Optical metrology for testing an all-composite 2-m-diameter mirror

Brian E. Catanzaro; James A. Thomas; Donald W. Small; Roger A. Johnston; Daniel D. Barber; Steven J. Connell; Shaun A. Whitmore; Eri J. Cohen

doi:10.1117/12.447303

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5 November 2001 Optical metrology for testing an all-composite 2-m-diameter mirror

Brian E. Catanzaro, James A. Thomas, Donald W. Small, Roger A. Johnston, Daniel D. Barber, Steven J. Connell, Shaun A. Whitmore, Eri J. Cohen

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Proceedings Volume 4444, Optomechanical Design and Engineering 2001; (2001) https://doi.org/10.1117/12.447303
Event: International Symposium on Optical Science and Technology, 2001, San Diego, CA, United States

Abstract

The Herschel Space Observatory (formerly known as FIRST) consists of a 3.5 m space telescope designed for use in the long IR and sub-millimeter wavebands. To demonstrate the viability of a carbon fiber composite telescope for this application, Composite Optics Incorporated (COI) manufactured a fast (F/1), large (2 m), lightweight (10.1 kg/m2) demonstration mirror. A key challenge in demonstrating the performance of this novel mirror was to characterize the surface accuracy at cryogenic (70 K) temperatures. A wide variety of optical metrology techniques were investigated and a brief survey of empirical test results and limitations of the various techniques will be presented in this paper. Two complementary infrared (IR) techniques operating at a wavelength of 10.6 microns were chosen for further development: (1) IR Twyman-Green Phase Shifting Interferometry (IR PSI) and (2) IR Shack-Hartmann (IR SH) Wavefront Sensing. Innovative design modifications made to an existing IR PSI to achieve high-resolution, scannable, infrared measurements of the composite mirror are described. The modified interferometer was capable of measuring surface gradients larger than 350 microradians. The design and results of measurements made with a custom-built IR SH Wavefront Sensor operating at 10.6 microns are also presented. A compact experimental setup permitting simultaneous operation of both the IR PSI and IR SH tools is shown. The advantages and the limitations of the two key IR metrology tools are discussed.

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Brian E. Catanzaro, James A. Thomas, Donald W. Small, Roger A. Johnston, Daniel D. Barber, Steven J. Connell, Shaun A. Whitmore, and Eri J. Cohen "Optical metrology for testing an all-composite 2-m-diameter mirror", Proc. SPIE 4444, Optomechanical Design and Engineering 2001, (5 November 2001); https://doi.org/10.1117/12.447303

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