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13 September 2012 Development of a 2D precision cryogenic chopper for METIS
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The Mid-infrared E-ELT Imager and Spectrograph, or METIS, is foreseen as the third instrument for the European Extremely Large Telescope (E-ELT). A key part of METIS is the Cold Chopper (MCC) which switches the optical beam between the target and a nearby reference sky during observation for elimination of the fluctuating IR background signal in post-processing. This paper discusses the development of the MCC demonstrator. The chopper mirror (Ø64mm) has to tip/tilt in 2D with a combined angle of up to 13.6mrad with 1.7μrad stability and repeatability within 5ms (95% duty cycle at 5Hz) at 80K. As these requirements cannot be met in the presence of friction or backlash, the mirror is guided by a monolithically integrated flexure mechanism. The angular position is actuated by three linear actuators and measured by three linear position sensors, resulting in a fast tip, tilt, and focus mirror. Using the third actuator to introduce symmetry, homogeneity in forces and heat flux is obtained. Both the actuators and the sensors are key components. A voice coil actuator had to be custom designed, to achieve the required acceleration force within the specified 1W heat load. The requirements for the displacement measurement can be met with a commercially available, fiber interferometry system. For integration of this system, stray light elimination is a critical design aspect and retro-reflectors have been used to reflect sufficient power into the fiber at large tip/tilt angles.
© (2012) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Sander L. Paalvast, Huub Janssen, Maurice Teuwen, Robert Huisman, Bernhard Brandl, Frank Molster, and Lars Venema "Development of a 2D precision cryogenic chopper for METIS", Proc. SPIE 8450, Modern Technologies in Space- and Ground-based Telescopes and Instrumentation II, 845020 (13 September 2012);

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