure 6.30 here Interferometry • Easiest to do with radio telescopes • Now possible with infrared and visible-light telescopes
- 53. What have learned? • How can we observe invisible light? – Telescopes for invisible light are usually modified versions of reflecting telescopes. – Many of the telescopes used for observing invisible light are in space. • How can multiple telescopes work together? – Linking multiple telescopes using interferometry enables them to produce the angular resolution of a much larger telescope.
Editor's Notes
- #10: Note that the text in the book does not mention CCDs, though figure 6.6 still makes reference to them. This should be mentioned to students.
- #18: Emphasize that at a great distance the lights would look like one light rather than two --- but we could still see them. (Students sometimes think that below a particular angular resolution we see nothing at all.)
- #25: Point students' attention to the human in the middle of the Keck mirror!
- #32: Optional if you wish to discuss buying your own telescope; also point students to Special Topic box on p. 177.