ABSTRACT. Stellar intensity scintillation in the optical was extensively studied at the astronomical observatory on La Palma (Canary Islands). Atmospheric turbulence causes "flying shadows" on the ground, and intensity fluctuations occur both because this pattern is carried by winds and is intrinsically changing. Temporal statistics and time changes were treated in Paper I, and the dependence on optical wavelength in Paper II. This paper discusses the structure of these flying shadows and analyzes the scintillation signals recorded in telescopes of different size and with different (secondary-mirror) obscurations. Using scintillation theory, a sequence of power spectra measured for smaller apertures is extrapolated up to very large (8 m) telescopes. Apodized apertures (with a gradual transmission falloff near the edges) are experimentally tested and modeled for suppressing the most rapid scintillation components. Double apertures determine the speed and direction of the flying shadows. Challenging photometry tasks (e.g., stellar microvariability) require methods for decreasing the scintillation "noise." The true source intensity I(l) may be segregated from the scintillation component in DI(t,l,x,y) postdetection computation, using physical modeling of the temporal, chromatic, and spatial properties of scintillation, rather than treating it as mere "noise." Such a scheme ideally requires multicolor high-speed (Շ10 ms) photometry on the flying shadows over the spatially resolved (Շ10 cm) telescope entrance pupil. Adaptive correction in real time of the two-dimensional intensity excursions across the telescope pupil also appears feasible, but would probably not offer photometric precision. However, such "second-order" adaptive optics, correcting not only the wavefront phase but also scintillation effects, is required for other critical tasks such as the direct imaging of extrasolar planets with large ground-based telescopes.

Stellar intensity scintillation in the optical was extensively studied at the astronomical observatory on La Palma (Canary Islands). Photon-counting detectors and digital signal processors recorded temporal auto-and cross-correlation functions, power spectra, and probability distributions. This first paper of a series treats the temporal properties of scintillation, ranging from microseconds to seasons of year. Previous studies, and the mechanisms producing scintillation are reviewed. Atmospheric turbulence causes 'flying shadows' on the ground, and intensity fluctuations occur both because this pattern is carried by winds, and is intrinsically changing. On very short time scales, a break in the correlation functions around 300 ¡¿s may be a signature of an inner scale (=3 mm in the shadow pattern at wind speeds of 10 m s -1 ). On millisecond time scales, the autocorrelation halfwidth decreases for smaller telescope apertures until -5 cm, when the 'flying shadows' become resolved. During any night, time scales and amplitudes evolve on scales of tens of minutes. In good summer conditions, the flying-shadow patterns are sufficiently regular and long-lived to show anti-correlation dips in autocorrelation functions, which in winter are smeared out by apparent wind shear. Recordings of intensity variance together with stellar speckle images suggest some correlation between good (angular) seeing and large scintillation. Near zenith, the temporal statistics (with up to twelfth-order moments measured) is best fitted by a Beta distribution of the second kind (F-distribution), although it is well approximated by log-normal functions, evolving with time.

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 aj increases for shorter wavelengths, at small zenith distances approximately consistent with a theoretical X -7/6 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 >/\) is most pronounced in very small apertures, but was measured up to 0 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 -100 ms cross-correlation delay between blue and red at Z = 60° ), 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 ( ^ 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.

In the paper "Atmospheric Intensity Scintillation of Stars. III. Effects for Different Telescope Apertures" by Dainis Dravins, Lennart Lindegren, Eva Mezey, and Andrew T. Young (PASP, 110, 610 [1998]), there is a typographical error on page 625, column (2), 17 lines from bottom. The expression giving the frequencies for which the previous equation ( 10) is valid has the superfluous characters "3D" on its right-hand side, which thus should read only "1 Hz." The error was caused in proof stage from inconsistencies in e-mail sending and receiving "standards.

No abstract