An Introduction to Astronomy Part III: Light and Telescopes

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An Introduction to AstronomyPart III Light
and Telescopes

• Lambert E. Murray, Ph.D.
• Professor of Physics

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The Electromagnetic Spectrum

• Visible light is only a small region of the
  electromagnetic spectrum.

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Types of Spectra

• Continuous Spectra
• This blackbody spectrum arises from the heating
  of an object.
• The temperature of the object determines its
  color.
• Emission Line Spectra
• This arises when electrons loose energy and emit
  radiation at wavelengths that are specific to the
  chemical makeup of the substance.
• Absorption Line Spectra
• This is the opposite of emission spectra - it
  arises when electrons gain energy and absorb
  radiation at wavelengths that are specific to the
  chemical makeup of the substance.

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Color Temperature and Stars
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Color Spectra
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Types of Spectra
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Hydrogen Absorption Spectra
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Atomic Structure

• Atoms consist of a very small, heavy nucleus made
  up of protons and neutrons surrounded by a
  cloud or electrons.
• Atoms are not charged they have the same number
  of protons as electrons.
• The chemical nature of atoms is determined by the
  number of protons.
• A neutron is approximately the same size as a
  proton, but has no charge.
• Atoms with the same number of protons, but
  different numbers of neutrons are called
  isotopes.
• Some isotopes are radioactive.

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Basic Atomic Structure
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Absorption and Emission
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Excitation and Ionization
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Spectroscopic Notation of Ions

• OI is the designation of neutral oxygen
• OII means one electron has been removed due to
  ionization
• OIII means two electrons have been removed by
  ionization
• In a hot gas we may find highly ionized elements,
  such as Fe XIV.
• Each ionic species has its own unique spectrum.

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Radioactive Isotopes

• Many elements have different isotopes
• Each isotope has the same chemical properties,
  but different numbers of neutrons in the nucleus.
• Some isotopes are unstable (radioactive).
• Radioactive isotopes change from one elementary
  species to another according to a radioactive
  decay rate characteristic of the parent isotope.

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Radioactive Decay
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Color Spectra
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Color and Temperature
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Color Temperature and Weins Law
Weins Law states that the wavelength at which
most of the light energy is emitted is given by
the equation lmax1/T
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Spectral Regions and Temperature
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The Stephan-Boltzmann Law
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The Inverse-Square Law
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Scattering of Blue Light by Dust
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The Doppler Shift

• This is the shift of wavelength and frequency as
  the source and receiver move toward or away from
  each other.
• A red-shift occurs when the source and receiver
  are moving away from each other.
• A blue-shift occurs when the source and receiver
  are moving toward each other.

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Using the Doppler Shift
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Possible Emission Spectrum of a Star
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Proper Motion and Radial Motion
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Radiation from the Sun

• The total amount of energy incident upon the
  earths outer atmosphere is called the solar
  constant 0.139 watt/sq-cm (about 10 inches
  square would give 100 watts). This is the total
  intensity for all wavelengths combined.
• As we have already pointed out, solar radiation
  in certain wavelengths is greater than in others
  and depends upon the surface temperature of the
  Sun.

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Solar Spectrum
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Optical Telescopes

• There are two principle types of optical
  telescopes
• Refractors
• Reflectors
• Refracting telescopes use lenses to bend and
  focus the light.
• Reflecting telescopes use mirrors to reflect and
  focus the light.

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An Astronomical Refracting Telescope
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A Refracting Telescope
This is a picture of the worlds largest
refracting telescope, found in the University of
Chicagos Yerkes Observatory in Williams Bay,
Wisconsin.
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A Reflecting Telescope
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Principle Objectives of the Telescope

• A telescope has two principle objectives
• To gather as much light from a star, planet, or
  planetary satellite as possible so that the image
  appears as bright as possible.
• To produce a magnified image
• Magnification may make the object appear bigger
• Magnification increases the ability to resolve
  different features which are close together.
• Both the light gathering capacity of a telescope
  and its ability to resolve small features depend
  upon the diameter of the objective lens or mirror
  of the telescope.

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Magnification
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Increasing the size of the telescope increases
the brightness and the resolution of the image.
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Improved Resolution with and Increase in Primary
Mirror Size
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Principle Disadvantages of Refracting Telescopes

• The light must pass through the lenses, thus the
  size of the lenses are limited because of the way
  they must be supported (around the edge).
• If the lenses are too big, gravity will make them
  sag and distort the image.
• Long focal length lenses are thinner, so the
  largest diameter refracting telescopes have very
  long focal lengths.
• A single lens will bend different wavelengths
  (colors) by slightly different amounts, creating
  chromatic aberration. This can be corrected
  using special lenses but these are much more
  difficult to manufacture and must be thicker and
  heavier, so that quality achromatic lenses must
  be smaller in size to prevent them from sagging.

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Correcting Chromatic Aberration
Two different kinds of glass, with different
refractive indexes must be used, and the lens
curvatures must be properly matched.
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Principle Advantages of A Reflecting Telescope

• A concave mirror can be supported over its entire
  back surface dimension. Such a mirror can
  therefore be made much larger than the largest
  refracting telescopes. However, if the size of
  the mirror is too large, gravitational
  distortions will occur when the mirrors
  orientation is changed.
• Since light does not pass through a mirror,
  mirrors are not subject to the problem of
  chromatic aberrations.

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Principle Disadvantages of Reflecting Telescopes

• The focal point of a concave mirror is in front
  of the mirror in the light path. Thus, the
  entire area of the mirror cannot be utilized to
  capture light from the object observed.
• It sometimes creates difficulties to locate the
  observer at the focal point of the mirror.

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Types of Reflectors
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Schematic of the Hale Reflecting Telescope
An example of a prime-focus reflecting telescope.
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Newtonian Reflecting Telescope
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A Hybrid Telescope
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Spherical Mirrors Exhibit Spherical Aberration
Inexpensive reflectors have spherical mirrors,
and thus suffer from spherical aberration.
Spherical lenses also exhibit spherical
aberrations.
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Research-Grade Telescopes Use Parabolic Mirrors
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Atmospheric Influence on Astronomy

• The atmosphere greatly influences what can be
  studied from the Earths surface.
• The atmosphere is not transparent in certain
  regions of the spectrum.
• There are several windows to the heavens
• An optical window
• An infrared window
• A radio window
• Turbulence in the atmosphere causes stars to
  twinkle and reduces the resolution of the
  telescopes.
• Seeing refers to atmospheric turbulence.
• Light pollution limits our ability to see faint
  objects.
• To overcome this limitation, we send satellites
  beyond the earths atmosphere.

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Absorption by Gases in the Atmosphere
Oxygen and Ozone absorb strongly in the
ultraviolet, while Carbon Dioxide and Water Vapor
absorb strongly in the infrared.
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SaturninVisibleRadio
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The Hubble Space Telescope
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Hubble Images vs. Ground-Based Images
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Hubble Images
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Color Images
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Toward the Next Generation Telescopes

• One way to make larger reflecting telescopes is
  to make many smaller mirrors act as a single,
  much larger mirror.
• These hybrid reflectors have electronic
  adjustments for each individual mirror.
• New techniques are allowing us to change the
  position of each individual element of these
  multi-element telescopes by small amounts to
  counteract the effects of the atmosphere.

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Multiple Mirrors can be Utilized to Act as a
Single, Large Mirror
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visible
radio
X-ray
infrared
Gamma ray
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End of Part III