Why are most research telescopes reflectors?

If you wanted to find something small in a dark basement, would you use a magnifying glass or find a way to get more light? Of course, you'd try to find more light. Astronomers do the same thing -- the Universe is very big and the light coming from other stars and galaxies is very dim. For example, the nearest large galaxy to our own, Andromeda (also called M31), is about six times the apparent diameter of the Moon, but the Moon seems much larger than Andromeda even through binoculars. Just enlarging the sky won't solve the problem -- you don't just need a magnifying glass to find something small in a dark basement. You need more light, so you need to make a telescope that will collect a lot of light. The bigger the diameter of the telescope you have, the more light you can collect, and the better the images you can take. But, you also need to have some way of focusing all the light you collect. Galileo's idea of using glass lenses to focus the light from stars was thought to be the best way to do this for some time.

BUT, one problem with glass lenses is that, like prisms, they don't bend light of all wavelengths evenly. Sometimes you want to break up the light, but lenses will break up the light even when you don't want it to. So, even if you make a lens very carefully, when you take a picture of a star with that single lens, you get a blob that is bluer on one edge and redder on the other edge. This phenomenon is called chromatic aberration. And there is nothing you can do about it! ...Well, almost nothing. You can minimize this effect if you are clever about it -- the 40-inch actually has two 40-inch lenses, made from two kinds of glass, crown and flint, which minimize chromatic aberration. You can also use special filters to let in only one color of light -- then all the light is aberrated in the same way (since it's monochromatic). In the case of the 40-inch, we use greenish or yellow filters, and the first stellar photograph through such a filter was taken in 1900.

Fortunately, mirrors reflect light of all (visible) wavelengths in exactly the same way, so if you have a reflecting telescope, you don't have to worry about chromatic aberration.

Another problem with lenses is that when they get very big they get very heavy. And, because you don't want to block any star light, you can only support a telescope lens around the edge of the lens, which is also the thinnest, most fragile part of the lens. You can't support the lens in any other way, otherwise you'll block the light you're trying to collect.

Mirrors, on the other hand, don't have to be free and unsupported on both sides because the light isn't going through them, it's only bouncing off one side of them. So mirrors can be heavy but you can support them well underneath and still not interfere with your light collection.

The Yerkes 40-inch refractor is the largest refracting telescope in the world; when it was completed (in 1897), it was the largest telescope in the world of any sort. Today, the largest telescope in the world is a reflector, the ten-meter, or 400-inch, Keck Telescope in Mauna Kea, Hawaii. The five-meter, or 200-inch, Hale Telescope at Palomar Mountain in California is the largest reflector in the continental United States. (For diameters of other large telescopes, see Big Eyes, a list of the world's largest optical telescopes.)

How much better are these telescopes at collecting light than the 40-inch? The light-collecting area of the telescope tube is proportional to the diameter of the tube squared. Since the 40-inch is nearly one meter, we can compare it to a 4-meter telescope like this:


so the 4-meter telescope is 16 times better at collecting light than a 1-meter. The Keck can gather 10x10, or 100 times more light than the Yerkes 40-inch refractor; the Hale Telescope can gather 5x5 = 25 times more light than the 40-inch. The University of Chicago collaborates on the operation of another observatory called Apache Point in New Mexico. The main telescope there is a 3.5-meter, and so it collects 3.5x3.5 = 12.25 times more light than the 40-inch can. The telescopes that the general public often buys are maybe 10 centimeters = 0.1 meters in diameter. So, comparing the 40-inch to the 10-centimeter :

the 40-inch is 100 times better at gathering light than the 10-centimeter! Clearly, light gathering power goes up fast as the size of your telescope increases -- for a little increase in diameter, you get a lot more light. In mathematical parlance, we say that it "goes as the square of the diameter."

SO, in conclusion, astronomers prefer reflecting telescopes because they can be made larger so that they collect more light, and because they don't have chromatic aberration.

This then begs the question: So why did they make the Yerkes 40-inch telescope a refractor and not a reflector? At the time of the construction of the 40-inch, no research-quality reflecting telescopes had ever been made, so people who were doing serious research only worked with refractors. At around the turn of the century, George W. Ritchey (working here at Yerkes) was one of several people who started to produce high-quality pictures from reflecting telescopes. (He was working on a 24-inch telescope; although we have a 24-inch today, the 24-inch he worked on was replaced more than 30 years ago.)


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Content originally generated in March 1995.
This file was last modified on 20 April 1999.