ATMOSPHERIC  SCINTILLATION


The Earth's turbulent atmosphere causes stars to "twinkle", i.e. to undergo rapid fluctuations in their intensity.  Already simple optical effects lead to image distortions (seeing), while only [differential] effects of higher order also cause the intensity to be modulated (scintillation). 
Scintillation studies

  Stellar intensity scintillation in the optical has been extensively studied at the Roque de los Muchachos Observatory on La Palma (Canary Islands), with measurements throughout some 25 full nights, during different seasons of year.

Figure: This drawing shows the front of the 60 cm telescope equipped with a rotatable aperture mask, permitting measurements through one or multiple openings of different size and shape.

  Our studies include:


   Flying shadows

  Short-exposure images of a telescope mirror illuminated by a bright star reveal rapidly moving "shadows".  With the unaided eye, such "flying shadows" can be glimpsed before and after a solar eclipse, when an uneclipsed solar crescent acts as the light source.  Then the "shadows" appear as elongated "bands" because of the brightness distribution of the solar crescent (shadow patterns from stars are isotropic).  Their motions are determined by wind components at various altitudes.


   Temporal statistics

  Temporal intensity variations occur because atmospheric winds carry the [intrinsically changing] shadow pattern across the telescope.  While the short-term statistics closely follow log-normal distributions, the longer-term changes between time of night and seasons of year reflect systematic weather changes.


   Color dependence

  Scintillation depends on wavelength.  For small apertures, the "flying shadows" on the Earth's surface are resolved. Here the fluctuations are more rapid (and have a greater amplitude) in the blue than in the red.  In large telescopes, these color differences nearly vanish.

  At zenith, the fluctuations in different colors are simultaneous, but shift out of phase with increasing zenith distance, due to atmospheric chromatic dispersion.  This timelag is independent of telescope size, and is one scintillation property that remains unchanged also in very large telescopes.


   Different telescopes

  Apertures of different size, shape, and central obscuration sample different portions of the flying-shadow patterns, and cause different scintillation properties.  Scintillation amplitude decreases in larger telescopes, while the most rapid scintillation components are especially reduced by apodized apertures.


   Eliminating scintillation?

  An adequate understanding of scintillation will enable second-order adaptive optics, correcting not only phase errors in the wavefront, but also its amplitude.  Such scintillation corrections will be required for the most critical ground-based observations, such as the direct imaging of exoplanets.

  Scintillation has a different dependence on atmospheric turbulence than [ordinary] seeing.  Special site selection (possibly Antarctica) might identify sites with a greatly reduced scintillation, such as probably required for ground-based studies of stellar non-radial oscillations in brightness.


   History of scintillation studies

  Scintillation was one of the earliest celestial phenomena studied by man.  Its cause has been sought for millennia, and not until the 17th century was its atmospheric origin realized.


   External links

  Astroclimatology (ESO)
  Sky Quality Group (IAC, Canary Islands)


Comments are welcome to dainis@astro.lu.se

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Updated JD 2,455,775