Measurement of the size of the isoplanatic patch using a phase-correcting telescope

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Abstract
In the presence of several-arc-second seeing at Mt. Wilson observatory, a flexible-mirror image-sharpening telescope produced diffraction-limited (0.5 arc s) images of the primary stars in the double star systems of Castor (α Gem), Algieba (γ Leo) and Almach (γ And). The images of both the primary and the companion star were simultaneously sharpened for Castor (separation 2 arc s) and Algieba (4 arc s) but not for Almach (10 arc s). Thus the size of the isoplanatic patch lay between 4 and 10 arc s. Using a simple model, we conclude that the bulk of the turbulent air responsible for the seeing lay between 1.1 and 1.7 km above ground.