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• Honors section meets TOMOROW EVENING (Wed Feb 16 @5:30pm in 6515 Sterling Hall).  See http://wisp.physics.wisc.edu/astro104/honors.html for details

• In 1910 Ernst Rutherford beamed alpha particles at thin gold foil
• thin gold foil like tissue paper
• alpha particles like bullets
• Some alpha particles bounced back!
• Rutherford concluded that there must be very dense things is the foil somewhere
• The atomic nucleus
• Neils Bohr worked out a model of the Hydrogen atom shortly thereafter
• one proton at the center
• one electron orbiting in certain allowed orbits
• Quantum mechanics predicts the positions of these orbits
• Emission line: electron jumps to lower orbit
• Falling in gives up energy
• Absorption line: electron jumps to a higher orbit
• climbing out takes energy
• Analogous processes in other atoms and molecules

Model of atom

Model of electron orbits

Photon Absorption and Emission

Term diagrams for Hydrogen and Sodium

• When something moves toward you, radiation emited from it has an apparently shorter wavelength
• away, longer wavelength

• Dl = change in wavelength
• l = wavelength
• v = relative velocity (speed)
• c = speed of radiation (in this case, light)

• Scientific instruments extend the senses
• Telescopes, microscopes = sight
• Radio, infrared, optical, UV, X-ray, Gamma-ray
• microphones = sound
• thermometers, scales = touch...
• Telescopes work by:
• collecting light more efficiently than our eye
• magnification (distant objects look closer)
• Two basic types of telescopes:
• Refracting

• Refraction--how lenses and prisms work
• light slows down when it goes through matter
• why your arm looks bent in a swimming pool
• how magnifying glasses work
• A curved piece of glass forms a lens
• focus of lens is where rays of light meet
• an extended object forms an image at the focal plane
• a focal plane works for projection onto film but not for your eye
• refracting telescopes need at least two lenses, objective and eyepiece
• spacing between lenses has to be just right (why you need a focus adjustment)
• Magnification = FL_(obj) / FL_(eye)
• telescope gathers more light than your eye
• Eye pupil = 0.5 cm diameter
• Galileo's telescope = 3 cm
• refracting telescope on roof = 15 cm
• Washburn observatory = 38 cm
• Lick (CA) = 91 cm
• Yerkes (Lake Geneva WI) = 101 cm
• Collecting area = p(d/2)²
• eye = 0.2 cm²
• Yerkes = 8000 cm²

^{Refraction diagram
Image diagram}

^{Refracting telescope schematic
Yerkes telescope and Lens}

• Principle of reflection: angle of reflection equals angle of incidence
• angles measured relative to the perpendicular to the surface
• Newton found that curved mirrors cause light to focus
• Types of reflecting telescopes:
• a) Newtonian--popular
• b) prime focus--how big would the primary have to be for you to be at the prime focus?
• c) Cassegrain--popular
• d) Coude--for special instruments that are too heavy to move with the telescope
• Secondary mirrors don't make a hole in the image
• Magnification = FL(primary) / FL(eye)

Focusing reflector
Reflecting Telescope Schematics

Secondary doesn't make a hole in image

• Problems with refractors:
• large refracting lenses hard to make: glass has to be good all the way through
• refracting elements must be mounted from edges
• refracting telescopes have to be long, since short focal length refractors are hard to make.  The largest telescope dome is for the largest refractor, not the largest reflector
• chromatic aberration
• Problems with refractors:
• Spherical aberration--bad focus
• Coma--off center is comet or teardrop-shaped
• Largest telescopes are reflectors
• Yerkes 0.1 m (largest refractor) WI/Chicago
• WIYN  3.5 m Wisconsin's own in AZ
• Hale   5 m Mt Palomar, CA
• Keck I & II  10 m, Mauna Kea, HI

Spherical Aberration

• Brief history:
• Galileo used his eyes and a pen in the 1600s
• Astronomers in the early 1900s used photographic plates (film)
• Modern astromoners use CCDs (charge-coupled devices--think camcorder)
• Photographic plates and CCDs allow for long exposure times (dim things become bright!)
• The eye records photons for 0.04 s before sending the picture to your brain
• Modern telescopes record photons for 1800 s (1/2 hour) or more at a time
• CCDs record information directly to the computer
• images from a WHOLE NIGHT can be added together (get your brain to do that!)

• Recall that visible light is only a tiny portion of the electromagnetic spectrum
• Special telescopes are needed to observe at other wavelengths
• Radio--metal dishes
• Infrared and UV--optical telescopes with special detectors above the atmosphere
• X-ray--grazing incidence optics, CCDs, above the atmosphere
• Gamma-Rays--no optics, only special detectors

Chandra image (X-ray)

X-ray Mirrors

GRO (Gamma Ray Observatory)

• most of the information we know about the planets comes from spectrographs
• a spectrum is worth a thousand pictures

Grating Spectrograph

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