Page 319 - ABCTE Study Guide_Neat

There are three major functions of an astronomical telescope. They are (in order of importance):

- Light-gathering power

Think about trying to fill a canteen with water during a rain shower by allowing rain to fall into the
opening of the canteen. Inserting a funnel into the opening of the canteen will allow you to fill it more
quickly. The funnel increases the water-gathering power of the canteen. A funnel with a larger area will
have more water gathering power. Analogously, telescopes act as light funnels to gather light from a
larger area down to the smaller area of the eyepiece or instrument on the telescope.

The light-gathering power of a telescope then allows astronomers to observe fainter stars or galaxies
than would be possible with less light-gathering power. The light-gathering power of a telescope
depends on the area, or the square of the diameter, of the primary mirror or lens. The primary purpose
of most large astronomical telescopes is increased light-gathering power.

- Resolving power

The resolving power of a telescope is the ability to see sharp detail. A telescope with good resolving
power allows us to see smaller details than a telescope with poor resolving power. The resolving power
depends on the diameter of the primary mirror or lens.

However, turbulence in Earth's atmosphere limits the resolving power of large telescopes. The reason
that images from the Hubble Space Telescope are so sharp is that the resolving power is not limited by
the atmosphere.

- Magnifying power

The magnifying power of a telescope is simply how many times bigger the image appears than it
actually is. Magnification does nothing to improve the quality of the image; hence it is less important