Lecture 7: Optics and Telescopes

Refraction of Light

  • The speed of light in a vacuum is 3 x 108 m/s.
  • When photons enter a material, such as water or glass, they move at a slower speed and change the direction of travel.
  • This change in velocity is called refraction
Figure 6-1

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Refracting Telescopes

  • Refracting telescopes use the refraction of light in a glass lens to create an image of an astronomical object.
  • If both sides of the lens are convex, parallel light rays focus at a point.
  • The distance from the lens to the focal point is called the focal length of the lens.
Figure 6-2
  • A refracting telescope has two lenses:
    1) A large lense called the objective lense
    2) A small lense called the eyepiece
  • FO = Focal Length of the Objective Lens.
  • FE = Focal Length of the Eyepiece Lens.
Figure 6-5
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  • The magnification of a telescope is given by:
    Magnification = FO/FE
  • By changing the eyepiece of a telescope, you can change the magnification.
  • Although magnifying an image helps you see it better, there is generally a degradation of the quality of the image under high magnification.

Light-gathering Power

  • The light-gathering power is proportional to the square of the objective lens' diameter.
  • Light-gathering power is generally the most important function of a telescope.
  • The larger a telescope's objective lens is, the more photons from the source it can gather. The result is a brighter and more detailed image.
Similar to Figure 6-6
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Problems with Refracting Telescopes

  1. Chromatic abberation
    • The red and blue light focusses at different locations creating coloured halos.
    • This occurs because the path taken by blue light through glass bends more than red light.
    • This can be corrected using compound lenses made of different types of glass.
    Figure 6-7
  2. Lens distortion
    • A very large glass lens will tend to become distorted due to its own weight.
    • The largest refracting telescope (built in the late 1800's) has a diameter of only 40 inches because of this problem.

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Reflection of Light

  • A material such as a mirror which does not allow light to travel through it, reflects the light instead.
  • The angle of reflection equals the angle of incidence.
Figure 6-9

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Reflecting Telescopes

  • Reflecting telescopes use a concave mirror instead of an objective lens.
Figure 6-10
  • The large (> 8 m) research telescopes are all reflecting telescopes.
  • A secondary mirror must be installed so that the light can get to your eye.
Figure 6-11

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Problems with Reflecting Telescopes

  • Spherical Abberation is caused because only a parabolic surface focusses to a point.
  • Spherical mirrors cause light from different parts of the mirror to focus at different points causing a blurred image.
  • Some telescopes correct for spherical abberation by adding a correcting lens in front of the mirror.
  • Due to a computer programming error, the Hubble Space telescope originally suffered from spherical abberation until the correcting lens was installed.
Figure 6-13

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Angular Resolution: Diffraction

  • Due to the wave nature of light, light waves spread when they pass through a small area like a telescope aperture.
  • If there were perfect seeing conditions and perfect optics, the smallest angle that could be resolved (due to diffraction) is

    &lambda - light wavelength, DA - diameter of aperture, &alphaDiff - angular resolution
  • The larger the telescope's objective lens, the smaller the angles you can resolve.
  • At shorter wavelengths, you can resolve smaller angles.

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Let us do some experiment

We need the following formulae
  • Small angle formula
  • Diffraction resolution limit

Diffraction of the green laser beam of the laser pointer

wavelength &lambda = aperture DA=

Beam spread of the green laser beam

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Angular Resolution: Seeing

  • Telescopes usually don't see angles as small as predicted above.
  • The turbulent atmosphere refracts light in a random way causing blurriness.
  • If the atmosphere is very still so that there isn't much blurriness, then we say that the seeing is good.
  • Telescopes are often built on top of high mountains, so there is less atmosphere for the star's light to travel through
  • Another solution: put the telescope above the atmosphere, like the Hubble Space Telescope.
  • Another solution: adaptive optics. Monitor a guide star and change the shape of the objective mirror to compensate for the atmosphere's turbulence.

The best telescopes are put where the seeing is good. Such places can get crowded !
List of the largest optical telescopes

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Telescopes for other Wavelengths

  • Our atmosphere absorbs gamma-rays, X-rays, most UV and Infrared light.
  • Earth-based telescopes can only detect visible light, some UV and IR and radio waves.
  • Orbiting telescopes are used for other wavelengths.
Figure 6-27

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Some examples of telescopes for other wavelengths:

Radio Wavelengths

  • The Dominion Radio Astrophysical Observatory is located at Penticton B.C.
  • This is a group of seven 9m parabolic radio antennae.
  • The signals from the telescopes are combined using a technique called interferometry to create an image equivalent to one much larger (600 m) telescope.
  • These radio telescopes are used to study the Hydrogen and other gas clouds in our Galaxy.
Other radio telescopes: Arecibo (single dish), VLA, VLBA VLBI , etc
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Infrared Wavelengths:

The Spitzer Space Telescope

  • The Spitzer telescope is 0.85 m
  • It detects light with wavelengths from 3 micro meters to 200 micro meters.
  • Most of this light is blocked by the Earth's atmosphere.
  • Infrared light is important because it can travel through regions with a lot of dust which blocks out light at visible wavelengths.
  • This allows us to see into star-forming regions.
Drawing of the Spitzer Space Telescope
  • Picture on the left is the Trifid Nebula in visible light.
  • Pictures on the right are false colour images of the Trifid nebula showing the infrared emission.

Herschel Space Observatory

Trifid Nebula
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X-ray Wavelengths: The Chandra Space Telescope; XMM-Newton

  • Very hot objects ( T = 106 K) radiate X-rays.
  • X-ray telescopes are best for observing supernova remnants, neutron stars and black holes.
  • This image shows a supernova remnant (in false colour) so that different colours represent different photon energies.
  • At the centre is a tiny neutron star that was born in the supernova explosion.
Cass A supernova remnant

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Microwaves: WMAP ; Planck

  • Universe is filled with 2.725 K blackbody radiation.
  • Peak of ~3K blackbody radiation is in microwave range, around 1 mm wavelength.
  • This radiation is a remnant for hot early days of our Universe, just after the Big Bang.
  • Study of the details of this Cosmic Microwave Background Radiation is a precision tool to learn about our Universe in its early days.
WMAP 5-year CMB map
Temperature differences in different directions on the sky are tiny,
&Delta T = 10-4 K, by very informative !

Next lecture: The Sun's Interior
Read Chapter 16, 8th Ed. pages 403 - 413 or 3rd Ed. 377 - 385