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

14 Sep 2000ASTR103, GMU, Dr. Correll2 Light and Telescopes The Nature of Light Optics and Telescopes Radio Astronomy--and Beyond

14 Sep 2000ASTR103, GMU, Dr. Correll3 The 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

14 Sep 2000ASTR103, GMU, Dr. Correll4 The 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

14 Sep 2000ASTR103, GMU, Dr. Correll5 Light--Waves or Particles? Newton believed light a particle phenomenon Thomas Young shows light behaves like waves (1801)

14 Sep 2000ASTR103, GMU, Dr. Correll6 Electromagnetic Waves Waves are described by –wavelength, (lambda) –frequency, (nu) –speed, v = –and they transport energy Electromagnetic waves result from the motion of electrons

14 Sep 2000ASTR103, GMU, Dr. Correll7 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 e-e- h

14 Sep 2000ASTR103, GMU, Dr. Correll8 Electromagnetic Spectrum High energy photons Low energy photons nm--nanometer--equal to meters

14 Sep 2000ASTR103, GMU, Dr. Correll9 Electromagnetic 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

14 Sep 2000ASTR103, GMU, Dr. Correll10 Atmospheric Windows Most electromagnetic energy absorbed by the atmosphere, but radio waves and visible light pass through

14 Sep 2000ASTR103, GMU, Dr. Correll11 Light and Telescopes The Nature of Light Optics and Telescopes Radio Astronomy--and Beyond

14 Sep 2000ASTR103, GMU, Dr. Correll12 Astronomical Telescopes Two basic types: –Refracting-- through a glass lens –Reflecting--from a mirror

14 Sep 2000ASTR103, GMU, Dr. Correll13 Refraction Refraction--light bends, or changes direction, when crossing an interface between different media, for example, air and glass

14 Sep 2000ASTR103, GMU, Dr. Correll14 Lenses Thus a lens collects light from a distant object and concentrates it at a focused image –every lens has a characteristic focal length

14 Sep 2000ASTR103, GMU, Dr. Correll15 Refracting Telescope Essential elements of a refracting telescope

14 Sep 2000ASTR103, GMU, Dr. Correll16 Telescope 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

14 Sep 2000ASTR103, GMU, Dr. Correll17 Resolution Reveals Details

14 Sep 2000ASTR103, GMU, Dr. Correll18 Complications with Refractors Chromatic Aberration-- different colors of light refract at different angles and thus have different focal points

14 Sep 2000ASTR103, GMU, Dr. Correll19 Complications with Refractors Spherical Aberration--best shape for lens surface is parabolic; this is difficult to make

14 Sep 2000ASTR103, GMU, Dr. Correll20 Reflecting Telescopes Reflecting mirrors do not suffer chromatic aberration since all colors reflect at the same angles Spherical aberrations must still be corrected

14 Sep 2000ASTR103, GMU, Dr. Correll21 Reflecting Telescopes Some standard configurations

14 Sep 2000ASTR103, GMU, Dr. Correll22 Reflecting Telescopes Typical home telescope

14 Sep 2000ASTR103, GMU, Dr. Correll23 Atmospheric 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

14 Sep 2000ASTR103, GMU, Dr. Correll24 Atmospheric distortion GroundGround/AdaptiveHST

14 Sep 2000ASTR103, GMU, Dr. Correll25 Image Recording Sketches Film Electronic Data CCD

14 Sep 2000ASTR103, GMU, Dr. Correll26 Light and Telescopes The Nature of Light Optics and Telescopes Radio Astronomy--and Beyond

14 Sep 2000ASTR103, GMU, Dr. Correll27 Radio 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

14 Sep 2000ASTR103, GMU, Dr. Correll28 Radio 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

14 Sep 2000ASTR103, GMU, Dr. Correll29 Radio Telescopes

14 Sep 2000ASTR103, GMU, Dr. Correll30 Other wavelengths Infrared from high mountain tops, rockets, or satellites UV, X-ray and gamma rays from satellites

14 Sep 2000ASTR103, GMU, Dr. Correll31 Different perspectives Comparing information from different spectra gives a better understanding of what’s going on

14 Sep 2000ASTR103, GMU, Dr. Correll32 Questions for study How are radio telescopes similar and different from visible light telescopes?