The Mid-infrared E-ELT Imager and Spectrograph, or METIS, is foreseen as an early 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 characterization of the fluctuating IR background
signal in post-processing. This paper discusses the development and characterization of the realized MCC demonstrator.
The chopper mirror (Ø64mm) should 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 introduces symmetry, and thus homogeneity in forces and heat flux.
In an earlier paper, Ref. , the design of the chopper and the breadboard level testing of the key components were
discussed. Since then, the chopper design has been revised to implement the lessons learned from the breadboard test and
a demonstrator has been realized. This demonstrator has undergone an elaborate test program for characterization and
performance validation in a cryogenic environment, as discussed in this paper.