SCIDAR

SCIDAR, short for scintillation detection and ranging, is an optical remote-sensing technique used to characterize atmospheric turbulence by analyzing the scintillation (intensity) fluctuations of starlight as it passes through the Earth’s atmosphere. It provides vertical profiles of the refractive-index structure constant, Cn^2(h), and, in many implementations, wind-speed information as a function of altitude h, typically from near the ground to tens of kilometers above it. The data are especially valuable for understanding how turbulence degrades astronomical seeing and for informing adaptive optics systems.

Principle of operation is based on observing the scintillation patterns produced by a bright binary star with a telescope. The two stars’ light traverses turbulent layers at different angles, creating correlated intensity fluctuations in the detector image. By calculating the cross-covariance of the scintillation patterns (and how it shifts with time), one can invert the measurements to obtain the distribution of Cn^2 with altitude and, in some setups, the corresponding wind velocities. The technique effectively converts temporal fluctuations into a vertical turbulence profile.

Variants of SCIDAR include Generalized SCIDAR, which extends sensitivity to lower and higher altitudes by altering the measurement plane, and Lateral SCIDAR, which emphasizes near-ground turbulence. These methods can be complemented by time-series analysis to extract wind fields.

Applications span astronomical adaptive optics, site-testing and characterization of seeing, and atmospheric research. Limitations include the need for suitable bright binary stars, complex data analysis, and limited sky coverage due to stellar availability.