02 light and telescopes mc neely 2008

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  • 1. Astronomy Light & Telescopes Edwin Hubble and the 48-inch Palomar Telescope in 1949

2. Light

  • A form of wave motion
  • Waves:
    • Rise and fall
    • Transfer energy, but not material
    • Features : Crest, trough, wavelength, frequency
  • Photon : Light can also behave as a particle named a photon
  • Frequency : The number of waves that pass a fixed point in a given time

3. Waves 4. Visible Light

  • Human eye responds to visible light which is just one portion of the electromagnetic spectrum
  • Visible Light = 4000-7000 Angstroms
  • Visible Spectrum: ROYGBIV

5. Wavelength Relationship 6. EM Spectrum Short wavelengths Long wavelengths 7. Speed of Light

  • The speed of light is represented as c in Einsteins famous equation (E=mc 2 )
  • c = 186,000 miles per second (300,000 km per second)
  • Speed limit of the universe, nothing can travel faster

8. Light Years

  • Light Year : Distance measure of light travel in one year, about 6 trillion miles
  • Light year is a measure of distance
  • Light from sun = 8 light minutes

9. Light Travel Time

  • Light from nearest star = 4.3 light years
  • Diameter of Milky Way Galaxy = 100,000 ly
  • Distance to Andromeda Galaxy = 2.3 million ly
  • Distance to Virgo Galaxy Cluster = 50 million ly

10. Time Travel

  • Light takes time to travel through space
  • The farther away we look in distance, the further back we look in time
  • Ex: The star Sirius lies 8 light years away.
  • When we look at Sirius, we are seeing the star as it was 8 years ago

11. Types of Telescopes

  • Three types :
  • Refractors
    • Use lenses to collect light
  • Reflectors
    • Use mirrors to collect light
  • Compound
    • Both lenses and mirrors

12. Telescope Designs http://www.aw-wrdsmth.com/scuttlebutt/telescope-daigram.jpg 13. Refractors Department store refractor Modern APO refractor 14. Types of Reflectors

  • The Newtonian reflector was first designed by Isaac Newton and uses two mirrors to collect light
  • In recent years, Newtonians have been popular in the Dobsonian design where the telescope tube is mounted like a cannon

Newtons original telescope http://telescopemaking.org/images/newtontele.jpg 15. Newtonian & Dobsonian Meade Dobsonian telescope http://www.nachohat.org/images/static/meade_starfinder.jpg Eyepiece Newtonian optical diagram John Dobson 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. 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. Properties of Telescopes

  • Objective : Main mirror or lens
  • Aperture : Diameter of the objective, determines amount of light gathered by the scope
  • Eyepiece : Set of small magnifying lenses that forms the image viewed through a telescope
  • Focal Length : Distance from the objective to the image in the eyepiece

19. Refractor Objective Lens http://www.rocketroberts.com/astro/refractor.htm 20. Magnification

  • Magnification =
  • Telescope focal lengthEyepiece focal length
  • Ex : 2800mm focal length Schmidt Cassegrain telescope, with 32mm and 25 mm focal length eyepieces:
  • 2800mm32mm = 87.5x
  • 2800mm16mm = 112x

21. Useful Magnification

  • Highest useful magnification usually equals 50 times the aperture of the scope in inches:
  • Useful magnification = 50 * Aperture (inches)
  • Ex : What is the highest useful magnification of a 2.4-inch department store telescope and a 6-inch reflecting telescope?
  • 2.4-in * 50 = 120x
  • 6-in * 50 = 300x

22. Telescope Formula

  • A useful relationship for describing telescopes is the following:
  • f/number =
  • Focal lengthAperture
  • Compare :
    • 8-inch reflecting telescope of 900mm focal length
    • 70 mm refractor of 480 mm focal length

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. Two Scopes 25. Telescopes and Light Collecting

  • Small increases in aperture can dramatically improve telescopic views
  • This is because area is proportional to thesquareof a telescopes diameter
  • Telescopes promoted as having highmagnificationare meant to deceive consumers becauseapertureis the true way to access a telescopes ability

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. Telescope Aberrations

  • Chromatic : Inability of alensto focus all colors of the spectrum.
    • Ex: Color error or chromatic aberration in refracting telescopes
  • Spherical : Inability of amirrorto reflect all light to a single point.
    • Ex: Poorly made reflecting telescope mirrors.
    • Original Hubble Space Telescope mirror

28. Binoculars

  • Useful for stargazing
  • Two telescope tubes mounted side to side
  • Usually have fixed magnifications
  • Ex: Pair labeled 7x50, means 7x magnification, front objective lenses of 50mm diameter

Milky Way starfield 29. Telescope Seeing

  • The term seeing refers to the steadiness of the atmosphere overhead
  • Poor atmospheric seeing produces twinkling (star scintillation)
  • Unsteady air produces poor telescope images without sharp focus
  • Telescopes need to acclimate to outside temperature

http://en.wikipedia.org/wiki/Astronomical_seeing Lunar crater Clavius in poor seeing 30. Star Scintillation This montage of photographs shows how a single stars image is distorted over time by atmospheric seeing or turbulence Ideal star image 31. Why do Stars Twinkle? Turbulent air causes a stars image to distort 32. Light Pollution

  • Stargazing is difficult in the city
  • Excess artificial light that enters the night sky is termed light pollution
  • Observatories are built in remote places away from cities if possible

http://www.apstas.com/astrotas/glow.jpg 33. Effects of LP http://www.spaceweather.com/swpod2003/20aug03/Carlson1.jpg 34. Kitt Peak LP The view from Kitt Peak National Observatory of the Tuscon, Arizona skyline in 1959 The same skyline in 1972 35. US at Night http://archives.cnn.com/2000/US/08/24/dimming.the.lights.ap/large.usa.lights.jpg 36. Eastern US http://www.seds.org/~aschultz/images/light-pollution/us_nite.gif 37. Europe http://www.clocktower.demon.co.uk/stockgrove/light/europe.jpg

  • Notice how brightness can indicate wealth and development; Poor countries have much less outdoor lighting

38. Earth at Night (Click Below) http://veimages.gsfc.nasa.gov//1438/earth_lights_lrg.jpg 39. Good and Bad Lighting

  • Good light fixtures shine their light only toward the ground, not toward your eyes or the sky
  • A bad light fixture is one in which the uncovered bulb is visible

40. Light Fixtures

  • Billboards that emit light straight into the sky are bad

41. Observatories

  • Observatories provide a permanent installation to house a telescope
  • Modern, professional observatories are usually located on mountain tops to take advantage of better seeing
  • Ex : Keck Observatory on Mauna Kea in Hawaii
  • Amateur astronomers build backyard observatories of many types

42. Keck Observatory http://www.wainscoat.com/astronomy/keck-moonlight.jpg

  • The twin Keck 10-meter telescopes are the largest in the world
  • The telescopes are located on the 14,000 foot elevation summit of Mauna Kea in Hawaii
  • Each telescope uses a mirror composed of 36 hexagonal segments arranged in a mosaic pattern
  • The individual mirrors act together like a single mirror

43. Keck Mirror http://www.astro.ucla.edu/~seth/albums/images/mirror3.jpg 44. Backyard Observatory Roof rolls off for easy access to sky SCT mounted on a permanent pier aka Mini Keck 45. Summary : Telescope Formulas

  • Magnification=
  • Telescope Focal Length (mm)Eyepiece Focal Length (mm)
  • Useful Magnification=
  • 50 * Aperture (in)