An Etymological Dictionary of Astronomy and Astrophysics
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In an imaging system, the dependence of its performance on field angle. In other words, the spatial variability of the → point spread function for off-axis sources.

→ an- + → isoplanatism.

In an imaging system, the unvarying of the → point spread function over an extended field of view.

From isoplanat(ic), → isoplanatic, + → -ism.