Design of a beam switch for the mm-wave receiver at Onsala Space Observatory

Abstract

Ground-based astronomical observations at millimetre wavelengths are very challenging due to the strong influence of the rapidly fluctuating water content in the Earth's atmosphere. Calibration to compensate for atmospheric disturbances and also to the receiver’s limited stability is done by switching rapidly between the source under observation and cold space. Prior to the upgrade of the 3 mm wavelength receiver at Onsala Space Observatory an evaluation of different designs of the beam switch is needed. The work was done at the Group for Advanced Receiver Development (GARD) at the department for Earth and Space Sciences at Chalmers University of Technology. GARD develops receivers for mm- and submillimetre waves used for radio astronomy and environmental sciences. Five different switching methods have been evaluated and one of them has been analyzed in detail. A 3D model was made in the CAD system Autodesk Inventor and the mechanical analysis of different structural deformation was carried out in ANSYS. The simulations carried out in ANSYS were, static structural directional deformation during acceleration, transient structural directional deformation during acceleration, modal analysis and static structural stress equivalent (von-Mises). The results confirmed that there were very small deformations and vibrations in the direction normal to the mirror plane. Further, the results showed that the Self-Resonant Frequency (SRF) is well above the frequency of the mechanical movement and it can be concluded that there is no risk for oscillations. The structural stress analysis also showed that the material used in the design, an aluminium alloy, is an appropriate choice for the application. Drive units for the beam switch were evaluated and partly analyzed. Different alternatives for the drives can be electrical motor or pneumatic actuator. A possible solution to obtain a smooth operation, i.e. smooth change in direction of the mirror, is to employ a crankshaft mechanism. Critical parts of the system that have to be further evaluated through testing are the tolerances for the guideways and limits in speed and lifetime for the pneumatic actuators. Sections that will not be dealt with are the climate impact on the design, which consists of variations in temperature and humidity, the energy consumption and systems environmental impact on nature.