2. OPTICAL TELESCOPES

The telescope was invented in Holland at the beginning of the 17^th C and soon afterwards (1609), Galileo began to use it to observe the sky. Immediately he made three great discoveries: the moons of Jupiter, the phases of Venus, and the mountains of the Moon. Today, you can buy a telescope comparable to Galileo's telescope for about $20 at Toys 'R Us. It had a lens of diameter 2.6 cm, and a second smaller lens called an eyepiece. (One lens would be enough if you put film at the focus. But when you look through a telescope, you need an eyepiece to undo the focusing effect of the lens of your eye.) A telescope with lenses is called a refracting telescope.

In 1668, Isaac Newton constructed the first reflecting telescope, which used a mirror instead of a lens for the primary focusing element. His design, called a Newtonian telescope, is still in common use today. Reflecting telescopes have a technical problem: the prime focus is located inconveniently in the path of the incoming light. This might be OK if the detecting element is a small CCD. But if the detector is large (and modern instruments are), it is desirable to employ at least one secondary mirror to deflect the focus to a point either out through the side of the telescope, as in the Newtonian design, or back through a hole in the primary mirror, as in the Cassegrain design, as shown below. The main telescopes of the Sommers-Bausch observatory on campus use the Cassegrain design.

Four designs for the focus of reflecting telescopes (source).

During the 18^th and 19^th centuries most of the best telescopes were refractors, with a few notable exceptions. But by the 20^th century, it was clear that reflectors were the preferred design for large telescopes (there are several reasons why mirrors are preferable to lenses, but the main one is that a very large lens is simply too heavy and expensive). The first really great modern telescope was the 2.4-meter diameter telescope on Mt. Wilson, California that was completed in 1917. Using this telescope, Edwin Hubble discovered in 1929 that the entire universe was expanding. The next giant step was the completion in 1947 of the 5-meter Hale telescope on Mt. Palomar, California, which was the world's best telescope* until 1993, when the 10-meter Keck-I telescope on the top of Mauna Kea, Hawaii was completed. (*In the late 1970s, the Soviet Union completed a 6-m telescope, but it never did perform as well as the 5-m Hale telescope.)

Today, there are some 15 telescopes in operation or nearing completion with mirror diameter greater than 6.5 meters (see table below, which has links to the home pages of each facility). The increase in astronomical observing power is enormous.

The competition to build ever bigger telescopes is continuing. Astronomers in California are developing plans for an even bigger optical telescope, with a mirror diameter of 30 meters -- about the size of a baseball diamond. And, not to be outdone, the European Southern Observatory is designing an even bigger telescope, called OWL (for "OverWhelmingly Large telescope"). Plans (some tentative, some pretty firm) to build several other large new telescopes are listed here. We have come a long way since Galileo!

In addition to these giant ground-based optical telescopes, we have the Hubble Space Telescope (HST). The HST primary mirror has a diameter of 2.4 meters, the same as the Mt. Wilson telescope and much smaller than the Keck telescopes. Nevertheless, it can see objects fainter and more distant than the Keck telescopes because it is in orbit, above the light and distortion of the Earth's atmosphere. The HST also has the advantage that it can observe the sky at ultraviolet wavelengths that are totally blocked by the Earth's atmosphere.

The HST is by far the most expensive telescope ever built. It cost $2 billion to build (about the cost of two B-1 bombers) and more than $100 million per year to operate. In contrast, the two Keck telescopes cost about $80 million each. It's far more expensive to build things to be launched into space than it is to build things on the ground.

In about ten years, astronomers hope to launch an even bigger telescope in space, called the James Webb Space Telescope (JWST). The JWST will have a primary mirror of diameter at least 5 meters, and will be able to see even fainter objects than the HST. NASA is supporting design studies of this telescope, but cannot begin to build it until Congress approves the project.

Why build these great telescopes? As I mentioned in lesson 1, one of the main themes of science is to explore nature beyond the limitations of our natural senses. Of all of our sense organs, our eyes are the most precise. But, as is obvious to everybody who has used binoculars, there is plenty of room for improvement. What are the limitations of your vision and how do we go beyond these limitations?

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Last modified December 28, 2002
Copyright by Richard McCray