Telescopes

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Telescopes

• How do they work?
• Chapter 5

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1. History
2. Lenses Hardware
3. Reflecting Telescopes
4. Refracting Telescopes
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History

• Hans Lippershey Middleburg, Holland
• invented the refractor telescope in 1608
• Galileo
• Used a small 30X scope, was the first to use a
  telescope in astronomy. Galileo's designs used a
  combination of convex and concave lenses.
• Observed the moon, began the modern age of
  Astronomy where measurement more important than
  philosophy.

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Galileo noticed

• moons orbiting Jupiter
• phases of Venus
• craters on the moon
• sunspots

This was strong evidence that Copernicus was
right although Galileo wasnt willing to die for
it.
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History

• Kepler
• improved the design to have two convex lenses,
  which made the image upside-down. Kepler's design
  is still the major design of refractors today,
  with a few later improvements in the lenses and
  the glass to make them.

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Why cant you see an object that is far away?

• The answer is simple the object does not take up
  much space on your eyes screen (retina).
• For example, at 150 feet the writing on a dime
  does not cover enough pixels on your retinal
  sensor for you to read the writing.
• This can be corrected by bending the light with
  lenses.

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Lenses

• The lens in your eyes works like a glass lens.
  The light bends as it goes through a different
  medium.
• Light rays are bent when they intersect glass a
  curved surface can produce an image.
• In your eye, the image is then focused at the
  retina.

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How does this apply to telescopes?

• If you had a bigger eye, you could collect more
  light from the object. This image could be
  magnified so it stretches out over more pixels in
  your retina.
• In a telescope, two pieces make this possible
• the objective lens (refractor telescopes) or
  primary mirror (reflecting telescopes)
• the eye piece

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• The objective lens (in refractors) or primary
  mirror (in reflectors) collects lots of light
  from a distant object and brings that light, or
  image, to a point or focus.
• An eyepiece lens takes the bright light from the
  focus of the objective lens or primary mirror and
  "spreads it out" (magnifies it) to take up a
  large portion of the retina. This is the same
  principle that a magnifying glass (lens) uses it
  takes a small image on the paper and spreads it
  out over the retina of your eye so that it looks
  big.

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How they really work

• Scopes gather light through the objective (mirror
  or lens)
• bigger is better because it gathers more light
• The ability to see faint objects increases
  proportionally with the square of the radius of
  the objective
• They focuses light
• changing the eyepiece changes the magnification
• magnification is the ratio of the focal length of
  the objective to the focal length of the eyepiece

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Diagram of a simple telescope. Parallel light rays enter from the left, pass through the objective lens, come to a focus at the focal plane, and exit through the eyepiece lens. The focal length of the objective is F, and the focal length of the eyepiece is f. www.ifa.hawaii.edu
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• When you combine an objective lens or primary
  mirror with an eyepiece, you have a telescope.
• Again, the basic idea is to collect lots of light
  to form a bright image inside the telescope, and
  then use something like a magnifying glass to
  magnify (enlarge) that bright image so that it
  takes up a lot of space on your retina.

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A telescope has two general properties

• how well it can collect the light
• (the aperature)
• how much it can magnify the image
• (the magnification)

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The Aperture

• A telescope's ability to collect light is
  directly related to the diameter of the lens or
  mirror -- the aperture -- that is used to gather
  light. Generally, the larger the aperture, the
  more light the telescope collects and brings to
  focus, and the brighter the final image.

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Magnification

• The telescope's magnification, its ability to
  enlarge an image, depends on the combination of
  lenses used. The eyepiece performs the
  magnification. Since any magnification can be
  achieved by almost any telescope by using
  different eyepieces, aperture is a more important
  feature than magnification

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A closer look at eyepieces
                                                                                                                        View through an eyepiece. Note that the image is upside-down.
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Eyepiece

• The purposes of the eyepiece are to
• produce and allow you to change the telescope's
  magnification
• produce a sharp image
• provide comfortable eye relief (the distance
  between your eye and the eyepiece when the image
  is in focus)
• determine the telescope's field of view
• apparent - how much of the sky, in degrees, is
  seen edge-to-edge through the eyepiece alone
  (specified on the eyepiece)
• true or real - how much of the sky can be seen
  when that eyepiece is placed in the telescope
  (true field apparent field/magnification)

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Filters

• Filters are pieces of glass or plastic that you
  can place in the barrel of an eyepiece to
  restrict the wavelengths of light that come
  through in the image.
• Set of filters for viewing, including a light
  pollution filter (left) and colored filters for
  enhancing contrast in planetary images.
• Filters can be used to
• enhance the viewing of faint sky objects in
  light-polluted skies
• enhance the contrast of fine features and details
  on the moon and planets
• safely view the sun

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There are 2 main types of Telescopes

• Refractor telescopes, which use glass lenses
• Reflector telescopes, which use mirrors instead
  of lenses.
• Both types accomplish exactly the same thing, but
  in completely different ways.

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Refractor Telescopes

• Refractors are the type of telescope that most of
  us are familiar with. They have the following
  parts
• a long tube, made of metal, plastic, or wood
• a glass combination lens at the front end
  (objective lens)
• a second glass combination lens (eyepiece)
• Refracting telescopes focus light rays by bending
  them with glass.

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This is the simplest telescope design you could
have. A big lens gathers the light and directs it
to a focal point and a small lens brings the
image to your eye.
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Advantages and Disadvantages

• Easy to use and reliable
• Excellent for lunar, planetary and binary star
  observing especially in larger apertures.
• High contrast images with no secondary mirror or
  diagonal obstruction.
• Sealed optical tube reduces image degrading air
  currents and protects optics.

• More expensive per inch of aperture
• Heavier, longer and bulkier than equivalent
  aperture Newtonians and catadioptrics.
• Small apertures
• Less suited for viewing small and faint deep sky
  objects.
• Color aberration due to colors of light bending
  different amounts.

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Refracting telescopes are not used for
astronomical research (anymore) because they are
large and have heavy lenses (i.e. expensive).
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Reflecting Telescopes

• History
• Isaac Newton developed the reflector about 1680,
  in response to the chromatic aberration (rainbow
  halo) problem that plagued refractors during his
  time. Instead of using a lens to gather light,
  Newton used a curved, metal mirror (primary
  mirror) to collect the light and reflect it to a
  focus. Because the mirror reflected light back
  into the tube, he had to use a small, flat mirror
  (secondary mirror) in the focal path of the
  primary mirror to deflect the image out through
  the side of the tube, to the eyepiece otherwise,
  his head would get in the way of incoming light.
• In 1722, John Hadley developed a design that used
  parabolic mirrors, and there were various
  improvements in mirror-making. The Newtonian
  reflector was a highly successful design, and
  remains one of the most popular telescope designs
  in use today.

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Advantages and disadvantages

• Lowest cost per inch of aperture
• Reasonably compact and portable up to focal
  lengths of 1000mm.
• Excellent for faint deep sky objects such as
  remote galaxies, nebulae and star clusters.
• Reasonably good for lunar and planetary work.
• Low in optical aberrations.

• Open optical tube design allows image-degrading
  air currents and air contaminants
• More fragile
• Large apertures (over 8") are bulky, heavy and
  tend to be expensive.
• Slight light loss due to secondary mirror
  obstruction when compared with refractors.

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Reflecting telescopes focus light by bending it
with mirrors
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A third type

• But I said there were 2 types I lied ?
• Sort of.
• Catadioptrics
• Uses both mirrors and lenses to collect the image
• Schmidt-Cassegrain
• Maksutov-Cassegrain

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Catadioptric telescopes

• Best all-around, all-purpose telescope design.
• Combines the optical advantages of both lenses
  and mirrors while canceling their disadvantages.
• Sharp images over a wide field.
• Excellent for deep sky observing or
  astrophotography with fast films or CCDs.
• Very good for lunar, planetary and binary star
  observing or photography.
• Closed tube design reduces image degrading air
  currents.
• Most are extremely compact and portable.
• Large apertures at reasonable prices and less
  expensive than equivalent aperture refractors.

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Schmidtt-Cassegrain
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Problems with earth-based telescopes

• Earths atmosphere reflects certain wavelengths
• x-rays, gamma rays and most UV light is not
  transmitted by our atmosphere
• Earths atmosphere blurs images
• the bending of light by the atmosphere depends on
  the temperature of the air
• twinkling (shimmering) effect
• Light pollution
• Solution? Put the telescope in space.

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Hubble Space Telescope
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Not everything is visible
www.yorku.ca/eye/spectrum.gif

• Many modern day telescopes do not use visible
  light to collect images.
• Radio telescopes, x-ray telescopes and infrared
  (IR) telescopes have become a staple of modern
  day astronomy, producing some amazing images.

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Chandra X-ray Observatory
Compton Gamma Ray Observatory
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Disadvantages of space-based telescopes

• Expensive to launch and maintain
• Difficult to repair
• Low lifetime

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Examples of space-based telescopes

• Hubble Space Telescope
• 3 times better resolution
• can see fainter objects
• Chandra X-ray Observatory
• Compton Gamma-Ray Observatory
• International Gamma Ray Astrophysics Laboratory
  (INTEGRAL)
• Nuclear Spectroscopic Telescope Array (NuSTAR)
• Swift Gamma Ray Burst Explorer

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Famous Telescopes

• Keck Telescope
• http//www.jpl.nasa.gov/events/lectures/dec04.cfm
• Kitt Peak Observatory
• http//www.jpl.nasa.gov/events/lectures/dec04.cfm

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Very Large Array (VLA)radio telescopes