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