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Observing Space : Telescopes — Class 9 Maharashtra SSC Science revision notes

A 3-page visual revision sheet for Observing Space : Telescopes, written for Class 9 Maharashtra SSC Science. Formulas, diagrams and the traps that cost marks, laid out to be read the night before rather than the term before. Free, and it opens inside CapCompass.

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Observing Space : Telescopes in one look

gathering light from far away, the two ways of doing it, and why the best telescopes leave the ground

What a telescope is actually for

  • a telescope's first job is to gather light, not to magnify. A distant object is faint long before it is small
  • the aperture, the diameter of the main lens or mirror, decides how much light is collected
  • light gathered goes as the square of the aperture, so doubling the diameter collects four times as much
  • resolving power is the ability to show two close objects as separate, and it too improves with aperture
  • magnification is the least important of the three, since magnifying a faint blurred image only gives a larger faint blurred image
  • so a bigger telescope is better mainly because it is bigger, not because it magnifies more

The two kinds of telescope

Refracting
  • uses a lens to gather and bend the light
  • light passes through the glass
  • suffers chromatic aberration, coloured fringes, since colours bend differently
  • a large lens sags under its own weight, being supported only at the edge
  • so refractors are limited to modest sizes
Reflecting
  • uses a curved mirror to gather and focus the light
  • light bounces off the surface
  • no chromatic aberration, since reflection treats all colours alike
  • a mirror can be supported from behind across its whole area
  • so every large modern telescope is a reflector

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