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Telescopes

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Telescopes

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Telescopes

  1. 1. Astronomy & Space Science Association in University of Kelaniya Astro Night 2014 Lecture 2 Telescopes and Optical Instrumentations Eranga Jayashantha Hon. mentd Dip in Astronomy & Astrophysics ( IOAA – China ) National Representative at Space Generation Advisory Council ( UN ) Sri Lanka national Astrophysics Olympiad Team Trainer at Institute of Physics Sri Lanka
  2. 2. 2 Telescopes Why we use telescopes? • To see better detail. • To see fainter objects. • To observe objects by using various wavelengths. • To detect special elementary particles
  3. 3. Telescopes can be divided into several types based on few classifications • 1) Based on detecting wavelength ( Optical Telescopes , Radio telescopes, IR Telescopes etc) • 2) Based on optical mechanism – I ) Optic System ;  Refracting Telescopes  i) Astronomical Telescope ii) Galileo Telescope  Reflecting Telescopes  i) Newtonian Telescope ii) Cassegrain Telescope iii) Gregorion Telescope – II) Catadioptric System ; i) Schmidth Cassegrain Telescope ii) Maksutove Cassegrain Telescope
  4. 4. Astr121 Lecture 5 4 Refracting Basic telescope type #1 Use lenses to create the image. Refract = To bend the light as it passes through some material. (glass, plastic)
  5. 5. Astr121 Lecture 5 5 Reflecting Basic telescope type #2. Use curved mirrors to create the image. Reflect = To bounce the light off the surface of some material. (glass, metal)
  6. 6. Multi Wavelength Imagining
  7. 7. Astr121 Lecture 5 17
  8. 8. Astr121 Lecture 5 18
  9. 9. VERITA S
  10. 10. Astr121 Lecture 5 24
  11. 11. Disadvantages of Refracting Telescopes 25 • 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
  12. 12. Reflecting vs. Refracting Problem #2: Weight: Large lenses: Very heavy. Supported on the edges. Large mirrors: Can be thin. Supported on the back. Largest refracting (lens) telescope: Yerkes 1 meter 
  13. 13. Reflecting vs. Refracting Problem #3: Manufacturing: Thin lenses = very long focal lengths.  Problems designing and using such long telescopes. Thick lenses = short focal lengths.  Lots of glass: must be free of defects, will be heavy.
  14. 14. Bigger is Better Keck Telescope (actually 2 of them) Largest reflecting Visible/IR telescope: 10 meter mirror  Located on Mauna Kea, a dormant Hawaiian volcano, about 14,000 feet high.
  15. 15. 34 Light Collection Reason #1 for big telescopes: Collect more light. The larger the opening, the more light collected. Larger opening  more light  detect fainter objects.
  16. 16. Astr121 Lecture 5 35 Resolving Power Reason #2 for big telescopes: Resolving power: the ability to distinguish detail. Means how closely two objects can be seen separately. Not Resolved Barely resolved Resolved
  17. 17. 36 Angular Measurements Measuring angles: 360 degrees in a circle 1o 60 arc-minutes per degree 1’ 60 arc-seconds per arc-minute 1’’ Same angles: Or use radians: 2p radians = 360o
  18. 18. α = 2.3 ´ 10 5 λ D Astr121 Lecture 5 37 Resolution Resolution of a telescope is determined by the size of its aperture (opening). If D = opening diameter, Smallest angular separation: If l and D are in same units  a is in arc-seconds. Larger diameter telescope = Smaller resolution
  19. 19. 38 Resolution Examples α = 2.3 ´ 10 5 λ D For l = 500 nm (visible): D a Golf ball at: Human eye: 0.5 cm 23 arc-sec 300 m Small telescope: 10 cm 1 arc-sec 7.5 km Keck telescope: 10 m 0.01 arc-sec 725 km
  20. 20. 39 Resolution Mizar Mizar Alcor
  21. 21. 40 Magnification Magnification = ‘Zoom’ factor for a given image, set by eyepiece. It’s important, but it can’t overcome low resolution. – Information limited by resolution and ‘seeing’. – Magnification useful below these limits. Example: TV or newspaper picture made of tiny dots. – Dot size = resolution – Zooming in doesn’t make more detail, just bigger dots.
  22. 22. 41 Altitude Keck telescopes at 14,000 feet  Why? – Above the weather. – Above a lot of turbulent air. – Above a lot of absorption.
  23. 23. 42 “Seeing” Seeing: how stable and clear the atmosphere is. Turbulence in the atmosphere causes distortions: ‘Twinkling’ of stars. Left side: bad seeing. Right side: good seeing. Typical atmospheric limit is 1’’. At high altitudes you are above much of the turbulent air.
  24. 24. Atmospheric Absorption Certain wavelengths are strongly absorbed, so must get to high altitude to see them. Especially IR, UV. Astr121 Lecture 5 43
  25. 25. 44 Light Pollution Populated areas emit a lot of light, making the sky appear less dark and blocking out faint objects. Telescopes often located in remote, unpopulated areas.
  26. 26. 45 Summary Bigger telescopes better because: • Collect more light  see fainter objects • Better resolution  see smaller detail Observing problems: • “Seeing”  air turbulence • Absorption  atmosphere opaque at some wavelengths • Light pollution  blocks faint objects Solution: telescope at high altitude, usually away from population.
  27. 27. Active optics 46
  28. 28. Adaptive Optics 47
  29. 29. 48
  30. 30. 49
  31. 31. 50
  32. 32. 51
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  34. 34. 53

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