Active thermal figure control for the TOPS II primary mirror

Roger Angel; Tae Kang; Brian Cuerden; Olivier Guyon; Phil Stahl

doi:10.1117/12.734930

19 September 2007 Active thermal figure control for the TOPS II primary mirror

Roger Angel, Tae Kang, Brian Cuerden, Olivier Guyon, Phil Stahl

Proceedings Volume 6693, Techniques and Instrumentation for Detection of Exoplanets III; 669313 (2007) https://doi.org/10.1117/12.734930
Event: Optical Engineering + Applications, 2007, San Diego, California, United States

Abstract

TOPS (Telescope to Observe Planetary Systems) is the first coronagraphic telescope concept designed specifically to take advantage of Guyon's method of Phase Induced Amplitude Apodization PIAA).¹ The TOPS primary mirror may incorporates active figure control to help achieve the desired wavefront control to approximately 1 angstrom RMS accurate across the spectral bandwidth. Direct correction of the primary figure avoids the need for a separate small deformable mirror. Because of Fresnel propagation, correction at a separate surface can introduce serious chromatic errors unless it is precisely conjugated to the primary. Active primary control also reduces complexity and mass and increases system throughput, and will likely enable a full system test to the 10-10 level in the 1 g environment before launch. We plan to use thermal actuators with no mechanical disturbance, using radiative heating or cooling fingers distributed inside the cells of a honeycomb mirror. The glass would have very small but finite coefficient of expansion of ~ 5x10^-8/C. Low order modes would be controlled by front-to-back gradients and high order modes by local rib expansion and contraction. Finite element models indicate that for a mirror with n cells up to n Zernike modes can be corrected to better than 90% fidelity, with still higher accuracy for the lower modes. An initial demonstration has been made with a borosilicate honeycomb mirror. Interferometric measurements show a single cell influence function with 300 nm stroke and ~5 minute time constant.

Citation Download Citation

Roger Angel, Tae Kang, Brian Cuerden, Olivier Guyon, and Phil Stahl "Active thermal figure control for the TOPS II primary mirror", Proc. SPIE 6693, Techniques and Instrumentation for Detection of Exoplanets III, 669313 (19 September 2007); https://doi.org/10.1117/12.734930

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