|Description:|| METIS, the future mid-infrared instrument for the European-Extremely Large Telescope (E-ELT), has to observe the IR radiation from celestial objects through a "hot" atmosphere via a "hot" telescope. In order to prevent further heat contributions, the instrument (and detectors) itself will be cooled to 30-80K (10-40K). For obtaining images with good contrast, METIS relies critically on its capability to subtract the highly variable (thermal) background from the image to a high level of accuracy.|
The Cold Chopper is one of the components to make that possible. It allows the instrument to look at a source, and then quickly at a location close to the science object to measure the background. The efficiency of the observation depends on the time the system is locked (very stable) on its designated position. The transition time between the two positions should be as short as possible.
The chopper's stability and reproducibility is better than 1.7 micro-radians, with a maximum chop throw of 13.6 milli-radians. The transition time should be less than 5 ms!
The hardware of a demonstrator (see photo) consists of the opto-mechanical system, cryogenic voice coils (specially designed) and a laser interferometer metrology system to measure the position of the mirror. At SRON in Groningen, the chopper is now being tested cryogenically and integrated with its control system, which should provide the required accuracy and speed. The photo shows the chopper unit itself. The mirror in the centre is a fixed reference mirror. In the animation, the 2D character of this chopper is clearly illustrated.
The project is a collaboration between Janssen Precision Engineering B.V. in Maastricht, SRON in Groningen, the NOVA-ASTRON group, together with TNO acting as consultant. See also http://esfri.strw.leidenuniv.nl/index.php?option=com_content&view=article&id=56&Itemid=62