Cryogenic Derotator

Make a very accurate mechanism that can rotate in cryogenic environments

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We want to design, build and test 2 demonstrators for the METIS derotator. One that should learn us more about what is possible without active control, and another one for what is possible with active control.

Analyse the requirements

Design the proto-types and test setups

Build the proto-types

Test the prototypes and document the results.

Involved Partners
NOVA and CEA-Saclay


Due to the construction of the telescope and the way the light is fed into the instruments, the image of the sky (slowly) rotates on the detector during an observation.


Rotation of the field in the sky over the night. Picture taken from

When nothing will be done against this, it will result in smearing of the astronomical sources in your image and therefore reduction in sensitivity, both in intensity and in spatial information.


Long duration exposure resulting from the rotation of the Earth. Picture taken from

To keep the detector co-rotating is very difficult, therefore a solution that often is used is to either rotate the whole instrument, or build a special “derotator” inside the instrument. Such a derotator is a set of mirrors that nicely counter-rotate the image, so that it will remain stable on the detector. A derotator for METIS will have to fulfill some extreme requirements with respect to stability and operating conditions (in vacuum at about 80K). The goal of this project is two-fold. We want to design, build and test 2 demonstrators. One that should learn us more about what is possible without active control, and another one for what is possible with active control.

For both cases we start with a design of the test setup, including the inclusion of the bearings, mechanical structure and measurement systems to measure all wobbles and deviations. Afterwards the system will be built and tested, with of course special attention to the stability performances and control of the system. The results will be documented.

The active control set-up has been designed, built and tested at room temperature.

Test setup for the active control.

From these results it can be concluded that the required performance can be achieved. However this test-setup was only tested at room temperature, because the components were not yet suitable for a cryo-vacuum environment. However, this seems a problem that can be solved.


Results of the active test set-up.

For the passive system the project is delayed with respect to the original planning. Several concepts have been looked at on component level. There are two promising concepts. These concepts will be designed, build and tested, also in a representative environment.

Involved Partners

The work will be done by NOVA and CEA-Saclay, the French partner in METIS.

Recent Publications

"A Design Study of a Cryogenic High Accurate Derotator",
M.R. de Koe,
master thesis, Delft University of Technology, 2013



Active controled mechanism
Continuously checking if the correction applied is correct and when necessary apply a new correction.


The correction applied to overcome the rotation of your field of view (for a telescope on the earth this is the rotation of the Earth.

Passive mechanism
Mechanism that is applied but not continuously checked for its prformance. Manufacturing tolerances should make that unnecessary.

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