Lec13-Mar03 - Quiz Ques3on#1 Lenses produce images...

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Unformatted text preview: 3/3/10 Quiz Ques3on #1 Lenses produce images by the process of Astro 109 Lecture 13: Op3cs and Telescopes A.  B.  C.  D.  E.  March 3 reflec3on refrac3on polariza3on sublima3on elimina3on Mar. 3 Quiz Ques3on #2 Quiz Ques3on #3 Suppose the angular separa3on of two stars on the sky is smaller than the angular resolu3on of your eyes. How would the stars appear to you? The most important reason to put an ultraviolet telescope in space is to A.  B.  C.  D.  E.  A.  B.  C.  D.  You would clearly see two stars. The two stars would look like a single point of light. You would not be able to see the stars at all. You would see only the larger of the two stars, and not the smaller one. E.  You would see only the redder of the two stars, and not the bluer one. Mar. 3 reduce opera3ng costs. reduce construc3on costs. avoid light pollu3on. avoid absorp3on by Earth’s atmosphere. avoid sunlight, since it’s always night in space. Mar. 3 Key Concepts Refrac3on •  refrac3ng and reflec3ng telescopes Light travels at a slower speed in a dense substance. •  angular resolu3on: diffrac3on, atmospheric blurring •  observing at different wavelengths •  telescopes in space Mar. 3 The index of refrac3on characterizes how fast light moves in a dense substance, and how much it bends. Mar. 3 1 3/3/10 Straight and curved glass Refrac3ng telescope Or put a camera here. Mar. 3 Mar. 3 World’s largest refractor Chroma3c aberra3on Mar. 3 Mar. 3 Reflec3on Straight mirror Mar. 3 Reflec3ng telescope (Newtonian) Curved mirror Mar. 3 2 3/3/10 Reflec3ng telescopes (other designs) Discussion Ques3on Reflec3ng telescopes have many advantages over refractors. Which of the following is not a true advantage? A.  The primary op3cal element needs to be polished on just one side. B.  A mirror can be supported all across the back. C.  A reflector has no chroma3c aberra3on. D.  In a refractor, some light is absorbed by the lens. E.  Reflectors have more light gathering power than refractors of the same aperture. Mar. 3 Mar. 3 Gemini North Observatory •  8 ­m mirror •  Cassegrain design •  one in Hawaii, one in Chile Mar. 3 8.4 ­m mirror blank, fresh out of the oven Mar. 3 Mauna Kea, Hawaii Southern African Large Telescope Mar. 3 Mar. 3 3 3/3/10 Angular resolu3on Discussion Ques3on What is the main reason for building op3cal observatories on tall mountains? Two limita3ons: A.  Land is cheap there. B.  The height of the mountain puts telescopes closer to the stars. C.  The thinner air allows light to travel faster. D.  Mountaintops are above the turbulent lower layers of the atmosphere, allowing sharper images. E.  No real reason – astronomers just like mountains. Mar. 3 •  “Diffrac3on” by telescope op3cs •  Blurring by atmospheric turbulence Mar. 3 Diffrac3on aperture http://www.noao.edu/education/gsmt/seeing" Seeing, or “Twinkle, Twinkle, Liele Star” •  Mainly limited by size of telescope •  Smallest angle that can be resolved – depends on wavelength of light λ θ = 2.5 × 105 arcsec D Mar. 3 •  Caused by turbulence in the atmosphere •  Main limita3on of ground ­based op3cal telescopes Mar. 3 Laser guide star Cameras: Photographic film vs. CCDs Adap3ve op3cs Deform mirror to “unblur ” the image. (Photo nega3ves) Mar. 3 Mar. 3 4 3/3/10 Two ways to show spectra Spectrograph Mar. 3 Mar. 3 Radio telescope Green Bank Telescope (West Virginia) Parkes Radio Telescope (Australia) Mar. 3 Mar. 3 Arecibo Observatory (Puerto Rico) Discussion Ques3on If you wonder why radio telescope are so big, think about this: Which of the following has the best angular resolu3on? λ θ∝ D A.  Keck 10 ­m telescope observing at 1 micron (10 ­6 m), with adap3ve op3cs B.  Spitzer Space Telescope (0.85 ­m) observing at 8 microns (8×10 ­6 m) C.  Green Bank Telescope (100 ­m) observing at 10 cm Mar. 3 Mar. 3 5 3/3/10 Radio Interferometer: Very Large Array (VLA, New Mexico) Mar. 3 Atmospheric transparency Mar. 3 Hubble Space Telescope Discussion Ques3on In addi3on to being in space, the Spitzer telescope is ac3vely cooled. Why do astronomers need to cool infrared telescopes? Spitzer Space Telescope (IR) Chandra A.  B.  C.  D.  E.  Telescopes in Space Fermi Gamma ­ray Space Telescope X ­ray It improves the angular resolu3on. It increases the magnifica3on. It reduces light emieed by the telescope itself. It makes the mirror reflect beeer. So the telescopes won’t melt. XMM Mar. 3 Mar. 3 HW ques3ons 2.  Explain why a flat piece of glass does not bring light to a focus while a curved piece of glass can. 6. Do most professional astronomers actually look through their telescopes? Why or why not? 10. Explain some of the disadvantages of refrac3ng telescopes compared to reflec3ng telescopes. 11.  What kind of telescope would you use if you wanted to take a color photograph en3rely free of chroma3c aberra3on? Explain your answer. HW ques3ons 15.  What is diffrac3on? Why does it limit the angular resolu3on of a telescope? What other physical phenomenon is ooen a more important restric3on on angular resolu3on? 18. What is light pollu3on? What effects does it have on the opera3on of telescopes? What can be done to minimize these effects? 24. Why are radio telescopes so large? Why does a single radio telescope have poorer angular resolu3on than a large op3cal telescope? How can the resolu3on be improved by making simultaneous observa3ons with several radio telescopes? 28. Why must astronomers use satellites and Earth ­orbi3ng observatories to study the heavens at X ­ray and gamma ­ray wavelengths? Mar. 3 Mar. 3 6 ...
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This note was uploaded on 09/15/2011 for the course PHYS 109 taught by Professor Pryor during the Spring '09 term at Rutgers.

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