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Quote:It's the temperature fluctuations in the air along the line of sight of the star light. The temperature change causes slight changes in refractive index (i.e. speed of light changes) bending the light a little. The turbulence makes the bending change quickly with time. Turbulence without temperature changes would not blur light. For example in the wake of a car traveling down the street, there is a lot of turbulence. Light is not blurred except close to the car exhaust with its hotter temperature.
Chuck D. Clawson, MI, USAProud owner of: Orion XT10i w/ Moonlite focuser & ZOC mirror, Celestron Ultima 8 PEC, Celestron C5+, Meade 90mm f/11 achromat (Model #390), and some random accessories (see bio).
Quote:Turbulence without temperature changes would not blur light. For example in the wake of a car traveling down the street, there is a lot of turbulence. Light is not blurred except close to the car exhaust with its hotter temperature.
Quote: Except during active convection, where city block to town-sized cells of heated air are rising like helium balloons, air motion is mostly horizontal, with a vertical component induced by shear.
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Quote:Quote:Turbulence without temperature changes would not blur light. For example in the wake of a car traveling down the street, there is a lot of turbulence. Light is not blurred except close to the car exhaust with its hotter temperature.I don't think that's quite correct. Turbulence induces localised changes in air pressure, and hence density and refractive index. This is how Schlieren photography works, and it's quite independent of changes in temperature.
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Quote:The concept of 'cells' in the discussion of seeing may be convenient for the discussion of relative scale, but it does not provide an understanding of the phenomenon. Indeed, it can be downright misleading, for it fosters a picture of the atmosphere organized into discrete bubble-like packets. The atmosphere is highly stratified, in both density and temperature. Except during active convection, where city block to town-sized cells of heated air are rising like helium balloons, air motion is mostly horizontal, with a vertical component induced by shear. Under most conditions it's the shear which imparts a vertical undulation to any one constant density surface. These undulations occur over a range of scales, and when considered over some (and especially the full) depth of the atmosphere being viewed through, the net effect could well be considered a fractal.
In any *physical* sense, the concept of 'seeing cells' is foreign to me. Except perhaps in very restricted conditions, such as when considering, for example, heat plumes rising from the observer. On an atmospheric scale, the awesome length of the column of air peered through can hardly allow for density variations perpendicular to the line of sight to be simplified to the point of being characterized as cells.
Quote:the use of the term is properly used to demonstrate that variations in density (primarily temperature driven) from location to location in the atmosphere is what causes the seeing effects. If there were no differences in density between locations in the atmosphere, there would be no seeing effects.
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Quote: This seems like the most convincing explanation so far, because it does not make the distorting diffraction effects underlying bad seeing dependent on either the enormous differential between the speed of turbulent air vs the speed of incoming light (much too small for the extent of effects observed) or the size differential between air movement and the wavelength of light (much too large for the extent of actual effects observed). Instead, at any given instant, it explains the atmospheric effect on light at any near-instantaneous moment as passing through a random jumble of imperfect lens elements (the density cells).
If there is turbulence there will be small differences in the index of refraction and as you surmise, it's like looking through a 60 mile long (or maybe much longer) lens that is never stable, never consistent.
It was a 20x power, 0.3° field of view telescope, sporting a singlet objective stopped down to 15 mm that profoundly changed our perspective on earth's place in the universe.
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