Verification and in situ calibration of large-aperture null correctors for convex aspheric mirrors

• Published in: Measurement : journal of the International Measurement Confederation 2017-08, Vol.106, p.79-87
• Main Authors: Xue, Shuai, Chen, Shanyong, Tian, Ye, Lu, Jinfeng, Hu, Hao
• Format: Article
• Language: English
• Subjects: Aperture; Calibration; Convex analysis; Convex mirrors; Interferometry; Mirrors; Null test; Statistics; Surface metrology
• ISSN: 0263-2241; 1873-412X
• DOI: 10.1016/j.measurement.2017.04.033

•The large-aperture null for convex aspheric mirror is in-situ calibrated by a wisely designed small-aperture null.•The presented method finally traces errors of a large-aperture null to small aperture flat and spherical surfaces.•Good material or high precision of the large-aperture null lens is not required; hence cost is reduced dramatically. Interferometric test of large convex aspheres with high accuracy is still an urgent problem. Adopting a large-aperture null corrector is usually inevitable, and it is imperative to certify the large-aperture null optics in case incorrect final shape of the aspheric mirror is obtained. Moreover, it is necessary to calibrate and remove errors of the large-aperture null lens and trace errors to surfaces that are easy to obtain high accuracy. For these purpose, this paper presents an in situ calibrated null test method. The large-aperture null corrector for convex aspheric mirror is verified and calibrated by a wisely designed small aperture certifying null whose surfaces are either flat or spherical which are easy to be fabricated, measured and assembled. A redundant test is implemented by a Zygo VeriFire Asphere interferometer for cross test. Compared with existing methods of certificating null correctors which utilize an expensive CGH or a self-aligning aspherical mirror, the presented method finally traces errors of a large-aperture null corrector to small aperture flat and spherical surfaces thus costs are expected to be saved dramatically.