Astronomy Packet: 5
Whether you use a telescope with two lenses or one with a mirror and a lens depends on what you want to do. Both have many advantages and disadvantages.
For small telescopes such as the kind sailors use to look for a distant shore, or the kind used on a hunting rifle, a telescope with two lenses makes more sense. That's because this type of telescope is more powerful than telescopes with mirrors. However, they are also much heavier. All the large telescopes in the world use mirrors because large mirrors weigh less than large lenses.
Another serious problem with lenses is that as light travels through the glass it is distorted slightly. These aberrations cause red light to focus at a different point than blue light. This means the observer must constantly change their focus back and forth in order to see both the reds and the blues. A mirror does not have this problem because the light never passes through the glass, instead it bounces off.
Cassegrain telescopes, which we learned about in Packet #4, use a lens for more power, and then they also use a mirror to get rid of the aberrations. It is also possible to buy specially designed eyepieces which clear up the color aberrations caused by lenses.
Assignment # 5
Describe some of the advantages and disadvantages of using lenses or mirrors in a telescope.