What instrument, intended to verify the mirror's shape, was installed incorrectly during ground testing?

Answer

Null corrector

The critical failure that allowed the flawed mirror to pass ground inspection was the incorrect installation of the null corrector. This specialized optical device is designed to compensate for the curvature of the mirror being tested, allowing technicians to measure the surface against a reference standard representing the ideal shape. The error occurred because the null corrector was mounted approximately 1.3 millimeters too close to the mirror surface. This misalignment caused the testing software and technicians to receive and process readings indicating the mirror was perfectly formed when, in reality, the measuring system itself was reading a slightly distorted profile, thereby locking in the incorrect specifications for the final polishing stages.

What instrument, intended to verify the mirror's shape, was installed incorrectly during ground testing?

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