When people think of telescopes, reflecting telescopes are usually the last things that come to mind. Rather, people tend to think of a straight tube with lenses at either side. This is unfortunate, since there are several ways to bend light so that it is magnified to the point where a person on the ground can make out the features on Jupiter, the billowing gasses of a distant nebula, or the light of distant stars. In fact, if a stargazer wants to peer into the inky voids of space with a scope of manageable size, a reflecting telescope will serve them well.
Reflecting telescopes use mirrors to magnify images, allowing the image to be enlarged a few times before coming through the eyepiece. This is simpler to implement than a standard, straight-through refracting telescope, as it allows several smaller adjustments to the image than a few, very large changes with lenses. And, by doing this with mirrors, the design of the telescope can be made much more compact, allowing more magnifications of an image in a smaller telescope.
In order to understand reflecting telescopes, you must begin by understanding the fact that there are two methods of focusing light so that objects seem closer. The first method is through lenses, which use refracting to bend light and focus it. The second method is with mirrors, which can be shaped to reflect light in such as way that small, distant objects appear to be very close and easily visible.
There are two general designs for reflecting telescopes: the Newtonian and the Cassegrain and its variants. The first type, Newtonian, is a very simple design and it is very popular with amateurs who want to home-build a telescope. In the Newtonian design, there is one large mirror at the base of a long tube, and the mirror is focused onto a flat mirror that redirects the image toward an eyepiece. This design was originally created by Isaac Newton, and it was the first successful design for a reflecting telescope.
The second type of reflecting telescope, the Cassegrain and its variants, uses two mirrors to create the image. One large mirror is set up at the base of a tube, with a smaller mirror facing it at the top of the tube. The light comes in through the top, is focused by the larger mirror, and reflected back by the smaller mirror and sent through a hole in the larger mirror and on to the eyepiece. This makes the Cassegrain telescope look like a refracting telescope, though they function very differently. However, their ultimate aim is the same: to allow people to see things that are very far away.
When looking for a telescope, reflecting telescopes are some of the best that can be found. They are effective, easy to use and, in some cases, easy to build – making them the preferred choice for professional astronomers and backyard hobbyists. So, when thinking about telescopes, don’t just think picture the lenses that usually come to mind, think about remarkable properties of mirrors and reflecting telescopes.