14 Sep 2000ASTR103, GMU, Dr. Correll1 ASTR 103--Week 3.
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Transcript of 14 Sep 2000ASTR103, GMU, Dr. Correll1 ASTR 103--Week 3.
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14 Sep 2000 ASTR103, GMU, Dr. Correll 1
ASTR 103--Week 3ASTR 103--Week 3
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14 Sep 2000 ASTR103, GMU, Dr. Correll 2
Light and TelescopesLight and Telescopes
• The Nature of Light
• Optics and Telescopes
• Radio Astronomy--and Beyond
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The Nature of LightThe Nature of Light
• Early discoveries– White light comprised of a spectrum of colors
• Isaac Newton demonstrated colors inherent in the light, not added by the prism
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The Nature of LightThe Nature of Light
– Light travels at a finite speed, c, not instantaneously
• Romer measures delay in Jupiter’s lunar eclipses
• 7 minutes from Sunlight to reach Earth
• 0.13 seconds for light to go around the world
mi/sec 10x86.1
km/sec 10x0.35
5
c
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Light--Waves or Particles?Light--Waves or Particles?
• Newton believed light a particle phenomenon• Thomas Young shows light behaves like waves
(1801)
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Electromagnetic WavesElectromagnetic Waves
• Waves are described by– wavelength, (lambda)– frequency, (nu)– speed, v =– and they transport energy
• Electromagnetic waves result from the motion of electrons
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14 Sep 2000 ASTR103, GMU, Dr. Correll 7
Light--Waves or Particles?Light--Waves or Particles?
• Albert Einstein shows that light contains energy in discrete packets– Photons--particles of light
and energy
• Photon energy increases with frequency, decreases with wavelength
hchE
e-
h
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Electromagnetic SpectrumElectromagnetic Spectrum
High energy photons
Low energy photons
nm--nanometer--equal to 10-9 meters
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Electromagnetic SpectrumElectromagnetic Spectrum
• Different spectral types correspond to different physical phenomenon– Gamma rays and X rays
generated by extreme energy
– visible light by atomic and molecular processes
– infrared light generated by heat and molecular processes
– radio waves generated by motion of free electrons and ions
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Atmospheric WindowsAtmospheric Windows
• Most electromagnetic energy absorbed by the atmosphere, but radio waves and visible light pass through
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14 Sep 2000 ASTR103, GMU, Dr. Correll 11
Light and TelescopesLight and Telescopes
• The Nature of Light
• Optics and Telescopes
• Radio Astronomy--and Beyond
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14 Sep 2000 ASTR103, GMU, Dr. Correll 12
Astronomical TelescopesAstronomical Telescopes
• Two basic types:– Refracting--
through a glass lens
– Reflecting--from a mirror
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RefractionRefraction
• Refraction--light bends, or changes direction, when crossing an interface between different media, for example, air and glass
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LensesLenses
• Thus a lens collects light from a distant object and concentrates it at a focused image– every lens has a
characteristic focal length
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Refracting TelescopeRefracting Telescope
• Essential elements of a refracting telescope
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Telescope PerformanceTelescope Performance
• Brighten (Aperture)– Larger objective lenses and longer exposure
times gather more light
• Resolve (Resolving Power)– Larger objective lenses and properly curved
surfaces improve resolution
• Magnify (Magnification)– M=focal length of objective divided by focal
length of eyepiece– Least important measure of performance
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Resolution Reveals DetailsResolution Reveals Details
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Complications with RefractorsComplications with Refractors
• Chromatic Aberration--different colors of light refract at different angles and thus have different focal points
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Complications with RefractorsComplications with Refractors
• Spherical Aberration--best shape for lens surface is parabolic; this is difficult to make
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Reflecting TelescopesReflecting Telescopes
• Reflecting mirrors do not suffer chromatic aberration since all colors reflect at the same angles
• Spherical aberrations must still be corrected
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Reflecting TelescopesReflecting Telescopes
• Some standard configurations
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Reflecting TelescopesReflecting Telescopes
• Typical home telescope
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Atmospheric distortionAtmospheric distortion
• Visible light propagates through the atmosphere, but is distorted– Twinkle, twinkle little star…– Telescopes in space don’t
suffer this problem– Hubble Space Telescope
• Ground-based telescopes can now be corrected with flexible optics and computer corrections to remove distortion
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Atmospheric distortionAtmospheric distortion
Ground Ground/Adaptive HST
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Image RecordingImage Recording
• Sketches• Film• Electronic Data
CCD
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14 Sep 2000 ASTR103, GMU, Dr. Correll 26
Light and TelescopesLight and Telescopes
• The Nature of Light
• Optics and Telescopes
• Radio Astronomy--and Beyond
![Page 27: 14 Sep 2000ASTR103, GMU, Dr. Correll1 ASTR 103--Week 3.](https://reader035.fdocuments.us/reader035/viewer/2022062322/5697bfdb1a28abf838cb0732/html5/thumbnails/27.jpg)
14 Sep 2000 ASTR103, GMU, Dr. Correll 27
Radio TelescopesRadio Telescopes
• Radio Waves have long wavelenths (cm and m)– objectives need to be
big, but not polished– “image” data recorded
with antenna instead of CCDs
• First radio telescopes in 1930s by Karl Jansky at Bell Labs
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14 Sep 2000 ASTR103, GMU, Dr. Correll 28
Radio TelescopesRadio Telescopes
• By interfering, or combining wave patterns, of signals from multiple radio telescopes much higher resolution images can be made– makes a “synthetic”
objective much larger than each part
– VLA in New Mexico (at right)
– VLBA from around the globe
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Radio TelescopesRadio Telescopes
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Other wavelengthsOther wavelengths
• Infrared from high mountain tops, rockets, or satellites
• UV, X-ray and gamma rays from satellites
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Different perspectivesDifferent perspectives
• Comparing information from different spectra gives a better understanding of what’s going on
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Questions for studyQuestions for study
• How are radio telescopes similar and different from visible light telescopes?