Reflecting Telescope: Background
If you look around at the telescopes people are using to look at the stars these days, chances are you'll see that many of them are reflecting telescopes.
A reflecting telescope is a popular option because it provides excellent performance. It also avoids some of the inadequacies of other types of telescopes. On top of all of that, it can be very inexpensive in relation to its quality.
If you're thinking about a reflecting telescope, but you're just not sure what it is or how it works, we've got the answer to the common question: What is a reflecting telescope? We'll give you some background on this popular choice for amateur astronomers, as well as talk about why a reflecting telescope can be a great option.
Reflecting Telescope Definition
A reflecting telescope, also known as a reflector telescope, is a telescope that uses a mirror or series of mirrors to gather and focus an image. In a reflecting telescope, curved mirrors take the place of the lenses that get used in refracting telescopes or catadioptric telescopes.
History of the Reflecting Telescope
The idea for the reflecting telescope came about as early as the tenth century. Even then, philosophers surmised that curved mirrors could be used to achieve the same effect as the lenses that telescopes used at that time.
Not long after the invention of the refracting telescope, Galileo and others theorized that they could use a curved mirror instead of a lens to gather an image.
There are some reports that a working reflecting telescope was created in the early 17th century. One scientist used a curved bronze mirror to convey the image to the viewer, but all indications are that it didn't work terribly well.
The first actual working telescope is generally credited to the English astronomist, philosopher, and all-around genius Sir Isaac Newton in the later part of the 17th century. Newton's telescope used a spherical ground metal primary mirror, along with a small diagonal mirror to gather and reflect the image to the viewer. Not coincidentally, this design has been dubbed the Newtonian telescope. You can read more about its design and function here.
Despite Newton's having been credited with the creation of the working reflecting telescope in 1668, the metal mirrors created a less-than-ideal design that wan't improved upon for about 100 years. By that time, production methods relating to the mirrors and their coatings created a design that had more practical use.
The late 20th century has seen the development of more advancements, such as adaptive optics and lucky imaging, which have improved the function of the reflecting telescope, and have solved some of the more difficult problems associated with astronomy such as astronomical seeing.
How a Reflecting Telescope Works
Part of the beauty of the reflecting telescope is that it can be exceedingly simple to put together. In fact, if you're looking to build your own telescope, chances are it will be a reflecting telescope.
In this sort of telescope, light enters the tube and reflects off of a concave mirror. The concavity of the mirror causes the light rays to converge on some focal point. If you have the luxury (and the space!) of using a gigantic telescope, that focal point is the viewer. Usually, however, the focal point is another mirror, which reflects the light out to the side of a (much) smaller telescope, where the viewer is waiting. The light rays can also be reflected off another mirror.
Here's a diagram that shows just how elegant this can be:
Image source: Wikimedia Commons
That's it. When you use one, you're truly working with 10th-century technology enhanced by 20th-century manufacturing methods. It's a powerful combination.
Why Reflecting Telescopes Are So Effective For Astronomy
One of the most important benefits of reflecting telescopes is that they are capable of showing a broader spectrum of light than some other types of telescope. Other types of telescopes, such as refracting telescopes or catadioptric telescopes, pass light waves through a lens. Because the material that makes up that lens can absorb certain wavelengths of light, the viewer who uses those types of telescopes will be viewing an image lacking in that wavelength.
Mirrors only have one surface that comes into contact with the light. Thus, a mirror need only have that one surface free of imperfections. A lens, on the other hand, must be free of imperfections and flaws throughout. The light passes through the entirety of the object. Advantage: mirrors.
Problems With Reflecting Telescopes
But the reflecting telescope isn't perfect. There are certain shortcomings that you must consider.
As we talked about previously, because the focal length of the reflected image would require a gigantic telescope, there is usually a secondary mirror that reflects the image to the viewer. This introduces additional opportunities for imperfections, and in fact can prevent some of the light from reaching the primary mirror in the first place.
A mirror also introduces "spherical aberration", which results from the fact that the rays reflected from the edge can't come to the same focal point as the rays reflected from the center. To resolve this problem, most reflecting telescopes use paraboloid mirrors, which can result axis aberrations for images near the edge of the field of view.
Types of Reflecting Telescopes
As with so many things whose design is simple, the reflecting telescope can be adapted in a surprising number of ways. Here are just a few of the amazing array of types of reflecting telescopes:
- Gregorian telescope: Uses a concave secondary mirror to reflect the image back through a hole in the primary mirror.
- Newtonian telescope: As discussed above, this uses a diagonal secondary mirror to reflect the image to the side of the telescope.
- Cassegrain telescope: Uses a hyperbolic secondary mirror to reflect the image back through a hole in the parabolic primary mirror.
- Ritchey-Chretien: Uses two hyperbolic mirrors. Its flat focal plane makes it ideal for wide field and photographic observing.
- Three-mirror anastigmat telescope: Uses a third mirror to eliminate distortion.
- Dall-Kirkham telescope: A type of Cassegrain telescope that uses a concave elliptical primary mirror and a convex spherical secondary mirror.
- Off-axis types: Use varying methods of moving or eliminating the secondary mirror to keep it from obstructing the light rays on the way to the primary mirror.
If you're looking for a telescope, there's a good chance a reflecting telescope will serve your needs very well. Given the vast variety of options, and the high fidelity with which they render images, reflecting telescopes are a fascinating and effective option for stargazing.