Telescopes

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Telescopes
Chapter Four
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• Astronomers, like other scientists, rely on
  observations to study their subjects
• But, astronomers, cannot directly probe the
  objects they study because of the vast distances
  between them and Earth
• In order to study these celestial objects,
  astronomers can only passively collect radiation
  emitted by these bodies

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• The problems lies within the fact that many
  objects are so far away that they radiation Earth
  receives from them is extremely faint
• To solve this problem, instruments are use to
  measure the brightness, the spectrum, and the
  position of the objects to high precision

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• To collect enough light to detect remote
  galaxies, astronomers use telescopes with mirrors
  to size of swimming pools
• To avoid the blurring and blocking effects of out
  atmosphere, astronomers use orbiting
  observatories
• Observatories in out space
• Computers are used to analyze and display
  observations

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4.1

• Our eyes cannot see extremely faint objects or
  fine details at great distances
• Telescopes help by collecting more light for our
  eye and by increasing the detail discernible

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• Collecting Power
• The ability to collect photons (light)
• Our eye is only so big, so it can only collect a
  limited number of photons
• Astronomer over come this limit by collecting
  photons with a telescope, which then funnels the
  photons to our eye

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• The bigger the telescopes collecting area, the
  more photons it gathers, leading to brighter
  images
• So we can see dim objects that are invisible to
  telescopes with small collecting areas

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• Because the collecting area of a circular
  collector of radius r is pr2, a small increase in
  the radius of the collecting area gives a large
  increase in the number of photons caught
• So if you double the radius of the mirror or
  lens, the collecting area increases by a factor
  of 4
• 22 4

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• Once the light is collected, it then needs to be
  focused into an image or concentrated on a
  detector
• Telescopes in which light is collected and
  focused by a lens are called refracting
  telescopes

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• Refraction is the bending of light when it passes
  through on substance and enters another
• Refraction of light occurs when light enters a
  material of different density
• Think of how a spoon in a glass of water appears
  to bend when it enters the water

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• Lenses and mirrors are used to collect photons of
  light
• Lenses have many disadvantages in large
  telescopes
• Large diameter lenses are expensive
• Lenses must be supported around the outside so as
  to not block the light passing through it, which
  can lead to them sagging in the middle,
  distorting the images
• Lenses can focus light at different spots,
  creating images fringes with color
• Lenses can absorb short-wavelength light, making
  them useless for certain purposes

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• To avoid such problems with lenses, many modern
  telescopes use mirrors to collect light
• These types of telescopes are called reflecting
  telescopes

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• Advantages of reflecting telescopes
• Curved mirrors can focus light rays, creating an
  image just as well as a lens
• Since light does not pass through a mirror like a
  lens, a mirror can be supported from behind,
  reducing the sagging problem
• Also, mirrors do not absorb short-wavelength
  light

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• With the mirror at the back of the telescope, the
  light reflected and focused to the front of the
  telescope, blocking incoming light
• So a second mirror is needed to redirect the
  light either out of the side or out of the back
  of the telescope

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• Because when moving these large telescopes the
  mirrors or lenses can shift easily causing
  distortions, instead of using one large mirror,
  some telescopes use many small mirrors acting as
  one, which are less likely to shift

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• Resolving Power
• A telescopes ability to discern fine details
• Resolving power is limited by the wave nature of
  light
• If two stars are close together, for them to be
  discernible as separate images, their light waves
  must not get mixed up

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• Diffraction
• A disturbance of light as it passes though an
  opening or around an obstacle
• Light waves are diffracted as they enter a
  telescope
• Creating rings of light surrounding the source of
  light, limiting the visible detail
• So to solve this problem, you need a large enough
  telescope

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4.2

• Interferometer is a device consisting of two or
  more telescopes connected together to work as a
  single instrument
• Used to obtain a high resolving power, the
  ability to see small-scale features
• Interferometers may operate at radio, infrared,
  or visible wavelengths
• So if two mirrors are 100 meters apart, the
  interferometer has the same resolving power as a
  telescope 100 meters in diameter

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4.3

• Observatories
• Twin 10-meter Keck telescopes largest optical
  telescopes in the U.S. (in Hawaii)
• VLT (very large telescope) worlds largest
  optical telescope (in Chile)
• OWL (overwhelming large telescope) plans for a
  telescope with a collecting area that spans a
  football field

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4.4

• Detecting the Light
• Long ago, the detector was the eye of an
  astronomer who then wrote down the data and made
  sketches of objects being observed
• Before the 1980s, photographic film was used to
  record the light, but this took long periods of
  time
• Today, astronomers use electronic detectors,
  similar to those in camcorders, through a process
  that takes much less time than film

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4.5

• Observing at Nonvisible Wavelengths
• Remember visible light is only one of the many
  wave bands of electromagnetic radiation
• Many astronomical objects, like cold clouds of
  gas, do not radiate visible light, but do radiate
  large amounts of other forms of light, like radio
  energy
• Radio, infrared, and X-ray telescopes have the
  same fundamental purposes as optical telescopes
  to collect radiation and to resolve details

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• False-color pictures
• Used to depict observations made at nonvisible
  wavelengths
• The colors used represent the intensity of
  radiation that we cannot see

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• Gases in our atmosphere such as ozone, carbon
  dioxide, and water vapor strongly absorb
  infrared, ultraviolet, and shorter wavelengths
• Some infrared radiation can penetrate the
  atmosphere through regions called atmospheric
  windows

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4.6

• Observatories in Space
• Since our atmosphere absorbs much of the
  infrared, ultraviolet, and X-rays given off by
  objects, many telescopes are placed in orbit
  around Earth above the atmosphere
• The blurring of the atmosphere creates
  scintillation, or the twinkling of stars
• Slight variations in the atmospheres temperature
  can cause the path of the light from the star to
  change slightly

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• Space Observatories vs Ground-Based Observatories
• Ground-based telescopes will remain larger than
  orbiting telescopes
• Space telescopes can be expensive to build, put
  into place, and maintain
• Astronomers must find places free of light
  pollution for ground-based telescopes
• Used for communication, microwave ovens, and
  search for extraterrestrials