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What Underlying Physical Process Degrades Seeing?

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#1 FirstSight

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Posted 26 March 2013 - 02:02 PM

My question is deeper than the obvious macro-cause: various forms of atmospheric turbulence are responsible for degrading seeing (heat radiation, nonlaminar air flow, etc). The speed of light is 300,000 km/sec and even extremely turbulent air moving at a nonlaminar differential of 100 km/hr is still only about 0.028 km/sec, meaning that the speed of light is on the order of ten million times greater than the velocity of even very turbulent air. OTOH, the average wavelength of visible light (555 nm, somewhere in the green part of the spectrum), is on the order of a thousand times smaller than the displacement caused by very turbulent 100 km/hr air, but even with a very gentle to near stagnant 1 km/hr zephyr of wind, that's still on the order of ten times greater than the average wavelength of light. Also, these order of magnitude comparisons don't yet take into account the actual depth of the turbulent layer of air that light has to pass through, nor the fact that air which is relatively clean of aerosols is relatively transparent, although it does refract light (blue skies most of the day, reddish skies at dawn and dusk).

So is impaired seeing related to a disturbed form of refraction caused by the air movements? But if so, wouldn't that tend to induce shifts in the apparent color of stars and planets? How exactly does it cause bloating of star images and focus shift (on nights of poor seeing, more frequent focuser tweaking is often necessary to find best image).

#2 GlennLeDrew

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Posted 26 March 2013 - 02:13 PM

Think of air as a sometimes/often/nearly always poor quality optical element. Just as a static piece of poorly figured glass will 'bloat' an image point, so will the rather more dynamic air.

#3 MikeRatcliff

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Posted 26 March 2013 - 02:14 PM

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.

High altitude jet streams are associated with strong temperature differences between the colder north side and warmer south side. Likewise a hot mirror in a newtonian will generate heat bubbles coming off the mirror.

Mike

#4 David Knisely

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Posted 26 March 2013 - 02:22 PM

Seeing is mostly due to mild temperature variations and instability in rather small pockets of air. These change the mean density of the air over a small "cell" of air making the cell act as a sort of "lens" to distort the path of the incoming light over what an adjacent cell might do. The effect is a little like looking through a bad pane of glass, except that the bad "spots" in the glass are somewhat fluid and mobile. When temperature variations are small over a large area, the cells' density is fairly uniform from cell to cell (or the cells are very large) so seeing tends to be good. Areas like over large bodies of water with higher moisture content sometimes have really good seeing for this reason. When there is a lot of localized heating and convection, the cells can locally be notably different in density from each other, so the optical distortion from them tends to be greater and the resultant seeing is worse. Their movement also can result in more distortion of the light paths as they come in. It is a pretty complex subject, but it basically comes down to the slight light bending effect of small moving pockets of air that causes seeing. Clear skies to you.

#5 uniondrone

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Posted 26 March 2013 - 03:10 PM

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.


+1

It's all about refractive index difference between cold and warm air and the motion of air pockets with differing refractive indices.

#6 Jon Isaacs

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Posted 26 March 2013 - 03:25 PM

Chris:

I think others have supplied you with the basics. Here's a few things to remember:

Under ideal conditions, an 8 inch telescope is looking through a column of air that is 8 inches in diameter and about 60 miles long. When the light from anyone one star or point in the field enters the atmosphere, should be essentially perfect.

However it still has to travel through a column of air that is 8 inches in diameter and 60 miles long. And then some poor fool like you or me are going to look at image and try to magnify it to the theoretical limits of the optics were are using. That means if there are small variations in the way individual photons/waves/whatever travel through that column, we will see it at the eyepiece.

We are expecting very small levels of disturbance..

I like to think in terms of mirages... it's basically the same effect... Magnify a mirage about 250x and then be happy that Jupiter is near the zenith.

Jon

#7 GlennLeDrew

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Posted 26 March 2013 - 03:38 PM

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.

#8 PhilHerring

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Posted 26 March 2013 - 05:21 PM

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.

#9 Ed Holland

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Posted 26 March 2013 - 05:22 PM

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.


One can get a sense of this horizontal shear motion sometimes by defocusing a telescope aimed at a bright object like Jupiter.

Ed

#10 MikeRatcliff

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Posted 26 March 2013 - 06:35 PM

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.


Right, I was referring to lower air speeds where density fluctuations are not zero but pretty small. Schlieren works on density differences, which can be caused by temperatures, shock waves, differing material molecular weights, etc...

#11 jpcannavo

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Posted 26 March 2013 - 09:03 PM

An excellent reference:
http://www.handprint...RO/seeing1.html

#12 David Knisely

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Posted 27 March 2013 - 03:11 AM

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.


The cell size varies widely. The cells can be very small or the size of a small city block, but 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. The air parcels of differing density can be above or below each other or existing laterally and moved by the local air currents. They can be twisted or irregular or rising or falling, expanding or being disrupted, but the density variations cause changes in the local index of refraction along the path of the incoming light, causing the light to deviate from a theoretical straight-line path to the observer. The presence of particulate matter or aerosols in the air can scatter light and reduce contrast as well, but the diversion of the photons by small-scale refraction is probably the main dynamic blurring effect we see at the eyepiece. This is what primarily causes seeing, plain and simple. Clear skies to you.

#13 FirstSight

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Posted 27 March 2013 - 08:39 AM

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.


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). This also satisfactorily explains why color (wavelengths) themselves only tend to get noticeably distored when light from objects is passing through the atmosphere at very low angles; Jupiter and Venus display a near-kalideoscopic rainbow of distored colors when viewed only a couple of degrees above the horizon out over the ocean, but not (even in very bad seeing) when sixty degrees up.

#14 t.r.

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Posted 27 March 2013 - 09:09 AM

It truly is a wonder that we can see any appreciable detail of the planets at all for all the atmosphere's complexity!

#15 Jon Isaacs

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Posted 27 March 2013 - 10:25 AM

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 you are trying to think of it as some sort of doppler-like effect dependent on velocity differentials, that doesn't work.

It's probably better to think of it as a form Schlieren imaging or shadowgraphing that image can capture small differences in the index of refraction of the medium.

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.

Jon

#16 FirstSight

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Posted 27 March 2013 - 11:34 AM

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.


This is exactly the model I had in mind, in light of the informative posts so far in this thread. The problem I initially had coming into this thread was that there are troubling issues with modeling seeing as predominately frequency or velocity distortions, because of the hugely disproportionate relative scales involved. A model based on refraction induced by differential air density pockets (or cells) OTOH creates what, at any given instant, can be considered a random set of imperfect static lenses; even though these fluctuate quite a bit within any humanly perceivable time scale (thus we sometimes see not merely bloating or distortion, but twinkling), for purposes relative to light-speed, the set of lenses any given photon passes through are virtually static.

At least that's what I take from the thoughtful responses to this thread so far. Further explanations and corrections (or viable alternatives) to this line of thought are welcome.

#17 JKoelman

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Posted 27 March 2013 - 12:47 PM

The physics is straightforward: light follows the fastest path from the star to your telescope. And while the speed of light in vacuum is constant, the speed of light in air depends somewhat on the air density and air temperature. These are tiny effects, but enough for turbulent variations in density (and temperature) to cause slightly different instantaneous deflections in nearby light paths. Not by much, but by an amount that is sufficient to cause a blur in aperture-averaged images.

Adaptive optics corrects for such blur, using high speed opto-mechanical corrections across the full aperture of the telescope.

#18 De Lorme

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Posted 28 March 2013 - 12:14 AM

Johannes, For the simple of minds{me} could you explain futher about how to use adaptive optics and high speed
opt-mechanical corrections with the telescope. Or you could point me in the right direction. Thanks for the pateince
in explaning this.
What kind of experiments could we do to not control the
atmosphere but rather work with it.

Clear skies, De Lorme

#19 JKoelman

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Posted 28 March 2013 - 10:20 AM

Enjoy!

#20 De Lorme

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Posted 28 March 2013 - 01:15 PM

Thanks Johannes! That was really interesting!. De Lorme

#21 csrlice12

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Posted 30 March 2013 - 09:32 AM

We could all breathe deep at the same time.....less air, better seeing! :lol:






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