TELESCOPIC ASTRONOMY

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TELESCOPIC ASTRONOMY
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What is a telescope?

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What are telescopes used for?

• View distant objects
• Collect light

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First Telescope

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

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Galileos Telescope
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Optical Telescopes

• Optical telescopes use a lens and light.
• One type Refracting telescope
• uses two large
• lenses
• to gather and focus light.

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• Primary lens
• the main lens in a refracting
• telescope. It is also called an
• objective
• lens.

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

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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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Pros

• Simple design
• Minimal maintenance
• Good for planets and moons
• Good for photography
• High contrast

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Cons

• Costly
• Bulky and large (large focal length)
• Chromatic aberration

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Refraction Limitation

• 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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Refraction Limitation

• 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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• 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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Do we still use these types of telescopes??
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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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Reflecting Telescope

• 1666- Newton found that a prism breaks up white
  light into a rainbow of colours
• Telescope lenses do the same
• Creates haloes of coloured light around objects
  being viewed

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Newtons Telescope
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• Primary mirror
• the main mirror in a
• reflecting telescope. It is larger than the
  secondary mirror. It is also called an objective
  mirror.
• NOTE The primary mirror is in a different spot
  than the primary lens of a refracting telescope.

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• Secondary mirror
• the smaller mirror in a
• reflecting
• telescope. It directs the light through a small
  hole in the primary mirror to the eyepiece.

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Benefits of Reflecting Telescopes

• Less expensive. Only the front surface of the
  mirror must be ground.
• The mirror can be supported 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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Cons of Reflecting Telescopes

• More maintenance

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William Hershel (1757-1822)

• By late 1770s, he was making the best metallic
  mirrors and telescopes in the world.
• 1781- Discovered Uranus
• His telescope had a 125cm mirror and was 40ft in
  length

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Hershels Telescope
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Large Telescopes

• Up until mid-1800s telescopes were generally
  small and not very powerful
• 1838- Earl of Rosse, Ireland, taught himself art
  of 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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Rosses Telescope
Rosse is important because he built the largest
telescope the world had ever seen and he saw
further into space than anyone had before him.
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Cassegrain Telescope

• Specific type of reflecting telescope
• a wide-angle reflecting telescope
• the eyepiece or camera is mounted at the back end
  of the tube
• developed in 1672

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Schmidt-Cassegrain Telescope

• Specific type of reflecting telescope
• a wide-angle reflecting telescope (Cassegrain
  telescope) with a correcting lens that minimizes
  spherical aberration
• The correcting plate (a lens) was added in 1930
  by the Estonian astronomer and lens-maker Bernard
  Schmidt (1879-1935).

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New Generation Telescopes

• Refraction and Reflection telescopes are OPTICAL
  telescopes (uses light)
• New generation telescopes are RADIO telescopes
  (uses waves)
• Radio telescopes use dishes to pick up radio
  waves. Sensors on the dishes collect the waves
  and turn them into a picture.

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Radio Telescopes
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New Generation Telescopes

• 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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Keck Telescope
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Handicaps to Radio Telescopes

• 1) Poor resolution To improve resolution, two
  or more radio telescopes can be combined to
  improve the resolving power

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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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Powers of a Telescope

• Light-gathering power-
• This is the ability of a telescope to collect
• light.
• The larger the telescopes lens, the more light
  it can gather.

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Powers of a Telescope

• Resolving Power
• the ability of a telescope to reveal fine detail.
  
• The larger the telescope, the better its
  resolving power. However, optical quality of the
  lens 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.
• Magnification of a telescope can be changed by
  changing the eyepiece.

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• Astronomers identify telescopes by diameter
  because that determines both light-gathering
  power and resolving power.

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Telescopes and Observatories

• An observatory is a place for viewing the sky
  through telescopes.
• Observatories can be small with just a single
  telescope outside or they can be large buildings
  housing a number of telescopes, each in their own
  room.

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Hale Observatory, California
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• The traditional image of an observatory is
  probably that of a large building with domed
  rooms housing telescopes. But an observatory can
  be just a single room with a telescope, or
  nothing more than an open area with telescopes.

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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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Telescopes and the Scientific Method
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The Scientific Method
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The Scientific Methods 4 Steps

• Observation and description of a phenomenon.
• The observations are made visually or with the
  aid of scientific equipment (like a telescope).
• 2) Formulation of a hypothesis to explain the
  phenomenon (usually in the form of math or a
  causal mechanism or a mathematical relation.
• Test the hypothesis by analyzing the results of
  observations or by predicting and observing the
  existence of new phenomena that follow from the
  hypothesis. If experiments do not confirm the
  hypothesis, the hypothesis must be rejected or
  modified (Go back to Step 2).
• Establish a theory based on repeated verification
  of the results.