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Object

Null corrector

A circular object with a metallic rim and an orange-yellow triangular section at its center is shown against a black background.

An auxiliary optical device used during the manufacture of aspheric mirrors. It converts the complex curved wavefront expected from a finished mirror into a flat one that can be checked against a simple reference. The technique is extraordinarily sensitive — a correctly built null corrector can detect figure errors of a few nanometres — but it inherits any error in its own construction, as the Hubble programme discovered in 1990.

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