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TELESCOPES

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  1. TELESCOPES Palomar 200-in

  2. 8.4-m Mirror Blank for Large Binocular Telescope

  3. Polishing one LBT 8.4-m mirror

  4. The Electromagnetic Spectrum

  5. Functions of Telescopes • Collect more light --- depends on (diam)2 • Resolve sources better (see more detail) • Magnify images

  6. Telescope “Objectives”: Specially Shaped Main Optical Element Purpose: form an accurate representation of original scene at a “focus” • Lens ---> “refracting” telescope • Mirror ---> “reflecting” telescope

  7. Refraction: Bending of Light Rays at a Glass/Air Interface

  8. Reflection from a Smooth Surface

  9. Refracting Telescope Minimum 2 lenses needed for visual use Place detector here

  10. Image formation (Java demo)

  11. Reflecting Telescopes

  12. “Catadioptric” (Lens+Mirror) Design

  13. Image Inversion in Simple Telescope

  14. Telescope Performance Characteristics • Focal Ratio (f/ number) • Magnification ("power") • Field of view • Light Gathering Power • Resolution

  15. Focal Ratio (or f/number) • f/ number = Obj FL / Obj Diam • Smaller numbers give more concentrated light in focal plane (better for faint extended objects); allow shorter exposures with film/electronic detectors • Higher numbers have better resolution; better for high magnification (e.g. for planets)

  16. Magnification • Defined to be ratio of apparent angular size of image to original angular size (without telescope) • Mag = FL (telescope) / FL (eyepiece) • For Celestrons, Mag = 2034 mm/FLE (mm) • Moderate magnifications (<150) best

  17. Field of View • FOV = True angular diameter -- i.e. as viewed without telescope -- of field visible in eyepiece. • Usually quoted in degrees or minutes of arc • Depends on eyepiece used • Is smaller for higher magnification with given telescope

  18. Light Gathering Power • Most important attribute of telescope • Light collected is proportional to the area of the objective, or to Dobj2 • If the pupil diameter of your eye is 5mm, an 8" telescope collects (203/5)2 = 1600x more light

  19. AGAIN, JONES' SNEAKY COLLEAGUES AIMED THE TELESCOPE AT THE SUN

  20. Table by D. Haworth

  21. Table by D. Haworth

  22. Image Quality (Resolution) • Design optics to reduce "aberrations" -- e.g. chromatic, spherical, etc. • Optical figuring to intended shape: must be better than “1/4 wavelength” • Larger telescopes better because of “diffraction” of light waves • Turbulence in air strongly affects image blur. “Seeing” = size of blur.

  23. Chromatic Aberration (present in any refracting element)

  24. Spherical "Aberration"

  25. Parabola: perfect paraxial focus

  26. Parabola: "coma" aberration off-axis

  27. Longer focal lengths reduce chromatic & spherical aberration (Hevelius, ca. 1650)

  28. “Schmidt-Cassegrain” design uses a thin refractive corrector to eliminate spherical aberration from a spherically-shaped primary

  29. 8.4-m Mirror Blank for Large Binocular Telescope

  30. Polishing one LBT 8.4-m mirror

  31. “Diffraction” of Light Waves Ideal case Real waves

  32. Wave Tank Simulation

  33. 6" 20" Effects of diffraction on size of image of a double star in a telescope 200" 94"

  34. “Seeing” Caused by Atmospheric Turbulence

  35. Video of enlarged image of bright star in a large telescope. Image size/motion caused by Earth’s atmosphere.

  36. Telescope Designs: A Multitude • Optical design • Mounting design • Equatorial • Altitude-Azimuth

  37. ReflectingTelescope Designs

  38. “Catadioptric” (Lens+Mirror) Design

  39. ALL Large Telescopes areREFLECTORS • Why?

  40. Equatorial Mount Alt-Az Mount

  41. Example Altitude-Azimuth Mounts

  42. "Equatorial Mounts"

  43. McCormick 26-in Refractor, Equatorial Mount

  44. McCormick 26-in Lens (Doublet)

  45. 200-in Dedication (1948) (Largest equatorial mount for optical telescope; "horseshoe")

  46. Astronomer in 200-in Prime Focus Cage