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Proceedings Paper

Fabrication and testing of a high-precision concave spherical mirror
Author(s): Jan Burke; Katie Green; Wayne Stuart; Edita Puhanic; Achim Leistner; Bob Oreb
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Paper Abstract

CSIRO's Australian Centre for Precision Optics has recently finished the production of a high-precision concave spherical mirror. The specifications were very ambitious: numerical aperture 0.75; asphericity below 5.5 nm rms and 27.3 nm P-V. The available reference transmission sphere had to be calibrated to enable adequate accuracy. Due to the high numerical aperture of the mirror, sub-aperture measurements had to be stitched together to form a complete surface map of the mirror. Phase-shifting interferometry at high numerical aperture suffers from phase-step non-uniformity because of the large off-axis angles. We present what we believe to be a new interpretation of this phenomenon as a focus error, which clarifies where in the interferometer the phase-shift error occurs. We discuss the ball-averaging method for calibrating the reference transmission sphere and present results from the averaging process to ensure an uncertainty commensurate with the certification requirement. For carrying out the sub-aperture measurements, we constructed a two-axis gimbal mount to swivel the mirror around the focus of the test wavefront. If the centers of curvature of the transmission sphere and the mirror coincide, the mirror can be tilted without losing the interferogram. We present a simple and effective alignment method, which can be generally applied to optical tests where the wavefront comes to a focus. The mirror was coated with protected aluminum and tested in its mount. No effect on the sphericity error from the coating was found, and the specifications were exceeded by approximately 30%. We discuss subtleties of the stitching process on curved surfaces and report final results.

Paper Details

Date Published: 11 August 2008
PDF: 10 pages
Proc. SPIE 7064, Interferometry XIV: Applications, 70640E (11 August 2008); doi: 10.1117/12.793522
Show Author Affiliations
Jan Burke, CSIRO Materials Science and Engineering, Australian Ctr. for Precision Optics (Australia)
Katie Green, CSIRO Materials Science and Engineering, Australian Ctr. for Precision Optics (Australia)
Wayne Stuart, CSIRO Materials Science and Engineering, Australian Ctr. for Precision Optics (Australia)
Edita Puhanic, CSIRO Materials Science and Engineering, Australian Ctr. for Precision Optics (Australia)
Achim Leistner, CSIRO Materials Science and Engineering, Australian Ctr. for Precision Optics (Australia)
Bob Oreb, CSIRO Materials Science and Engineering, Australian Ctr. for Precision Optics (Australia)


Published in SPIE Proceedings Vol. 7064:
Interferometry XIV: Applications
Erik L. Novak; Wolfgang Osten; Christophe Gorecki, Editor(s)

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