TELESCOPIC ASTRONOMY

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TELESCOPIC ASTRONOMY
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First Telescope

• 1608- Hans Lippershey. Dutch lens grinder.
• 1609- Galileo. Built his own telescope, used it
  for scientific study.
• Mountains and valleys on Moon
• Moons of Jupiter
• Phases of Venus
• Saturns rings
• Sunspots

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Galileos Telescope
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Powers of a Telescope

• Light-gathering power-
• This is the ability of a telescope to collect
  light.

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Powers of a Telescope

• Resolving Power
• the ability of a telescope to reveal fine
• detail.

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• When light is focused into an image, a blurred
  fringe surrounds the image (diffraction fringe).
• We can never see any detail smaller than the
  fringe.
• Large diameter telescopes have small fringes and
  we can see smaller details. Therefore the larger
  the telescope, the better its resolving power.
• Optical quality and atmospheric conditions limit
  the detail we can see.

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Powers of a Telescope

• Magnifying power
• the ability to make the image
• bigger

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• Magnification of a telescope can be changed by
  changing the eyepiece. We cannot alter the
  telescopes light-gathering Or resolving power.
• Astronomers identify telescopes by diameter
  because that determines both light-gathering
  power and resolving power.

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Optical Telescopes

• Refracting telescope uses a large lens to gather
  and focus light.
• Reflecting telescope uses a large mirror

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Focal length the distance from the lens or
mirror to the image formed of a distant light
source
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• Primary lens
• the main lens in a refracting
• telescope. It is also called an
• objective
• lens.

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• Primary mirror
• the main mirror in a
• reflecting
• telescope. It is also called an
• objective
• mirror.

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• Eyepiece
• A small lens to magnify the image produced by the
• objective (primary)
• lens

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Chromatic Aberration

• When light is refracted through glass, shorter
  wavelengths bend more than longer wavelengths,
  and blue light comes to a focus closer to the
  lens than does red light.

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Chromatic Aberration

• If we focus on the blue image, the red image is
  out of focus and we see a red blur around the
  image. This color separation is called chromatic
  aberration.

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Achromatic Lens

• An achromatic lens is made of two components made
  of different kinds of glass and brings the two
  different wavelengths to the same focus. Other
  wavelengths are still out of focus.

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Yerkes Refracting Telescope

• Largest refracting telescope in the world is at
  Yerkes Observatory in Wisconsin
• Lens is 1m in diameter
• ½ tonne
• The glass sags under its own weight

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Newtons Reflecting Telescope

• 1666- Newton found that a prism breaks up white
  light into a rainbow of colours
• Telescope lenses do the same
• 1663- James Gregory designed a telescope with a
  large concave primary mirror and a smaller
  concave secondary mirror
• 1672- Newton modified the design, and it won huge
  acclaim

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Benefits of Reflecting Telescopes

• Less expensive. Only the front surface of the
  mirror must be ground.
• The glass doesnt need to be perfectly
  Transparent
• The mirror can be supported over its back surface
  to reduce sagging.
• They do not suffer from chromatic aberration
  because the light is reflected toward the focus
  before it can enter the glass.

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Four ways to look through reflecting telescopes
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Prime Focus
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Newtonian Focus
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Cassegrain Focus
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Schmidt-Cassegrain
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Hershels Telescope

• Late 1770s, William Hershel was making the best
  metallic mirrors and telescopes in the world.
• 1781- Discovered Uranus
• 1789 - Built a giant telescope which he used with
  his sister Caroline
• His telescope had a 125cm mirror
• 40ft in length

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Rosses Telescope

• 1838- Earl of Rosse, Ireland, taught himself
  mirror-making and built a 91cm telescope
• 1842- attempted to build a 181cm telescope but it
  broke when moved
• built another one that couldnt be moved

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Observatories

• 1874- 91cm telescope and observatory at
  University of California (James Lick)
• 1880- 76cm telescope in France
• 1897- 102cm telescope at Yerkes Observatory in
  Wisconsin
• 1908- 153cm telescope on Mount Wilson California
  (George Ellery Hale)
• 1917- 254cm telescope also built on Mount Wilson
  (John D. Hooker)
• 1948- 500cm mirror. Hale Observatory, Mount
  Palomar, California.

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Observatories are built on top of mountains
because

• air is thin and more transparent
• the sky is darker
• stars are brighter
• 4) wind blows smoothly over some mountaintops
• there is less pollution

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New Generation Telescopes
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Keck Telescope

• 1993 Keck telescope 1000cm mirror, made of
  smaller segments
• Photographic plates were more sensitive and
  permitted a permanent record of observations
• Photographic plates have since been replaced by
  electronic imaging devices

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A large mirror sags in the middle. To prevent
this

• Mirrors can be made very thick but they are very
  heavy and very costly.

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A large mirror sags in the middle. To prevent
this

• Spincasting an oven turns and molten glass
  flows outward in a mold to form a concave upper
  surface.

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A large mirror sags in the middle. To prevent
this

• A mirror can be made in
• segments.

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A large mirror sags in the middle. To prevent
this

• Thin mirrors (floppy mirrors) can have their
  shape controlled by a computer called active
  optics. They cool quickly to adjust to
  surrounding temperatures.

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Radio Telescopes

• Objects in space emit light waves of many
  different wavelengths.
• Radio Telescopes receive very long wavelengths
  (radio waves).
• 1937 first Radio telescope picks up long wave
  radio emissions from deep space

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Handicaps to Radio Telescopes

• Poor resolution
• To improve resolution, two or more radio
  telescopes can be combined to improve the
  resolving power (called a radio interferometer).
  Resolving power equals the separation of the
  telescopes.

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Handicaps to Radio Telescopes

• Low intensity
• In order to get strong signals focused on the
  antenna, the radio astronomer must build large
  collecting dishes. The largest dish is the 300 m
  dish at Arecibo, Puerto Rico.

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Handicaps to Radio Telescopes

• Interference
• This occurs because of poorly designed
  transmitters in Earth satellites to automobiles
  with faulty ignition systems.

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Space Telescopes

• Hubble
• Spitzer
• Kepler
• Webb
• Chandra
• Hershel
• Planck
• Fermi
• XMM-Newton