Atmospheric distortion

Atmospheric distortion is responsible for poor seeing, reddening, extinction and the adding of absorption lines to stellar spectra. To diminuish the effect of atmosphere, large telescopes are built on tall mountains or put in space....
Let's see in details the different kinds of atmospheric distortions.

Seeing

The air of the atmosphere is constantly in turbulent motion.
Light from a star is refracted in random directions over time periods of tens of milliseconds, by rapidly-moving pockets of air of varying densities and temperatures. For this reason images twinkle and are blurred to our eyes. This atmospheric effect is called seeing.
The highly magnified view of a telescope shows images of the star that dance about many times a second. For this reason, a longer exposure picture will show a fuzzy blob (called the ``seeing disk'') which is the size of the entire distribution of dancing images...

 


The effect of atmospheric distortion depends on the conditions. When the air is stable (little turbulence) we have good seeing and the twinkling is small. Details as small as 0.5 arc seconds can be seen when the seeing is good (still much larger than the theoretical resolving power of large research telescopes). Poor seeing happens when the air is turbulent so the images dance about and details smaller than 2 to 3 arc seconds cannot be seen.
A first solution to this problem is to locate telescopes on very high mountains or in space, since the more atmosphere there is above a telescope, the greater is the turbulent motion and the poorer is the seeing.
Another way to solve this problem can be speckle interferometry which can get rid of atmospheric distortion by taking many fast exposures of an object and using computers to shift them to a common center and process them to removes other noise and distortions caused by the atmosphere, telescope, and electronics...
Another technique called adaptive optics makes quick changes in the light path of the optics to compensate for the atmospheric turbulence. Before the focussed light from the objective reaches the camera, it bounces off a thin deformable mirror that can be adjusted thousands of times a second to reposition the multiple images back to the center.


Reddening and Extinction

All wavelengths (see technical card about light: wavelength) are scattered or absorbed by some amount. This effect is called extinction. Some wavelength suffer more extinction than others. For example, the water vapor in the air (and carbon dioxide in a lesser amount) absorb much of the infrared energy from space, and for this reason, some of the infrared band can only be observed from mountains above 2750 meters elevation...

In general, redder (longer wavelength) light is scattered less by atmosphere molecules and dust than bluer (shorter wavelength) light. This effect, known as reddening, is the main reason why the Sun appears orange or red close to the horizon (since in this position, the light coming from the Sun has to travel through a larger portion of atmosphere and a larger part of the blue radiation is reflected off by air).


Atmospheric lines

Gases in the Earth's atmosphere can introduce extra absorption lines into the spectra of celestial objects which have to be removed from the spectroscopy data..

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