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Telescopes

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Telescopes How do they work? Chapter 5 – PowerPoint PPT presentation

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Title: Telescopes


1
Telescopes
  • How do they work?
  • Chapter 5

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

4
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.
5
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.

6
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.

7
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.

8
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

9
  • 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.

10
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

11

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
12
  • 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.

13
A telescope has two general properties
  • how well it can collect the light
  • (the aperature)
  • how much it can magnify the image
  • (the magnification)

14
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.

15
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

16
A closer look at eyepieces
                                                                                                                        View through an eyepiece. Note that the image is upside-down.
17
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)

18
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

19
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.

20
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.

21
                                                                                                                                                                                                                                                
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.
22
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.

23
Refracting telescopes are not used for
astronomical research (anymore) because they are
large and have heavy lenses (i.e. expensive).
24
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.

25
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.

26
Reflecting telescopes focus light by bending it
with mirrors
27
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

28
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.

29
Schmidtt-Cassegrain
30
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.

31
Hubble Space Telescope
32
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.

33
Chandra X-ray Observatory
Compton Gamma Ray Observatory
34
Disadvantages of space-based telescopes
  • Expensive to launch and maintain
  • Difficult to repair
  • Low lifetime

35
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

36
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

37
Very Large Array (VLA)radio telescopes
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