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

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