Telescopes

• Real image


  light converges towards focal point and is inverted
• Relationship between power and focal length

  Long focal length = less power
• In lens equation, how does the distance of image of lens relate to the orientation of the image?

  if image is real v is positive, if image is virtual v is negative
• Refracting Telescopes

  2 converging lenses, objective lens forms real image of distant object, eye piece lens forms virtual image of object
• Reflecting Telescope

  parabolic concave mirror and convex mirror
• Chromatic Aberration

  image distortion in refracting telescopes where different wavelengths of light are refracted by different amounts
• What does chromatic aberration do to the image?

  edges of image appears coloured
• How does blue light compare to red in chromatic aberration?

  blue light has a higher refractive index so focuses closer to lens
• How can you fix chromatic aberration?

  introducing second diverging lens
• Spherical Aberration

  affects both reflectors and refractors, rays of light come to focus at different points due to curvature of mirror/lens
• What affect does spherical aberration have on the image?

  image becomes blurred
• How to fix spherical aberration?

  using parabolic lens/mirror
• Relationship Between Aperture and Resolving power

  Large aperture = good resolving power
• Resolving power depends on:

  • size of aperture
  • wavelength of radiation
• How does wavelength affect Resolving Power?

  large wavelength = poor resolving power
• Telescope in normal adjustment

  Real, inverted, magnified
• What does it mean to be in normal adjustment?


  when magnified image appears at infinity to the observer
• How is normal adjustment achieved?


  both lenses must be arranged so the focal points meet at the same point
• In terms of focal length what does normal adjustment mean?

  focal object greater than focal eyepiece
• Angular magnification equation

  focal object / focal eyepiece
• Angular Magnification in terms of angles

  Angle produced by emerging ray at eyepiece / angle produced by incident ray at object
• Lens equation

  1/focal length = 1/distance object to lens + 1/distance image to lens
• Magnification as ratio of heights

  height of image / height of objects

  OR

  distance lens to image / distance lens to object
• What is angular size?

  The objective lens forms a real image of the object. The eyepiece then magnifies this image. The angular size of the image viewed through the eyepiece is angular size determined by h / d
• Advantages of Reflecting telescopes

  • for same magnification they are cheaper and smaller
  • easier to make large mirrors than large lens
  • no chromatic aberration
  • better resolving power
• Structure- Differences between radio and optical telescopes

  • Radio telescopes use large parabolic dishes, whereas optical telescopes use lenses or mirrors
  • Radio telescope dishes are generally much larger to collect longer wavelengths
  • Radio telescopes require remote locations, while optical telescopes are often placed at high altitudes to reduce atmospheric interference
• Which has a higher resolving power, optical or radio telescopes?

  Optical telescopes as they take in shorter wavelengths of visible light
• What is the difference in collecting power for optical and radio telescopes?

  Both have collecting power proportional to the square of the diameter of their objective, so radio will provide a larger collecting power despite lower resolving power
• Minimum angular resolution

  The smallest angular separation between two point sources of light that a telescope can distinguish as two distinct objects
• What does it mean to have a smaller angular resolution?

  better resolving power, meaning the instrument can separate objects that are closer together
• What affects angular resolution?

  Aperture size and Wavelength
• Where do optical telescopes need to be placed?

  Light pollution requires optical telescopes to be away from centres of population.

  Distortions due to atmosphere require optical to be high up
• Where do radio telescopes need to be placed?

  located in a radio quiet area and can be at lower altitude