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

Abstract

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.

PSST: The Planet Search Survey Telescope

“…For the bright, high-S/N stars, which are of primary interest to us, the dominant noise sources are scintillation and shot (Poisson) noise from the target star itself. The contribution of scintillation was estimated using the equations in Dravins et al (1998) for an average air mass of 1.5. For fainter stars, sky photon noise is the dominant source of scatter, which is not surprising given the large area of the PSST pixels ( ).…”

“…One weakness of the wide-field, smallaperture approach to transit searches, however, is that the photometry is more affected by differential extinction across the field, making it more difficult to achieve the requisite differential photometric precision. Scintillation is also a significant noise source for a small system (Dravins et al 1998) and can be the dominant contributor for the brightest stars.…”

Stratospheric Observatory For Infrared Astronomy (SOFIA)

“…Mayor & Queloz 1995) and by larger field searches such as Kepler (Koch et al 1998 Becklin & Moon 1) Scintillation noise will be very low with HIPO. Extrapolating a commonly used ground based model (Dravins et al 1998) to stratospheric altitudes indicates a noise floor of 4x10-5 in 15 minute integration. The fact that SOFIA operates above the tropopause suggests that this model will break down, further suggesting that there will be in fact less scintillation noise (Sandell et al 2003) 2) Use of FLITECAM with HIPO, allows observations of the entire wavelength range from 0.3 to 5 J-Lm.…”

Scintillation correction for astronomical photometry on large and extremely large telescopes with tomographic atmospheric reconstruction

“…The scintillation index can be calculated from atmospheric and telescope parameters as (Dravins et al 1998) …”