02 Light And Telescopes Mc Neely 2008
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02 Light And Telescopes Mc Neely 2008






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  • pp. 170-171: #4, 5, 6, 7, 30, 31 Reading: pp. 148—153 (Sec 5.2)
  • Homework: p. 22-23, #5, 6, 17, 18, 29 Reading pp. 6-7 (How can we know…)
  • Homework pp. 196-197: #5, 9, 10, 11, 15, 21, 23, 26, 28, 30, 31, 35 Reading pp. 180-183 (What are the two basic designs…?) pp. 186-194 (Sec 6.3)
  • Reading p. 186 (Special Topic)
  • Reading p. 178 (Magnification and Telescopes)

02 Light And Telescopes Mc Neely 2008 02 Light And Telescopes Mc Neely 2008 Presentation Transcript

  • Astronomy Light & Telescopes Edwin Hubble and the 48-inch Palomar Telescope in 1949
  • 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
  • Waves
  • 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
  • Wavelength Relationship
  • EM Spectrum Short wavelengths Long wavelengths
  • Speed of Light
    • The speed of light is represented as “c” in Einstein’s 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
  • 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
  • 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
  • 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
  • Types of Telescopes
    • Three types :
    • Refractors
      • Use lenses to collect light
    • Reflectors
      • Use mirrors to collect light
    • Compound
      • Both lenses and mirrors
  • Telescope Designs http://www.aw-wrdsmth.com/scuttlebutt/telescope-daigram.jpg
  • Refractors “ Department store” refractor Modern APO refractor
  • 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
    Newton’s original telescope http://telescopemaking.org/images/newtontele.jpg
  • Newtonian & Dobsonian Meade Dobsonian telescope http://www.nachohat.org/images/static/meade_starfinder.jpg Eyepiece Newtonian optical diagram John Dobson
  • 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
  • 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
  • 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
  • Refractor Objective Lens http://www.rocketroberts.com/astro/refractor.htm
  • Magnification
    • Magnification =
    • Telescope focal length ÷ Eyepiece focal length
    • Ex : 2800mm focal length Schmidt Cassegrain telescope, with 32mm and 25 mm focal length eyepieces:
    • 2800mm ÷ 32mm = 87.5x
    • 2800mm ÷ 16mm = 112x
  • 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
  • Telescope Formula
    • A useful relationship for describing telescopes is the following:
    • f/number =
    • Focal length ÷ Aperture
    • Compare :
      • 8-inch reflecting telescope of 900mm focal length
      • 70 mm refractor of 480 mm focal length
  • 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
  • Two Scopes
  • Telescopes and Light Collecting
    • Small increases in aperture can dramatically improve telescopic views
    • This is because area is proportional to the square of a telescope’s diameter
    • Telescopes promoted as having high magnification are meant to deceive consumers because aperture is the true way to access a telescope’s ability
  • 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
  • Telescope Aberrations
    • Chromatic : Inability of a lens to focus all colors of the spectrum.
      • Ex: Color error or chromatic aberration in refracting telescopes
    • Spherical : Inability of a mirror to reflect all light to a single point.
      • Ex: Poorly made reflecting telescope mirrors.
      • Original Hubble Space Telescope mirror
  • 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
  • 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
  • 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
  • Why do Star’s Twinkle? Turbulent air causes a star’s image to distort
  • 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
  • Effects of LP http://www.spaceweather.com/swpod2003/20aug03/Carlson1.jpg
  • Kitt Peak LP The view from Kitt Peak National Observatory of the Tuscon, Arizona skyline in 1959 The same skyline in 1972
  • US at Night http://archives.cnn.com/2000/US/08/24/dimming.the.lights.ap/large.usa.lights.jpg
  • Eastern US http://www.seds.org/~aschultz/images/light-pollution/us_nite.gif
  • 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
  • Earth at Night (Click Below) http://veimages.gsfc.nasa.gov//1438/earth_lights_lrg.jpg
  • 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
  • Light Fixtures
    • Billboards that emit light straight into the sky are bad
  • 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
  • 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
  • Keck “Mirror” http://www.astro.ucla.edu/~seth/albums/images/mirror3.jpg
  • Backyard Observatory Roof rolls off for easy access to sky SCT mounted on a permanent pier aka Mini Keck
  • Summary : Telescope Formulas
    • Magnification =
    • Telescope Focal Length (mm) ÷ Eyepiece Focal Length (mm)
    • “ Useful” Magnification =
    • 50 * Aperture (in)
    • f/Number =
    • Telescope Focal length (mm) ÷ Telescope Aperture (mm)