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:
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 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
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.
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
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.
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
Sky Quality Group (IAC, Canary Islands)
Top of DD research pages
Updated JD 2,455,775