Scintillation - wavelength dependence

   Main publication

  D. Dravins,  L.Lindegren,  E.Mezey  &  A.T.Young

II.  Dependence on Optical Wavelength
PASP  109, 725­737 (1997)

ABSTRACT.  Atmospheric intensity scintillation of stars on milli- and microsecond time scales was extensively measured at the astronomical observatory on La Palma (Canary Islands).  Scintillation statistics and temporal changes were discussed in Paper I, while this paper shows how scintillation depends on optical wavelength.  Such effects originate from the changing refractive index of air, and from wavelength-dependent diffraction in atmospheric inhomogeneities.  The intensity variance sigma2 increases for shorter wavelengths, at small zenith distances approximately consistent with a theoretical lambda­7/6 slope, but with a tendency for a somewhat weaker dependence.  Scintillation in the blue is more rapid than in the red.  The increase with wavelength of autocorrelation time scales (roughly proportional to sqrt(lambda) is most pronounced in very small apertures, but was measured up to diameter 20 cm.  Scintillation at different wavelengths is not simultaneous: atmospheric chromatic dispersion stretches the atmospherically induced "flying shadows" into "flying spectra" on the ground.  As the "shadows" fly past the telescope aperture, a time delay appears between fluctuations at different wavelengths whenever the turbulence-carrying winds have components parallel to the direction of dispersion.  These effects increase with zenith distance (reaching approx. 100 ms cross-correlation delay between blue and red at Z = 60 degrees ), and also with increased wavelength difference.  This time delay between scintillation in different colors is a property of the atmospheric flying shadows, and thus a property that remains unchanged even in very large telescopes.  However, the wavelength dependence of scintillation amplitude and time scale is "fully" developed only in small telescope apertures (less than about 5 cm), the scales where the "flying shadows" on the Earth's surface become resolved.  Although these dependences rapidly vanish after averaging in larger apertures, an understanding of chromatic effects may still be needed for the most accurate photometric measurements.  These will probably require a sampling of the [stellar] signal with full spatial, temporal and chromatic resolution to segregate the scintillation signatures from those of astrophysical variability.

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
time displacement.

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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