PHYS102: Introduction to Electromagnetism

While microscopes allow us to look more deeply into the microcosmos, telescopes let us see into the vast distances of the cosmos on the largest scales. The problem we face when looking at distant objects is mainly related to the concept of intensity. Because light rays coming from distant stars or planets spread out on their way to us, the intensity of the light that can enter our eye from those objects is very low. When the intensity is too low for our retinal cells, they will not tell our brain that there is a point of light there at all.

The main point of a telescope is to simply catch more light by collecting the rays that each star emits with a large-diameter lens or mirror. After collecting the light, the trick is to then bend the rays in just the right way so they re-converge onto a single spot on our retina. When this is achieved, the intensity of the spot on your retina is much larger than before, when only the limited amount of light entering the small opening in your iris contributed to that spot.

And just as for a microscope, the optics in a telescope must also ensure that spots corresponding to different stars will appear at large enough spacings from each other on the retina, so that they can be perceived as individual points of light instead of just a blur. Watch this lecture, which details this process.