Extremely compact secondary mirror unit for the SOFIA Telescope capable of 6-degree-of-freedom alignment plus chopping

Lorenzo Zago; Pierre M. Genequand; Joseph Moerschell

doi:10.1117/12.319276

25 August 1998 Extremely compact secondary mirror unit for the SOFIA Telescope capable of 6-degree-of-freedom alignment plus chopping

Lorenzo Zago, Pierre M. Genequand, Joseph Moerschell

Author Affiliations +

Proceedings Volume 3352, Advanced Technology Optical/IR Telescopes VI; (1998) https://doi.org/10.1117/12.319276
Event: Astronomical Telescopes and Instrumentation, 1998, Kona, HI, United States

Abstract

SOFIA is a 2.5-m telescope to be carried on a special Boeing 747 for airborne observations at about 15'000 m. The paper describes the main features of the secondary mirror unit. The SOFIA secondary mirror needs active control for alignment along five degrees of freedom as well as for very fast chopping with a frequency up to 20 Hz. Moreover the general optical concept and the housing of the telescope inside a Boeing 747 have required the design of a very compact mechanism: indeed while the secondary mirror has a diameter of 350 mm the entire height of the secondary mirror unit (including the mirror) cannot be greater than 300 mm, which makes the SOFIA design much more compact than any other similar project. The objective is achieved by a very tight integration between a novel hexapod mechanism, in charge of tilt offsets and alignment along 3 axes, and a fast chopping mechanism based on advanced flexure structure technology. In the hexapod mechanism (which is in fact capable of 6-dof), the six linear actuators are arranged in an original geometry in order to leave as much space as possible to the overlying chopping system. Also, the actuators' `hinges' are here materialized by flexure elements. Three motorized levers are linked by flexure elements to the mirror isostatic interface as well as to a reaction ring for compensating angular momentum, which is mechanically driven together with the mirror. This a major difference from other designs (e.g. Keck or VLT) where the compensation mass is driven and controlled separately. The SOFIA solution obtains thus various advantages in term of used volume and has a simpler control system. Various details of the chopping mechanism are provided in the paper. Simulation preliminary results are also given.

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Lorenzo Zago, Pierre M. Genequand, and Joseph Moerschell "Extremely compact secondary mirror unit for the SOFIA Telescope capable of 6-degree-of-freedom alignment plus chopping", Proc. SPIE 3352, Advanced Technology Optical/IR Telescopes VI, (25 August 1998); https://doi.org/10.1117/12.319276

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