"It's not the things we don't know that cause the trouble, it's the things we know that ain't so." I love that quote and I wish I'd said it first, but Artemus Ward beat me to it by about a hundred and fifty years.

Less than fifty years ago, one thing every astronomer "knew" was that there was a limit to what a telescope could see when looking out into space. If you made a telescope's main lens or mirror bigger and bigger, it would collect more and more light but the degree of detail in what you saw would not increase. The limiting mirror or lens size is quite small, about ten inches, and beyond that you will get a brighter but not a sharper image.

The problem has nothing to do with the telescope's design or manufacture. The spoiler is the Earth's atmosphere, which is in constant small-scale turbulence. The moving air distorts the path of the light rays traveling through it, so that instead of appearing as a steady, sharp image, the target seems to be in small, random motion. The nursery rhyme has it right. When the target looks small, like a star, it will twinkle; when it is larger and more diffuse, like a planet or galaxy, fine detail will be blurred.

Twenty years ago that was the end of the story. If you wanted highly detailed images of objects in space, you had to place your telescope outside the Earth's atmosphere. That idea led to the orbiting Hubble Space Telescope, whose wonderful images have appeared on every TV channel and in every magazine. The Hubble pictures are far more detailed than any obtained by a telescope down here on Earth, even though the size of the Hubble's mirror, at 94 inches, is much smaller than the 200-inch mirror at Mount Palomar. The only road to detailed images of astronomical objects was surely the high road, through telescopes placed in orbit.

This "fact" turned out to be one of the things we know that ain't so. About fifteen years ago, a small group of scientists working on a quite different problem for the Strategic Defense Initiative ("Star Wars" to most people) came up with the idea of aiming a laser beam upward and measuring the way that its path was distorted in the atmosphere. Knowing what happened to the laser beam, the focus of the observing telescope mirror could be continuously, and rapidly, changed, so as to compensate for the changes in light path. The procedure, known as "adaptive optics," was tried. It worked spectacularly well. Today, ground- based telescopes are obtaining images of a crispness and clarity that a generation ago would have been considered impossible.

What else do we "know" that can't be done with ground-based telescopes today? Well, the Earth's atmosphere completely absorbs light of certain wavelengths. If we want to learn what is happening in space at those wavelengths, we still need orbiting telescopes. I certainly believe that is true. On the other hand, it may be just one more thing I know that ain't so.

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