View source for 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:
#light collection
#resolution
#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:

<math>n_1 \sin{\theta_1} = n_2 \sin{\theta_2} \,.</math>

{{TODO|Add basic refractor diagram}}
{{TODO|Add converging lens ray diagram}}

== 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

:<math>\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}\,.</math>

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 ==
=== The Rayleigh Criterion ===
=== Seeing ===


== Abberations ==
=== Spherical abberation ===
=== Chromatic abberation ===
=== Coma ===
=== Astigmatism ===
=== Distortion ===
=== Field curvature ===


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