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Chapter 4:
Atmospheric Turbulence - Bad Air
Bad, Bad Air
We need the atmosphere to sustain life. It is made of air, a conglomerate of different gaseous elements that is pretty much transparent to the light that we normally see. However, since it is a mixture, and getting more mixed all the time, we could do really good astronomy if it weren't there at all. In fact, Isaac Newton wrote about the problem in Opticks in 1730: "€œFor the Air through which we look upon the Stars, is in perpetual Tremor - But, these Stars do not twinkle when viewed through -large apertures. The only Remedy is a most serene and quiet Air, such as may perhaps be found on the tops of the highest Mountains above the grosser Clouds." In this basic and extremely perceptive observation, Newton acknowledged the problem, observed the phenomenon of aperture averaging of scintillation, and proposed a solution. Over 200 years before spaceflight, he suggested that we put telescopes on mountains to get into clearer air. We still take his advice, and we go one step further — we launch billion dollar satellites into space to really, really get above the grosser Clouds. Now, in the latter part of the 20th century, when we name computers after Newton himself, we have developed adaptive optics so that we don't really have to go to the tops of the highest mountains — we just have to be able to see around the tremors so that it looks like we are above the air. What is wrong with the air? It has an index of refraction near 1.0, which is pretty close to a vacuum; the problem is that it is not exactly 1.0. Pretty close doesn't make it. The index depends upon the wavelength of the electromagnetic wave passing through it. It depends upon temperature, pressure, and density. The less air there is, the more like a vacuum it is. Because the index of refraction is not uniform, it distorts the nice clean electromagnetic wave passing through it. While we use all sorts of lenses for many reasons, we really don't want to throw a handful of little lenses into the air in front of our telescope. The light coming through the various lenses would reach our focal plane, our eye, at different times. Being a wave, the light exhibits interference when it reaches our eye.
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