Atmospheric Intensity Scintillation of Stars. III. Effects for Different Telescope Apertures

Abstract

ABSTRACTStellar 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(λ) may be segregated from the scintillation component ΔI(t,λ,x,y) in 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 (≲10 ms) photometry on the flying shadows over the spatially resolved (≲10 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.