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Understanding Optical Telescopes: Reflectors vs. Refractors

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This chapter delves into the world of optical telescopes, focusing on the two primary types: refractors, which utilize lenses, and reflectors, which employ mirrors. It highlights the key components of telescopes, including the objective lens or mirror (aperture), focal length, and eyepiece. The text elaborates on important concepts like light-gathering power, magnifying power, and resolving power, explaining how these factors influence astronomical observations. Additionally, it discusses advanced telescopic technologies, including infrared and X-ray telescopes, and the challenges of observing high-energy photons.

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Understanding Optical Telescopes: Reflectors vs. Refractors

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  1. Chapter 4: Telescopes

  2. Optical telescopes: Reflectors and refractors E O Refractors use lenses E: eyepiece O: objective

  3. Reflectors use mirrors: Newtonian Cassegrain

  4. Objective (O) = Main light-gathering surface (lens or mirror) Aperture= a = Diameter of objective ("size" of telescope) Focal length= f = Distance from objective to image ("length" of telescope) Eyepiece (E) = Lens used to inspect image from objective

  5. 1) LIGHT GATHERING POWER (how faint you can see) • LGP ~ a2 (Bigger = better) • 2) MAGNIFYING POWER (how large image is) • MP = f(O) / f(E) (Bigger = better) • So can change this with eyepice choice, not a fundamental property of telescope • Practical limit  50 x O (in inches) • 3) RESOLVING POWER (size of detail, sharpness of focus) • Size of detail ~ 1/a (Smaller = better) • i.e., larger ‘a’ allows smaller details to be seen

  6. Resolution vs. magnification

  7. Airy disk. Light coming through a circular aperture

  8. Resolution vs. magnification

  9. Atmospheric seeing

  10. The planet Uranus imaged in the infrared

  11. Spitzer Space Telescope An infrared telescope in orbit around the Sun!

  12. 43 m diameter

  13. How to improve resolution? Radio interferometry The Very Large Array, Socorro, New Mexico

  14. VLBI

  15. Chandra Xray observatory

  16. X-ray telescope design (Chandra X-ray Observatory) It is very difficult to focus (redirect) high energy photons, because they are so readily absorbed by matter. Thus it is difficult to tell where they came from, and to reconstruct the image.

  17. Chandra image of X-rays emitted from hot gas surrounding galaxy Cygnus-A

  18. An Xray image of a black hole, by Chandra.

  19. X-ray image Visible (optical) Hot stars give off a lot more Xrays than cooler stars do.

  20. Observing gamma rays Space-based telescopes (Compton Gamma ray Observatory

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