Lesson Two : Tools of Optical Astronomy
Optical astronomers’ tools fall into two main categories: those that refract light using lenses and those that reflect light using mirrors. You are probably familiar with some everyday optical tools that people use, like magnifying glasses, eyeglasses, and binoculars. These tools all use lenses to bend light waves, making objects viewed through the lens seem bigger.The earliest telescopes were built using the same principle: a light wave coming from an object travels through a series of lenses and is bent in a specific way so that the observed object appears larger than it would if viewed with only the lens of the naked eye. Astronomers made immense progress with the invention of the telescope, but they needed larger telescopes to be able to clearly see objects that were fainter and farther away. They had trouble building these larger telescopes, because it was extremely difficult to produce large lenses that were of a high enough quality to be useful.
Astronomers solved this problem by using telescopes with mirrors in them. Curved mirrors reflect light waves at an angle, much the way light waves bend when passing through lenses. This means that mirrors can also be used to make images that are larger than the original image. Most large optical research telescopes today are reflecting telescopes, using a series of mirrors to allow astronomers to see into the heavens.