D. Dravins, L.Lindegren, E.Mezey & A.T.Young
ATMOSPHERIC INTENSITY SCINTILLATION OF STARS.
II. Dependence on Optical Wavelength
PASP 109, 725737 (1997)
Fig. 1. Wavelength dependence of
intensity variance sigma2, measured
with a 2.5 cm aperture at 400, 550 and 700 nm. The theoretically
expected slope of 7/6 is marked. The error bars are computed
from the full measurement sequence, which is somewhat conservative, since
part of the variations is not noise, but rather systematic changes
in the atmosphere.
Fig. 2. Autocorrelation functions
measured at 400 and 700 nm, for different telescope apertures. At
shorter optical wavelengths, the fluctuations are more rapid. The
effect is most pronounced for the smallest apertures, but could be followed
up to diameter 20 cm.
Fig. 3. Cross covariance between
intensity fluctuations at 400 and 700 nm, measured with a 20-cm diameter
aperture, and its zenith-angle dependence. Near the zenith the fluctuations
are simultaneous, but with increasing Z a time delay develops, seen
as a difference from the [symmetric] autocovariance function for 700 nm.
The effect is due to atmospheric dispersion, which stretches the "flying
shadows" into "flying spectra" on the ground. To enable a logarithmic
plot format, and show also the smaller detail, the quantity plotted is
the covariance plus a small number (epsilon = 0.01,
0.003, and 0.001 respectively); the actual zero-level is marked by dotted
lines in each panel.
Fig. 4. Cross correlations of atmospheric
intensity scintillation between different pairs of colors.
The time delays that develop at larger zenith angles depend upon the difference
in wavelength. Here, scintillation at 700 nm was successively cross
correlated with that simultaneously measured at 550, 400, and 365 nm.
With increasing wavelength difference, (a) the "agreement" (i.e. degree
of correlation) between scintillation in different colors decreases, and
(b) the time delay increases, visible as a shift of the correlation
maximum. In the violet, the dispersion of air changes rapidly with
wavelength, which explains the significant differences between the nearby
wavelengths of 365 and 400 nm. The functions were normalized to unity
for zero delay of the 700 nm autocorrelation, and the bold solid curves
show the relative power in the cross correlation. The thin solid
curves show the cross correlation normalized to unity at its maximum (similar
to the autocorrelation), thus more clearly revealing the magnitude of
Fig. 5. Cross covariance between intensity
fluctuations at 400 and 700 nm in the same region of the sky, and its dependence
upon aperture size. While the amount of temporal lag between colors
(Fig. 3) is a property of the atmosphere, and independent of the size of
the telescope, the magnitude of the cross covariance (solid curve) changes
with telescope size, and is most pronounced in small apertures (less
than about 5 cm). Dashed curves show the autocovariance at 400 nm;
dotted curves that at 700 nm. This wavelength difference largely
vanishes in greater apertures.
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