This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: In a new study published in Measurement, researchers from the Changchun Institute of Optics, Fine Mechanics and Physics of the Chinese Academy of Sciences have developed a calibration method that combines machine vision with interferometry. This method separates a mirror's fixed internal offset from changes in the test path alignment.
When applied to a four-meter-class monolithic mirror, the method produced a combined standard uncertainty of 31 micrometers. Large optical systems increasingly use monolithic mirrors that integrate several reflecting surfaces into a single component. When the primary and tertiary surfaces are manufactured on the same substrate, their relative position is fixed during fabrication.
However, a lateral displacement between their optical axes cannot be corrected later by adjusting separate mounts and can reduce imaging performance. Therefore, the offset needs to be measured during polishing and final inspection while the results can still guide the manufacturing process. Since the two surfaces share one structure, the error belongs to the mirror itself rather than an adjustable assembly.
A reliable measurement helps engineers assess polishing progress and determine if further correction is needed before completing the mirror. The central challenge is that interferometric testing responds to two different sources of error. One is the intrinsic offset between the mirror surfaces.
The other is a temporary lateral shift in the test beam when the interferometer is aligned with the mirror. Both sources of error can produce similar coma in the measured wavefront, so optical data alone cannot distinguish between them reliably. Additionally, conventional approaches may transfer measurement references through several mechanical stages, which allows small alignment errors to accumulate.
Repeated realignment can also make it harder to compare results from different manufacturing stages. To create an independent geometric reference, the researchers arranged four industrial cameras around the mirror along two orthogonal directions. The cameras observed the relationship between the crosshair projected by the interferometer and the mirror's reference features.
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