New capabilities for laser holographic testing during assembly and collimation of large segmented telescope mirrors

Abstract

Subject of study. Several novel options for interferometric testing of large aspheric segmented primary telescope mirrors during all phases of construction (assembly, collimation, and performance assessment) are proposed. In this paper, we will describe and validate new engineering design solutions for interferometric surface testing of large concave aspheric segmented telescope mirrors. Methods. All proposed test configurations are based on a reflecting optical compensator in the form of an axial synthetic holographic optical element or a reflective convex aspheric surface of revolution. We propose a quasi-autocollimative light path in which an optical compensator reverses the object wavefront; however, the object wavefront itself does not directly produce the image of the surface being tested in the plane where the interference and shadow patterns are recorded, thereby eliminating significant distortion-like errors. Main results. We provide the calculated parameters for quasi-autocollimative systems used to test the surfaces of the aspheric concave primary mirror segments of four well-known world-class telescopes: the Millimetron telescope, diameter 10 m; the James Webb Space Telescope, diameter 6.5 m; the Extremely Large Telescope (ELT), diameter 39.3 m; and the postponed European Southern Observatory conceptual design known as the Overwhelmingly Large Telescope (OLT), diameter 100 m. We show that simultaneous use of a series of similar coaxial optical compensators on the object leg of the interferometer will almost completely eliminate any restrictions related to the size, asphericity, and curvature of aspheric segments of primary mirrors in telescopes. Using a conical substrate for the working surface of the holographic compensator will substantially reduce the maximum spatial frequency of the compensator. The calculations were performed using Mathcad and Zemax. We propose that traditional non-autocollimative laser holographic test configurations without wavefront reversal be used on the object leg during the initial assembly of the primary mirror segments. Laser holographic topography is also suitable for this purpose. Practical significance. The novel ideas, methods, and systems engineering solutions proposed here are based on wavefront reversal by a reflecting optical compensator or a series (“chain”) of such compensators and will support in-process testing and certification testing of aspheric primary mirror segments, with interferometric accuracy, for any of the optical telescopes currently operating or under development, whether ground based or space based. These capabilities will support full-scale interferometric testing of the 12 m diameter Millimetron aspheric segmented primary mirror initially proposed by Academician N. S. Kardashev, but will also undoubtedly be especially important for space-based telescopes.