Air turbulence
A constant turbulence in the lower earth atmosphere is referred to as air turbulence , which is particularly noticeable when observing celestial bodies with larger telescopes . In addition, optical communication links between the ground station and space satellite also suffer from the same optical turbulence caused by the atmosphere.
In cold winter nights, the optical effects of atmospheric turbulence are freiäugig often significantly as a "twinkling of the stars" detectable. Stars near the horizon sparkle more strongly than those in the zenith , because the starlight on the horizon has to travel a much longer path in the atmosphere than at the zenith and thus experiences fewer optical disturbances in the direction of the zenith. In physical optics , the effect of the sparkle is called scintillation .
In addition a - usually only in telescope ascertainable - image motion of blurred (in English blurring called) Sternbildchens or star patch. This back and forth dancing takes place in the range of a few tenths of a second and in extreme cases can reach 5–7 "(0.002 °). All three effects (scintillation, image movement, image blurring) are summarized in astronomy under the term seeing .
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Formation of streaks due to heating is also called heat shimmer . In photography , the phenomenon occurs particularly when using long focal lengths . This can result in image blurring or actually straight edges are shown as wavy lines. Similar effects often occur in tunnel construction , when the measuring line has to run close to the tunnel wall in curves. This can be in small directional error arcsec area cause that the Markscheider as Seitenrefraktion referred.
causes
The air turbulence is primarily caused by temperature and pressure differences in the earth's atmosphere, which are mainly caused by the uneven warming of the earth's surface by the sun. By convection can laminar flows, but also turbulent hurricanes or jet streams emerge. The turbulent mixing of the atmosphere changes the refractive index and thus directly the optical properties of the atmosphere.
Often there are further disruptions if the light beam - for example in the vicinity of an observatory - sweeps past too slightly above or to the side of a warmer object ( house roof , chimney , rocks heated during the day , waste heat from engines, etc.). The construction of the observatory itself can also cause a slight deflection of light or a flicker if the dome and its interior are even warmer than the outside evening air.
Planets without flicker
When observing planets with free eyes, in contrast to stars, there is hardly any flickering ( scintillation ) to be seen - which can also be used as a marker for planets. The reason is that stars, due to their distance from Earth of more than 4 light years, only produce very small luminous disks at the level of the tropopause. For example, Betelgeuse can only generate an image (light disk) with a diameter of a few millimeters with an angular expansion of approx. 50 milli-arcseconds at a height of 10 km. The eye cannot focus at this height, so a cylindrical beam with the diameter determined by the size of the pupil of the eye is thrown onto the Netzhaus. In the dark, the pupil opening can be between 4 mm and 9 mm (see “ The human eye in numbers ”). Since the scintillation pattern generated by the earth's atmosphere varies precisely on such scales every millisecond, i.e. in the mm to cm range between light and dark, the viewer sees the stars twinkle.
When looking at Jupiter with a maximum extension of approx. 45 arcseconds, on the other hand, a cone-shaped light beam hits the eye, which corresponds to a 2.25 m diameter luminous disc even at a height of 10 km (tropopause). This image, which is still largely undisturbed by the atmosphere at a height of 10 km and illuminated by Jupiter, also generates a scintillation pattern at the observer's location. Compared to the constellation, about 60,000 (with a 9 mm eye pupil) to 300,000 (with a 4 mm eye pupil) overlap in the eye. The twinkling effect is thus significantly reduced and can hardly be detected.
As a rule, humans can only perceive objects with an angular expansion of less than 60 arc seconds as points without expansion.
