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Galileo’s telescope

Galileo’s telescope. Refractor or Reflector. Refraction. Refraction is the bending of light when it passes from one substance into another Your eye uses refraction to focus light. Refraction makes a pencil appear to be bent when placed in water.

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Galileo’s telescope

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  1. Galileo’s telescope

  2. Refractor or Reflector

  3. Refraction • Refraction is the bending of light when it passes from one substance into another • Your eye uses refraction to focus light

  4. Refraction makes a pencil appear to be bent when placed in water. When light passes through a glass slab it first refracts towards the normal then away from the normal.

  5. Example: Refraction at Sunset • Sun appears distorted at sunset because of how light bends in Earth’s atmosphere

  6. Refraction telescope images The main lens in a refracting telescope is called the primary lens (or the objective lens). This lens is a part of the telescope and is fixed.

  7. Problems with refractors. Light of differing frequency travel at different speeds in glass. This results in a spectrum from a prism and leads to chromatic aberration in lenses. Large achromatic lenses are expensive. Lenses must be supported at the edges. Large lenses sag in the middle under their own weight. Long focal length= long tubes. Costly mounts and domes.

  8. Chromatic Aberration

  9. Achromatic Lenses

  10. Yerkes Observatory Williams Bay Wis. Worlds largest at 40 inches in diameter 63ft long

  11. Reflectors Most large telescopes are reflectors. Isaac Newton presented a reflecting telescope to the Royal Society in 1671. Mirrors avoid chromatic aberration. Objective mirror instead of an objective lens. Early reflectors were made of polished metal alloys. Tarnished rapidly dimming images.

  12. Reflectors 2 1850 method for depositing silver on glass. Most telescopes became reflectors Still tarnished, normal glass changes shape with temperature changes 1940’s technique for casting Pyrex glass and aluminum coatings (more durable) Now Pyrex or Fused Quartz Palomar 200 inch mirror took 11 years to build and required 5 tons of glass.

  13. Reflecting telescope images Light in a reflecting telescope does not have to pass through glass at all. The main mirror in a reflecting telescope is called the primary mirror (objective mirror).

  14. Images are inverted

  15. Types of reflectors

  16. Prime or Cassegrain Prime focus is often used in photography. It places the observer in a cage. The Cassegrain focus is at the bottom and requires a secondary mirror.

  17. Cassegrain Telescopes 18” Telescope 41” Telescope

  18. Earl of Rosse’s mid –nineteenth century telescope Observers stood at the prime focus.

  19. Newtonian A Newtonian focus is inconvenient for large telescopes.

  20. SCT

  21. Telescope Mounts The choice of mount depends on the telescopes main function and size. Good optics are useless if you can’t control the telescope. Good control is useless if the optics are poor.

  22. Alt-Azimuth Mounts

  23. Equatorial Mounts

  24. Equatorial Mounts 200in

  25. Russian 6m First large telescope to use alt-azimuth mount. A key move to the new generation of telescopes. For a given aperture this permits a cheaper lighter structure in a more compact dome.

  26. Telescope Rating Characteristics Powers of a telescope Light gathering power Resolving power Magnifying power.

  27. Light Gather Power Low High Low High Magnification Low High Resolution

  28. Light Gathering Power A telescope is a light bucket. Like a bucket catching rain the diameter is the determining factor in hiw much light gets caught. Area= π r2 The large the radius, diameter, the greater the area. Bigger is better. The more light you collect the farther out you see.

  29. Resolving Power … ability of a telescope to see fine detail. Defined as the angular distance between two objects that are barely visible as separate. Due to the wave nature of light magnified images have diffraction fringes.

  30. Diffraction Limits The edge of a telescope acts as slits causing diffraction patterns. These are only seen at the highest magnification for a telescope.

  31. Overlap Closely spaced objects begin to overlap, becoming indistinguishable.

  32. Increasing resolving power. The smaller the number the better the resolution.

  33. Resolution formula Angular resolution = 0.25 * For visible wavelengths in the middle of the visible spectrum “ α” ≈ Note aperture is in the denominator. Large D smaller resolution. Bigger telescope is better.

  34. Magnifying Power Usually the big sell. Least important. It can be changed. Magnification is calculated by the focal length of the objective divided by the focal length of the eyepiece. To Increase magnification just change to a shorter focal length eyepiece. M =

  35. Maximum Magnification • As magnification of an instrument is increased the image will be dimmer. • As a rule of thumb the maximum magnification of a telescope can be found by • Mmax = 20(X/cm) *D(cm)

  36. F5_4 Focal Length of a Lens The focal length of a lens or mirror is the distance from the center of the lens to the image formed from an object placed at a great distance.

  37. Observing Problems: Light Pollution Many of the most interesting objects are dim. Bright skies wash out the images. The darker the sky the better.

  38. Seeing

  39. Seeing To reduce the effects of the atmosphere observatories are placed at high elevations and in regions where the air is dry.

  40. Paranal Observatory ESO The best seeing is from remote, high and dry locations.

  41. Telescopes New Generation telescopes use advances in technology to correct images for bad seeing conditions.

  42. New Generation Telescopes Large mirrors had to be made thick to avoid sagging. A mirror can be supported from the back. Traditional telescopes were big, heavy, and expensive. Control devices had to be massive too. High speed computers have helped to reduce costs and improve performance. Computer control makes alt-azimuth mounts usable. Computer control makes it possible to control the shape of thin mirrors rapidly. This reduces the costs of making the mirror, smaller mounts and allows for…

  43. Thin floppy mirrors • Mirrors are backed by movable pistons able to change the shape of the mirror quickly under computer control. • One of the Keck hexagonal mirror segments. • Thin mirrors are lighter require less support and they change temperature faster (less trouble with convection currents at surface)

  44. Floppy Mirrors

  45. Nordic 2.6 M Canary Islands 1989 First large instrument whose dome and primary mirror shape are continually adjusted for the sharpest possible image.

  46. Gemini 8.1 m mirrors Rapid control of mirror shapes makes it possible to correct some distortions caused by poor seeing. Real time mirror control achieved by 120 actuators under the mirror and 60 around the edge. Right: The Gemini mirrors have adaptive optics

  47. Adaptive optics AO allows this 1.5m telescope to reach 0.1” resolution

  48. Adaptive Optics Corrected Image

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