14 Sep 2000ASTR103, GMU, Dr. Correll1 ASTR 103--Week 3

14 Sep 2000 ASTR103, GMU, Dr. Correll 1

ASTR 103--Week 3ASTR 103--Week 3


Light and TelescopesLight and Telescopes

• The Nature of Light

• Optics and Telescopes

• Radio Astronomy--and Beyond


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


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


Light--Waves or Particles?Light--Waves or Particles?

• Newton believed light a particle phenomenon• Thomas Young shows light behaves like waves

(1801)


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


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


Electromagnetic SpectrumElectromagnetic Spectrum

High energy photons

Low energy photons

nm--nanometer--equal to 10-9 meters


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


Atmospheric WindowsAtmospheric Windows

• Most electromagnetic energy absorbed by the atmosphere, but radio waves and visible light pass through







Astronomical TelescopesAstronomical Telescopes

• Two basic types:– Refracting--

through a glass lens

– Reflecting--from a mirror


RefractionRefraction

• Refraction--light bends, or changes direction, when crossing an interface between different media, for example, air and glass


LensesLenses

• Thus a lens collects light from a distant object and concentrates it at a focused image– every lens has a

characteristic focal length


Refracting TelescopeRefracting Telescope

• Essential elements of a refracting telescope


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


Resolution Reveals DetailsResolution Reveals Details


Complications with RefractorsComplications with Refractors

• Chromatic Aberration--different colors of light refract at different angles and thus have different focal points


Complications with RefractorsComplications with Refractors

• Spherical Aberration--best shape for lens surface is parabolic; this is difficult to make


Reflecting TelescopesReflecting Telescopes

• Reflecting mirrors do not suffer chromatic aberration since all colors reflect at the same angles

• Spherical aberrations must still be corrected



• Some standard configurations



• Typical home telescope


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


Atmospheric distortionAtmospheric distortion

Ground Ground/Adaptive HST


Image RecordingImage Recording

• Sketches• Film• Electronic Data

CCD







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



• 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




Other wavelengthsOther wavelengths

• Infrared from high mountain tops, rockets, or satellites

• UV, X-ray and gamma rays from satellites


Different perspectivesDifferent perspectives

• Comparing information from different spectra gives a better understanding of what’s going on


Questions for studyQuestions for study

• How are radio telescopes similar and different from visible light telescopes?

Documents

14 Sep 2000ASTR103, GMU, Dr. Correll1 ASTR 103--Week 3