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.

Resolution

Abberations