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Telescopes

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Telescopes

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  1. Telescopes Amateur and Professional

  2. Galileo 1609

  3. The Moon as a World

  4. Jupiter has Moons

  5. Refracting telescopes

  6. Long focus refractors were awkward but suffered less from chromatic aberration

  7. Isaac Newton’s reflecting telescope Mirrors do not have chromatic aberration

  8. Reflecting telescope Objective mirrors instead of lenses

  9. Three Powers • Magnifying • Resolving • Light Gathering

  10. Magnifying Power • Ability to make objects appear larger in angular size • One can change the magnifying power of a telescope by changing the eyepiece used with it • Mag Power = focal length of objective divided by the focal length of the eyepiece

  11. Resolving Power • Ability to see fine detail • Depends on the diameter of the objective lens or mirror

  12. Light Gathering Power • The ability to make faint objects look brighter • Depends on the area of the objective lens or mirror • Thus a telescope with an objective lens 2 inches in diameter has 4 times the light gathering power of a telescope with a lens 1 inch in diameter

  13. Herschel & Lord Rosse

  14. 19th century: epoch of the large refractors

  15. Refracting telescopes Lick Vienna

  16. Yerkes Observatory Largest refracting telescope with a one meter objective

  17. 20th century Large Reflectors Come of Age Mount Wilson Observatory 1.5m (1908) and 2.5m (1918)

  18. Palomar 5-m(entered operation in 1948)

  19. 4 meter Reflecting telescope

  20. Objective Mirror

  21. Dome of 4 meter Kitt Peak

  22. Keck Telescopes

  23. SOAR Telescope 4.1 meter

  24. SOAR Telescope -- Cerro Pachon

  25. SOAR Observing Room

  26. SOAR Image of the planetary nebula NGC 2440

  27. MSU Campus Observatory

  28. Boller & Chivens reflecting telescope with a 24-inch objective mirror

  29. More on resolution • Eagle-eyed Dawes • The Dawes Limit R = 4.56/D Where R = resolution in seconds of arc D = diameter of objective in inches More appropriate for visible light and small telescopes

  30. A more general expression for the theoretical resolving power • Imagine that star images look like Airy disks

  31. Minimum Angle that can be resolved • R = 1.22 x 206,265 l / d R = resolution in seconds of arc l = wavelength of light d = diameter of the objective lens or mirror Note that the wavelength of light and the diameter of the objective should be in the same units

  32. Examples • For Visible light around 500nm Our 24-inch telescope R = 0.20 seconds This may be compared with the Dawes limit of 0.19 seconds But with large ground-based telescopes it is difficult to achieve this

  33. Astronomical “seeing” • Blurring effect of looking through air • Causes stars to twinkle and planetary detail to blur • At the SOAR site: good seeing means stellar images better than about 0.7 seconds of arc • In Michigan, good seeing means better than about 3 seconds of arc • Not to be confused with good transparency

  34. Bad seeing on this side Good seeing on this side

  35. Electromagnetic Spectrum

  36. Radio TelescopesArecibo

  37. Very Large Array

  38. Radio telescope resolution • = 1m d = 100m R = 2500 seconds = 42 minutes! Even though radio telescopes are much bigger, their resolving power is much worse than for optical telescopes Interferometric arrays get around this

  39. Very Large Array

  40. Interferometry Size of array = 10 km for a VLA This becomes the effective d Now R becomes 25 secsec for a 1-m wavelength For VLBI (very long baseline interfeormetry) the d = 10,000km and R = 0.025 seconds

  41. Observing from space • No clouds • Perfect seeing • Can see wavelengths of light blocked by the earth’s atmosphere

  42. Hubble Space Telescope

  43. Rooftop telescopes