Seeing

As astronomers we are always concerned with "seeing". The term "seeing" refers to how still the air is. When we look at a planet or other astronomical object, we look up through layers of Earth's atmosphere. This atmosphere is always moving. Light from the planet or star enters the atmosphere on its way to our eyes and is affected by these layers of atmosphere, which bends the light slightly in different directions as it travels toward us. This bending of light causes starlight to "twinkle" and the image of a planet to dance around and "boil" in our eyepieces. Have you ever noticed on a clear winter night after a relatively warm day, how the stars twinkle so vigorously? If you look at a bright star through a telescope on a night like that, you will notice how the star is not still at all. In fact, it may seem to bubble and roil, giving off sparkles of different colors. That is an example of poor seeing. If you were observing a pair of very close double stars, you might not be able to separate the two because they are so stirred up! Such poor seeing also affects planetary observing. Under the same conditions, the image of Jupiter in your eyepiece may appear to 'boil' and will not hold still no matter what you do. Of course, what is happening is radiative cooling. During the warm day the Earth absorbs heat. Then, when the Sun sets, this heat begins to radiate into the air as we cool down. These rising air currents also contribute to unsteadiness in the atmosphere.

Dr. Julius Benton gives a more formal definition of 'seeing'. According to Benton, "Astronomical seeing is the result of a number of very slight differences in the refractive index of air from one point to another, and such variations are directly related to density differences, normally associated with temperature gradients from one location to another. The observed effect of such random atmospheric deviations is an irregular distortion and motion of the image [511]." It is difficult to make a serious observation of Jupiter that will produce good data or images when seeing is poor.

Organizations like the A.L.P.O., B.A.A., and others have established arbitrary scales to estimate the seeing conditions during an observation. The A.L.P.O. uses a scale of 0-10 to indicate seeing conditions. Basically, the scale runs from 0, which is the worst possible seeing, to 10 which is perfect. Few of us ever see perfect seeing conditions. The A.L.P.O. scale gives some guidelines for these different levels of seeing and when useful work can be performed:

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