02 Light And Telescopes Mc Neely 2008


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02 Light And Telescopes Mc Neely 2008

  1. 1. Astronomy Light & Telescopes Edwin Hubble and the 48-inch Palomar Telescope in 1949
  2. 2. Light <ul><li>A form of wave motion </li></ul><ul><li>Waves: </li></ul><ul><ul><li>Rise and fall </li></ul></ul><ul><ul><li>Transfer energy, but not material </li></ul></ul><ul><ul><li>Features : Crest, trough, wavelength, frequency </li></ul></ul><ul><li>Photon : Light can also behave as a particle named a photon </li></ul><ul><li>Frequency : The number of waves that pass a fixed point in a given time </li></ul>
  3. 3. Waves
  4. 4. Visible Light <ul><li>Human eye responds to “visible light” which is just one portion of the electromagnetic spectrum </li></ul><ul><li>Visible Light = 4000-7000 Angstroms </li></ul><ul><li>Visible Spectrum: ROYGBIV </li></ul>
  5. 5. Wavelength Relationship
  6. 6. EM Spectrum Short wavelengths Long wavelengths
  7. 7. Speed of Light <ul><li>The speed of light is represented as “c” in Einstein’s famous equation (E=mc 2 ) </li></ul><ul><li>“ c” = 186,000 miles per second (300,000 km per second) </li></ul><ul><li>“ Speed limit of the universe,” nothing can travel faster </li></ul>
  8. 8. Light Years <ul><li>Light Year : Distance measure of light travel in one year, about 6 trillion miles </li></ul><ul><li>Light year is a measure of distance </li></ul><ul><li>Light from sun = 8 light minutes </li></ul>
  9. 9. Light Travel Time <ul><li>Light from nearest star = 4.3 light years </li></ul><ul><li>Diameter of Milky Way Galaxy = 100,000 ly </li></ul><ul><li>Distance to Andromeda Galaxy = 2.3 million ly </li></ul><ul><li>Distance to Virgo Galaxy Cluster = 50 million ly </li></ul>
  10. 10. Time Travel <ul><li>Light takes time to travel through space </li></ul><ul><li>The farther away we look in distance, the further back we look in time </li></ul><ul><li>Ex: The star Sirius lies 8 light years away. </li></ul><ul><li>When we look at Sirius, we are seeing the star as it was 8 years ago </li></ul>
  11. 11. Types of Telescopes <ul><li>Three types : </li></ul><ul><li>Refractors </li></ul><ul><ul><li>Use lenses to collect light </li></ul></ul><ul><li>Reflectors </li></ul><ul><ul><li>Use mirrors to collect light </li></ul></ul><ul><li>Compound </li></ul><ul><ul><li>Both lenses and mirrors </li></ul></ul>
  12. 12. Telescope Designs http://www.aw-wrdsmth.com/scuttlebutt/telescope-daigram.jpg
  13. 13. Refractors “ Department store” refractor Modern APO refractor
  14. 14. Types of Reflectors <ul><li>The Newtonian reflector was first designed by Isaac Newton and uses two mirrors to collect light </li></ul><ul><li>In recent years, Newtonians have been popular in the Dobsonian design where the telescope tube is mounted like a cannon </li></ul>Newton’s original telescope http://telescopemaking.org/images/newtontele.jpg
  15. 15. Newtonian & Dobsonian Meade Dobsonian telescope http://www.nachohat.org/images/static/meade_starfinder.jpg Eyepiece Newtonian optical diagram John Dobson
  16. 16. 6-in Newtonian on a Dobsonian Mount “ 6-in” indicates that the telescope uses a 6-inch diameter mirror as its main light gathering optic This Orion Telescopes XT6 is an excellent scope for beginners and is reasonably priced
  17. 17. Compound Scopes: Schmidt-Cassegrain Cutaway view of an SCT Maksutovs are similar yet use a more curved front lens Main mirror Lens Meade Telescopes 8-inch SCT Eyepiece
  18. 18. Properties of Telescopes <ul><li>Objective : Main mirror or lens </li></ul><ul><li>Aperture : Diameter of the objective, determines amount of light gathered by the scope </li></ul><ul><li>Eyepiece : Set of small magnifying lenses that forms the image viewed through a telescope </li></ul><ul><li>Focal Length : Distance from the objective to the image in the eyepiece </li></ul>
  19. 19. Refractor Objective Lens http://www.rocketroberts.com/astro/refractor.htm
  20. 20. Magnification <ul><li>Magnification = </li></ul><ul><li>Telescope focal length ÷ Eyepiece focal length </li></ul><ul><li>Ex : 2800mm focal length Schmidt Cassegrain telescope, with 32mm and 25 mm focal length eyepieces: </li></ul><ul><li>2800mm ÷ 32mm = 87.5x </li></ul><ul><li>2800mm ÷ 16mm = 112x </li></ul>
  21. 21. Useful Magnification <ul><li>Highest useful magnification usually equals 50 times the aperture of the scope in inches: </li></ul><ul><li>Useful magnification = 50 * Aperture (inches) </li></ul><ul><li>Ex : What is the highest useful magnification of a 2.4-inch department store telescope and a 6-inch reflecting telescope? </li></ul><ul><li>2.4-in * 50 = 120x </li></ul><ul><li>6-in * 50 = 300x </li></ul>
  22. 22. Telescope Formula <ul><li>A useful relationship for describing telescopes is the following: </li></ul><ul><li>f/number = </li></ul><ul><li>Focal length ÷ Aperture </li></ul><ul><li>Compare : </li></ul><ul><ul><li>8-inch reflecting telescope of 900mm focal length </li></ul></ul><ul><ul><li>70 mm refractor of 480 mm focal length </li></ul></ul>
  23. 23. Telescope Formula Examples 8-inch Reflector 2.7-inch Refractor Aperture (mm) 200mm 70mm Focal Length (mm) 900mm 480mm f/Number 900/200=f/4.5 480/70=f/6.8 Magnification (32mm eyepiece) 900/32=28x 480/32=15x
  24. 24. Two Scopes
  25. 25. Telescopes and Light Collecting <ul><li>Small increases in aperture can dramatically improve telescopic views </li></ul><ul><li>This is because area is proportional to the square of a telescope’s diameter </li></ul><ul><li>Telescopes promoted as having high magnification are meant to deceive consumers because aperture is the true way to access a telescope’s ability </li></ul>
  26. 26. Aperture Demo http://www.clarkvision.com/visastro/m51-apert/index.html The animation compares sketches of the Whirlpool Galaxy (M51) through 6, 8, and 12.5 in telescopes
  27. 27. Telescope Aberrations <ul><li>Chromatic : Inability of a lens to focus all colors of the spectrum. </li></ul><ul><ul><li>Ex: Color error or chromatic aberration in refracting telescopes </li></ul></ul><ul><li>Spherical : Inability of a mirror to reflect all light to a single point. </li></ul><ul><ul><li>Ex: Poorly made reflecting telescope mirrors. </li></ul></ul><ul><ul><li>Original Hubble Space Telescope mirror </li></ul></ul>
  28. 28. Binoculars <ul><li>Useful for stargazing </li></ul><ul><li>Two telescope tubes mounted side to side </li></ul><ul><li>Usually have fixed magnifications </li></ul><ul><li>Ex: Pair labeled 7x50, means 7x magnification, front objective lenses of 50mm diameter </li></ul>Milky Way starfield
  29. 29. Telescope “Seeing” <ul><li>The term “seeing” refers to the steadiness of the atmosphere overhead </li></ul><ul><li>Poor atmospheric seeing produces “twinkling” (star scintillation) </li></ul><ul><li>Unsteady air produces poor telescope images without sharp focus </li></ul><ul><li>Telescopes need to acclimate to outside temperature </li></ul>http://en.wikipedia.org/wiki/Astronomical_seeing Lunar crater Clavius in poor seeing
  30. 30. Star Scintillation This montage of photographs shows how a single star’s image is distorted over time by atmospheric seeing or turbulence Ideal star image
  31. 31. Why do Star’s Twinkle? Turbulent air causes a star’s image to distort
  32. 32. Light Pollution <ul><li>Stargazing is difficult in the city </li></ul><ul><li>Excess artificial light that enters the night sky is termed light pollution </li></ul><ul><li>Observatories are built in remote places away from cities if possible </li></ul>http://www.apstas.com/astrotas/glow.jpg
  33. 33. Effects of LP http://www.spaceweather.com/swpod2003/20aug03/Carlson1.jpg
  34. 34. Kitt Peak LP The view from Kitt Peak National Observatory of the Tuscon, Arizona skyline in 1959 The same skyline in 1972
  35. 35. US at Night http://archives.cnn.com/2000/US/08/24/dimming.the.lights.ap/large.usa.lights.jpg
  36. 36. Eastern US http://www.seds.org/~aschultz/images/light-pollution/us_nite.gif
  37. 37. Europe http://www.clocktower.demon.co.uk/stockgrove/light/europe.jpg <ul><li>Notice how brightness can indicate wealth and development; Poor countries have much less outdoor lighting </li></ul>
  38. 38. Earth at Night (Click Below) http://veimages.gsfc.nasa.gov//1438/earth_lights_lrg.jpg
  39. 39. Good and Bad Lighting <ul><li>Good light fixtures shine their light only toward the ground, not toward your eyes or the sky </li></ul><ul><li>A bad light fixture is one in which the uncovered bulb is visible </li></ul>
  40. 40. Light Fixtures <ul><li>Billboards that emit light straight into the sky are bad </li></ul>
  41. 41. Observatories <ul><li>Observatories provide a permanent installation to house a telescope </li></ul><ul><li>Modern, professional observatories are usually located on mountain tops to take advantage of better seeing </li></ul><ul><li>Ex : Keck Observatory on Mauna Kea in Hawaii </li></ul><ul><li>Amateur astronomers build backyard observatories of many types </li></ul>
  42. 42. Keck Observatory http://www.wainscoat.com/astronomy/keck-moonlight.jpg <ul><li>The twin Keck 10-meter telescopes are the largest in the world </li></ul><ul><li>The telescopes are located on the 14,000 foot elevation summit of Mauna Kea in Hawaii </li></ul><ul><li>Each telescope uses a “mirror” composed of 36 hexagonal segments arranged in a mosaic pattern </li></ul><ul><li>The individual mirrors act together like a single mirror </li></ul>
  43. 43. Keck “Mirror” http://www.astro.ucla.edu/~seth/albums/images/mirror3.jpg
  44. 44. Backyard Observatory Roof rolls off for easy access to sky SCT mounted on a permanent pier aka Mini Keck
  45. 45. Summary : Telescope Formulas <ul><li>Magnification = </li></ul><ul><li>Telescope Focal Length (mm) ÷ Eyepiece Focal Length (mm) </li></ul><ul><li>“ Useful” Magnification = </li></ul><ul><li>50 * Aperture (in) </li></ul><ul><li>f/Number = </li></ul><ul><li>Telescope Focal length (mm) ÷ Telescope Aperture (mm) </li></ul>