The Basics of Astronomy
Our Eyes in the Sky
So astronomers study light, but how do they observe this light? I’m sure you know the answer is Telescopes, the tool of any Astronomer. Telescopes are built off of similar principles that are at work in your eyes all the time. That is, using Lenses (or Mirrors) to bring light to a focus.
Your eyes consists on only one lenses (unlike in telescopes) and functions similarly to a camera. Light from an object enters your eye, and then encounters a Lens. This lens changes the directions of the varying rays of light, and causes an Image to be formed at the focal point of the Lenses. In your eye you want that focal point to be at your retina, otherwise you will soon be needing glasses.
Glasses actually work much like a Refracting Telescope (the other most basic design of telescope is the Reflecting Telescope) would. One lens brings light to a focus near the focal point of the second lens which allows the final image to be at the position desired. In a Telescope you want that Image to be at a point where you will then be able to form the Image in you eye, or at a camera.
A Reflecting Telescope works on much the same principles of bringing light to different focuses, but uses Mirrors rather than lenses to achieve this effect. Reflecting telescopes also have the advantage that they can be more compact due to the fact that they can bounce light back towards a different mirror, and don’t need a long tube like Refracting telescopes.
We need large telescopes in Astronomy due to a need for good Angular resolution. Angular Resolution is just the ability to see the separation between objects. Think of seeing a car’s headlights from far away, you will only be able to see one big light. As the car gets closer you can tell there are actually two lights on the car. Having large mirrors in Reflection telescopes allows us to resolve these kinds on discrepancies. Also having a large “light-collection area” allows Astronomers to capture as much incoming light as possible.
The wavelength of the Light that you are observing also makes a difference here as well, that’s why Radio Telescopes (you may know them better as Satellite Dishes) are typically so large.