Discovering the Universe

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Neil F. Comins William J. Kaufmann III
Discovering the Universe Eighth Edition
CHAPTER 3 Light and Telescopes
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WHAT DO YOU THINK?

• What is visible light?
• A wave? A particle? Energy?
• What is electromagnetic radiation?
• What are the main functions of telescopes?
• Why do all research telescopes use mirrors,
  rather than lenses, to collect light?
• Why do stars twinkle?

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In this chapter you will discover

• The connection between visible light, X-rays,
  radio waves, other types of electromagnetic
  radiation
• The debate over what light is and how Einstein
  resolved it
• How telescopes collect and focus light
• Why different types of telescopes are used for
  different types of research

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In this chapter you will discover

• The limitations of telescopes, especially those
  that use lenses to collect light
• What the new generations of land-based and
  space-based high-technology telescopes being
  developed can do
• How astronomers use the entire spectrum of
  electromagnetic radiation to observe the stars
  and other astronomical objects and events

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Color is property of light!
White light is composed of different colors when
shone through glass.
but the glass is not creating those colors!
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Observation Water Waves naturally interfere
create noticeable patterns
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Youngs Experiment (1801)
Light has a wavelike property, too!
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Observations of Nature

• Electricity acts through space over a distance
• Lightening, sparks on a doorknob
• Magnetism acts through space over a distance
• Two magnets attract or repel one another without
  touching

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More Observations

• If you spin a conductor in a magnetic field, you
  get electricity!
• Electric Generators _at_ dams windmills
• Portable gas generators
• If you run electricity into a coil, you get a
  magnet!
• Electromagnetic cranes
• Auto solenoids
• Electric Motors

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Maxwells Observation

• Change Electricity gt create magnetism
• Change Magnetism gt create electricity
• Continuously change both, continuously create
  radiation!
• Radiation created moves at c the speed of
  light!

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Electro-magnetic radiation!
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Roemers observations of Jupiters Moons
Eclipses demonstrated light moves at a finite
speed
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Non-visible light (beyond the red end of the
spectrum) has energy, too!
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The entire EM Spectrum
What we see is only a small part of what there
is!
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EM Spectrum Varies by
Size (wavelength, color) Energy How the waves are
detected But not. How fast they move through
space!
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Atmospheric Windows to the stars universe
Visible Radio light
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Different types of Reflecting Telescopes
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Small Telescope image of Andromeda Galaxy
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Photographs vs. CCD chips vs. Multi-color
filtered CCD composite images
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Refracting Telescopes bend light through lenses
Heavy glass lenses, bending different colors to
different points (Chromatic aberration)
imperfections in glass, limit practical size
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Functions of Telescopes!

1. Gather Light
2. Resolve Sharp Details
3. Magnify Resulting Images

Regardless of Wavelength range size
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Orion in UV, Infrared, Optical Wavelengths
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1 Function Gathering Light

• Depends upon the size of the objective mirror or
  lens.
• Light gathering area increases with SQUARE of the
  diameter
• 10 m telescope gather 4x more light than 5m
• Subject to interference from other sources!

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2 Function Resolution

• Depends upon the size of the objective mirror or
  lens.
• Better resolution with more light
• Depends upon wavelength of light, too!
• Smaller wavelengths provide smaller details
• UV images have more detail than Radio
• Also subject to interference

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Radio Telescopes gather long-wave, low-energy
light Poor resolution unless made LARGE!
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• Seeing is the ability to resolve small details
• Affected by
• Imperfections in optics (shapes of
  lenses/mirrors)
• Atmospheric motion, density, temperature,
  moisture
• Improved by
• Adaptive optics subtracting out the atmospheric
  effects
• Getting above atmosphere!

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Improve seeing by getting above the atmosphere
(and gather more types of light, too!)
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1
2
3

1. Ground-based image of Neptune
2. Ground-based image with adaptive optics
3. Hubble Space Telescope image

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3 Function Magnification

• Least important
• Without a bright, sharp image, no use!
• Bigger, Dimmer, Fuzzier!
• Depends upon EYEPIECE used
• Small scopes 50-500 each
• Easily swapped to magnify images
• Depends upon telescope geometry, too

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Active Adaptive Optics!

• Active optics (1980s)
• Put actuators on segmented mirrors to bend
  them to the right shape
• Keck, NTT, VLT Telescopes
• Adaptive optics (1990s to present)
• Deform mirror in real time to compensate for
  atmospheric motion
• Laser Guide Stars

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VLT in Chile

• (4) combined 8.2 m telescopes
• Tracking motions of stars at Milky Way Center

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SALT in Africa

• Largest current single surface scope

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Next Generation Space Telescope

• NASAs next great observatory
• Bigger than Hubble

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Seeing in Stereo!
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Interferometry Combining signals simultaneously
from 2 or more scopes
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Visible Radio wave views of Saturn
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Why build telescopes at all?
We already have enough! Why do we need a more
detailed picture of Mars? Who cares? This cost
100 Million dollars? Youve got to be kidding
me
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Summary The Nature Of Light

• Photons, units of vibrating electric and magnetic
  fields, all carry energy through space at the
  same speed, the speed of light (300,000 km/s in a
  vacuum, slower in any medium).
• Radio waves, microwaves, infrared radiation,
  visible light, ultraviolet radiation, X rays, and
  gamma rays are the forms of electromagnetic
  radiation. They travel as photons, sometimes
  behaving as particles, sometimes as waves.

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The Nature Of Light

• Visible light occupies only a small portion of
  the electromagnetic spectrum.
• The wavelength of a visible light photon is
  associated with its color. Wavelengths of visible
  light range from about 400 nm for violet light to
  700 nm for red light.
• Infrared radiation and radio waves have
  wavelengths longer than those of visible light.
  Ultraviolet radiation, X rays, and gamma rays
  have wavelengths that are shorter.

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Optics and Telescopes

• A telescopes most important function is to
  gather as much light as possible. Its second
  function is to reveal the observed object in as
  much detail as possible. Often the least
  important function of a telescope is to magnify
  objects.
• Reflecting telescopes, or reflectors, produce
  images by reflecting light rays from concave
  mirrors to a focal point or focal plane.

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Optics and Telescopes

• Refracting telescopes, or refractors, produce
  images by bending light rays as they pass through
  glass lenses. Glass impurity, opacity to certain
  wavelengths, and structural difficulties make it
  inadvisable to build extremely large refractors.
  
• Reflectors are not subject to the problems that
  limit the usefulness of refractors.
• Earth-based telescopes are being built with
  active and adaptive optics. These advanced
  technologies yield resolving power comparable to
  the Hubble Space Telescope.

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Nonoptical Astronomy

• Radio telescopes have large, reflecting antennas
  (dishes) that are used to focus radio waves.
• Very sharp radio images are produced with arrays
  of radio telescopes linked together in a
  technique called interferometry.
• Earths atmosphere is fairly transparent to most
  visible light and radio waves, along with some
  infrared and ultraviolet radiation arriving from
  space, but it absorbs much of the electromagnetic
  radiation at other wavelengths.

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Nonoptical Astronomy

• For observations at other wavelengths,
  astronomers mostly depend upon telescopes carried
  above the atmosphere by rockets. Satellite-based
  observatories are giving us a wealth of new
  information about the universe and permitting
  coordinated observation of the sky at all
  wavelengths.
• Charge-coupled devices (CCDs) record images on
  many telescopes used between infrared and X-ray
  wavelengths.

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Key Terms
infrared radiation interferometry light-gathering
power magnification Newtonian reflector objective
lens photon pixel primary mirror prime
focus radio telescope radio wave reflecting
telescope reflection
refracting telescope Schmidt corrector
plate secondary mirror seeing disk spectrum spheri
cal aberration twinkling ultraviolet
(UV) radiation very-long-baseline interferometry
(VLBI) wavelength X ray
active optics adaptive optics angular
resolution Cassegrain focus charge-coupled
device coudé focus electromagnetic
radiation electromagnetic spectrum eyepiece
lens focal length focal plane focal
point frequency gamma ray
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WHAT DID YOU THINK?

• What is light?
• Lightmore properly visible light, is one form
  of electromagnetic radiation. All electromagnetic
  radiation (radio waves, microwaves, infrared
  radiation, visible light, ultraviolet radiation,
  X rays, and gamma rays) has both wave and
  particle properties.

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WHAT DID YOU THINK?

• What type of electromagnetic radiation is most
  dangerous to life?
• Gamma rays have the highest energies of all
  photons, so they are the most dangerous to life.
  However, ultraviolet radiation from the Sun is
  the most common everyday form of dangerous
  electromagnetic radiation that we encounter.

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WHAT DID YOU THINK?

• What is the main purpose of a telescope?
• A telescope is designed primarily to collect as
  much light as possible.

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WHAT DID YOU THINK?

• Why do all research telescopes use mirrors,
  rather than lenses, to collect light?
• Telescopes that use lenses have more problems,
  such as chromatic aberration, internal defects,
  complex shapes, and distortion from sagging, than
  do telescopes that use mirrors.

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WHAT DID YOU THINK?

• Why do stars twinkle?
• Rapid changes in the density of Earths
  atmosphere cause passing starlight to change
  direction, making stars appear to twinkle.