General Astronomy/Telescopes/Basic Optics

Although the Greek origin of the word telescope means far-seeing, "see something far away" is not a good description of the function of a telescope. If an object is bright enough, no matter how far it is, we can see it. In fact, telescopes serve three main functions:

1. light collection
2. resolution
3. magnification

Before understanding how telescopes work to achieve these three goals, we take a pause here and study some principals that the optics work.

Reflection and Refraction

The most primitive type of a telescope is a refracting telescope (also called a refractor), which is used by Galileo back in the 1600s. Later, Newton designed and built a reflecting telescope (also called a reflector) making use of mirrors. We first focus on refracting telescopes. A basic refractor consists of a converging lens which focus light onto the focal plane at which detectors and instruments are then placed. As discussed in the previous chapter, the path a light ray passing through a lens can be understood by the Snell's law of refraction:

$n_{1}\sin {\theta _{1}}=n_{2}\sin {\theta _{2}}\,.$

Magnification

Magnification in a telescope can be given by the following formula: mag=focal length of the objective lens÷focal length of the eyepiece lens

Light Gathering Power

Since most objects in the sky are very dim, a telescope effectively collects and converges light for our viewing. How dim are they? A brief calculation helps explain.

The Sun is at a distance of 1 AU (~1.5e+11 m) away, the light coming out "spread" over a spherical surface (actually it is the energy that spreads over a surface, which we call the flux). Suppose we have a telescope with a 2 m (diameter) aperture, then the fraction of light collected from the Sun is

${\frac {\mbox{light collecting area}}{\mbox{area of spherical surface}}}={\frac {\pi (1)^{2}}{4\pi (1.5\times 10^{11})^{2}}}\simeq 1.1\times 10^{-23}\,.$

For objects beyond the solar system, they are millions, if not billions, times further away from us. Therefore, astronomers prefer to build larger telescopes for greater light collecting power.