Abstract
Adaptive optics provides a method for improving telescope imaging affected by atmospheric seeing distortions, but the differences in path length fluctuations through the atmosphere to two or more separate telescopes continues to limit the quality of stellar interferometry, and unfortunately is not ameliorated by adaptive optics. Some corrections to such fluctuations can be made by atmospheric density measurements near the ground, particularly since in some cases a substantial fraction of path length fluctuations occur in the atmosphere within 20-30 m of ground level. It is shown that more extensive corrections can be made by measurements of backscattered lidar radiation sent along the direction of the telescopes' observation. It is proposed that Rayleigh or Raman backscattering be used to measure changes in atmospheric density or index of refraction. Rayleigh scattering by molecules provides more intensity than does Raman and can allow path length corrections over distances of a few hundred meters to somewhat more than a kilometer with a fractional accuracy as good as 10-6 of the total atmospheric delay. Such measurements can substantially assist stellar interferometry. Details of how this might be done, and of likely errors and difficulties, are discussed quantitatively.
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