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Astronomy 121 Lecture 5

Astronomy 121 Lecture 5. Today: Telescope basics Optical astronomy [ Ch 6.2 ]. Telescopes. Why use telescopes? To see better detail. To see fainter objects. To see other wavelengths. Two basic telescope designs: Refracting Reflecting. Refracting. Basic telescope type #1

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Astronomy 121 Lecture 5

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  1. Astronomy 121Lecture 5 Today: Telescope basics Optical astronomy [ Ch 6.2 ] Astr121 Lecture 5

  2. Telescopes Why use telescopes? • To see better detail. • To see fainter objects. • To see other wavelengths. Two basic telescope designs: • Refracting • Reflecting Astr121 Lecture 5

  3. Refracting Basic telescope type #1 Use lenses to create the image. Refract = To bend the light as it passes through some material. (glass, plastic) Astr121 Lecture 5

  4. 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) Astr121 Lecture 5

  5. Reflecting vs. Refracting Problems with refracting telescopes: #1: Chromatic aberration: Lens acts like a prism  Different colors are focused differently: Astr121 Lecture 5

  6. 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  Astr121 Lecture 5

  7. 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. Astr121 Lecture 5

  8. 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. Astr121 Lecture 5

  9. 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. Astr121 Lecture 5

  10. 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 Astr121 Lecture 5

  11. 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 Astr121 Lecture 5

  12. 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 Astr121 Lecture 5

  13. Resolution Examples For l = 500 nm (visible): D a Golf ball at: Astr121 Lecture 5

  14. Resolution Alcor Mizar Mizar Astr121 Lecture 5

  15. 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. Astr121 Lecture 5

  16. Altitude Keck telescopes at 14,000 feet  Why? • Above the weather. • Above a lot of turbulent air. • Above a lot of absorption. Astr121 Lecture 5

  17. “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. Astr121 Lecture 5

  18. Atmospheric Absorption Certain wavelengths are strongly absorbed, so must get to high altitude to see them. Especially IR, UV. Astr121 Lecture 5

  19. 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. Astr121 Lecture 5

  20. 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. Astr121 Lecture 5

  21. Next Time: Telescopes at Other Wavelengths Space Astronomy [ Ch. 6.4, 6.5 ] Astr121 Lecture 5

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