Chapter 5: Telescopes


  • Galileo made the first astronomical telescope in 1609 \rightarrow revolutionary impact back then.
  • Telescopes are still being improved today.
  • 10 largest telescopes have all been built since 1993.
  • Much larger telescopes are being built today.
  • We can also send telescopes into space and to objects in the solar system.

Seeing Light

  • We see by the light that enters the pupil of our eye.
  • Pupil is a circle with an area = πR2pupil.
  • Our pupil’s maximum diameter is about 8 mm when it expands - “adapts” to total darkness.
  • Pupil’s maximum area is about 50 mm2.

The Light “Gain” of a telescope

  • A telescope gives us a pupil bigger than our eye.
  • Today’s largest telescope’s diameter = 10.4 m = 10,400 mm (same unit as our pupil).

Resolving Power

  • Resolving power means seeing finer details, NOT just larger images = magnification.
  • Chapter 2 we learned that we observe angles.
  • Resolving power means that we can separate features just a tiny angle apart - “angular resolution”.
  • Angle depends on (the wavelength)/(diameter of pupil or telescope).
  • Bigger diameter \rightarrow smaller angle - better resolution.

Units for Angles

  • We normally measure angles in degrees with 360o = full circle - one degree is a very large angle for astronomy.
  • Therefore, we create smaller angular units.
  • Minutes of arc = 1’ = 1o/60.
  • Seconds of arc = 1’ = 1’/60 = 1o/3600.
  • Measuring:
  • Angles in seconds of arc.
  • Wavelength in nm.
  • Telescope diameter in centimeters.
  • Angle of Resolution (“) = 0.025*(𝛌(nm)/D(cm))
  • NOTE: Using a shorter wavelength and/or a larger diameter gives a smaller angle - better resolution.

Resolving Power

  • Using the pupil of our eye we can resolve angles ≥ 1’.
  • Using small telescopes (Dtel >> Dpupil) we can resolve angles ≥ 1” = 60x better.
  • This is how Galileo made his important discoveries beginning in 1609.
  • Using today’s telescopes (D is about 10 m).

Atmospheric Blurring

  • Light passing through Earth’s atmosphere is constantly shifter by movement of the air.
  • This causes the stars to twinkle.
  • Through the telescope the images are blurred preventing us from seeing finer detail \rightarrow increased brightness is the major gain of telescopes on Earth.

Types of Telescopes

  • Lens telescope - Galileo’s telescope.
  • Light transmitter from air to glass/water/plastic changes direction - “refracts”.
  • Shaping the glass into a lens brings the light to a focus at a point - refracting telescope.

Refracting Telescopes

  • Reflecting telescopes have several problems:
  • A lens with a diameter more than 1 m is so heavy it sags and cracks under its weight.
  • Thick glass absorbs light.
  • A lens focuses light of different colours at different locations.
  • Therefore, astronomers developed another telescope:
  • Mirror telescope - reflecting telescope.
  • Light reflects from a shiny surface - mirror.
  • Shaping the mirror into a curve reflects light to focus.
  • NOTE: The shiny surface is on the front of the mirror so the light does not go through any glass.
  • Because the light does not go through the mirror, it can be supported on the back as well as the sides \rightarrow much larger mirror diameter.

Other Wavelengths

  • In chapter 4 we learned that astronomers are observing all forms of light from radio waves to gamma-rays.
  • Reflecting telescopes are used for all forms of radiation.

Location of Observatories

  • Putting telescopes in space is VERY expensive.
  • For telescopes on Earth to be as productive as possible, they must be at the very best locations.
  • Far from cities to avoid “light pollution”.
  • Light pollution includes city lights.
  • Very dry locations with few clouds.
  • High mountains to be above as much atmospheric blurring as possible.

Today’s Telescopes

  • Largest possible single mirror has a diameter of 8.4 m.
  • Largest diameter are too heavy to support.
  • To construct larger mirrors:
  • Use many smaller (about 1m) mirrors to assemble a large mirror - like a puzzle or mosaic - a “segmented mirror” because you have a lot of segments.
  • Use 2 or more large mirrors to combine.
  • Recall: Resolution angle depends on the wavelength divided by the diameter of the telescope.
  • Problem: radio waves are very long, need an impossibly HUGE diameter to resolve detail.
  • Solution: combine separate telescopes to observe one object - interferometer.
  • Combined together their separation - “diameter” of a single telescope.
  • So, resolution angle depends on wavelength divided by separation of the telescope.

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