An isoplanatic patch is an angular region over which approximately the same wavefront correction can improve images affected by atmospheric turbulence. Two sight lines separated by angle pass through positions separated by roughly at height , so a correction measured toward one star need not match another direction.
In adaptive optics, this makes the angular distance to a reference source physically relevant. Comparing corrected images at increasing angular separations tests the useful patch for that atmosphere and wavelength. The patch is a statement about similar aberrations, not about equal source brightness.
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