![]() ![]() The air's light-bending power, or refractive index, depends on its density and therefore its temperature. ![]() Such a change results from a temperature difference of just 0.2° Celsius.Īdd the miles of air that the light wave traverses before it even gets to the telescope, and it's a wonder that we can see any detail at all on objects above our atmosphere. ![]() But if the refractive power of the air down one part of the telescope tube differs from the rest by more than just one part in 1,600, the ¼-wave tolerance will be breached. In an ideal world the air would affect every part of a light wave equally. Clearly the air is an important optical element. But that same light wave, in traversing just three feet of air inside a telescope tube, is retarded by about 400 wavelengths compared to where it would be if the telescope contained a vacuum. The usual definition of an optically "good" telescope is one that keeps all parts of a light wave entering it nicely squared up to within quarter-wavelength accuracy by the time the wave comes to focus. Alan Adler took these pictures during two minutes with his 8-inch Newtonian reflector. These photos show the double star Zeta Aquarii (which has a separation of 2 arcseconds) being messed up by atmospheric seeing, which varies from moment to moment. ![]()
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