The textbook web resources for Chapter 4 are here.

Telescopes enable us to see faint, distant objects because of two properties that they have:

1. Light-Gathering Power
  - Telescopes use lenses or mirrors to "collect" photons of light
     and concentrate them for our eyes.  This allows us to see very
     faint objects.
  - The larger the lens/mirror, the more photons get collected, and
     the more powerful the telescope is.
  - Telescopes are often named/described by the radius of their
     light-gathering area.

  Refracting Telescopes (aka Refractors) use lenses to bend
  light waves into a focus point.  The bending of light is called
  refraction.  This works because light slows down as it moves
  through different substances. 
  The drawbacks to using a refracting telescope include...
  - the great expense of making large lenses.
  - big lenses sag over time, distorting the image they produce
  - most transparent materials focus different colors to slightly
    different places, creating a "chromatic aberration" (color fringes
    around the image)
  - some lens materials absorb short-wavelength light

  Reflecting Telescopes (aka Reflectors) use curved mirrors to
  gather and focus light. Most astronomers use reflectors
  - the mirrors focus as efficiently as lenses do
  - the mirrors can be supported from behind, eliminating sagging
  - since light doesn't pass through the mirror, all colors pass
     through equally well with no absorption of short wavelengths
  Multimirror instruments are reflectors that use several small
  mirrors, each of which can be adjusted to focus an image.  These
  are often used to counteract the bending of a mirror as the
  telescope is moved from one position to another.

2. Resolving Power
  Resolution is the ability to discern fine details.  We can
  sometimes think of it as the ability to see small, close objects as
  separate, rather than blurred together.

  Diffraction is a disturbance of light waves as they pass through
  an opening or around an obstacle.  This limits the ability to see
  fine details through a telescope - because the waves are
  diffracted as they pass through the opening of the scope.  

  The larger the opening through which the light passes, the less of
  an effect diffraction will have.  This is the idea behind an
  interferometer.  An interferometer uses two or more telescopes
  that are set a distance apart.  The light from each scope is
  collected and sent to a common detector (often linked to a
  computer).  Conflicting light waves from the two scopes cancel 
  each other out - this eliminates some of the blurring and gives a
 greater resolution.

Effects of the Atmosphere
  Different types of telescopes observe different wavelengths of
  light, including visible light, infrared, ultraviolet, radio waves, and
  X-rays.  Some of these wavelengths are absorbed by the
  atmosphere, so it is better to observe the from space using an
  orbiting telescope.

  Scintillation is the "twinkling" of stars.  It is
  caused by  variations in the density of the atmosphere.  These
  density  variations are caused by temperature differences - cooler
  air is  denser, warmer air is less dense.  Light is refracted a
  bit  differently from each layer, causing the image to flicker or

   Seeing is a measure of how "steady" the atmosphere is during

  astronomical observations.  The more the image is distorted by
  scintillation, the more the image is distorted and the worse the
  "seeing" is.

  Seeing can be improved by comparing an observed object to a
  reference star and using a computer to eliminate the distortion, or
  by a process called adaptive optics.  With adaptive optics, tiny
  mechanical actuators constantly adjust the shape of the mirror to
  cancel out the distortion.

  Observatories in space (space telescopes) eliminate the effects of
  scintillation and bad seeing, because the light they gather does not
  pass through the atmosphere.

  Light pollution is an excess of background light that interferes
  with our ability to see the stars and other astronomical objects. 



Homework from the Text:

  • Read Chapter 4 (pg. 119-143).
  • Do Review Questions #1-8.
Subpages (1): Files for Telescopes