Radioastronomers have launched an international initiative whose objective is the construction of a radiotelescope with a collective surface collectrice equivalent to one square kilometre. This instrument, named SKA (Square Kilometer Array), would allow astronomers to reach several objectives, such as determine the age of the universe (the period when the first stars were formed) and to understand the origin and evolution of cosmic magnetism. In addition to a collective surface of 1 km2, some of the other main constraints imposed upon the project by radioastronomers include the cost of development (fixed at 1 billion US$) and the coverage of the largest possible part of the sky (up to 60 degrees from zenith).

    Given that a unique surface of 1 km2 is practically impossible to achieve, the instrument will be composed of several individual stations linked by fiber optics to obtain an equivalent collective surface. Several countries (Australia, Canada, China, Europe and the United States) have developed concepts and Canada's contribution was proposed by a group of university and industry researchers, supervised by the Federal Radioastrophysics Observatory (A CNRC institute located in Penticton, B. C.).

    The concept, named LAR (Large Adaptative Reflector) is illustrated in the figures below and is composed of three elements:

    1. A low-curvature deformable parabolic reflector located near the ground. This reflector enables wabes to be reflected towards a focal point. It is actuated in such a way as to direct the curvature towards the part of the sky which one wishes to observe.
    2. A radio receptor which must be positioned at the focal point to capture the signal coming from the reflected sky via a primary collector.
    3. A positioning mechanism capable of moving and orienting the radio receptor in accordance with the position of the focal point determined by the shape of the reflector.
    Fig. 1: Schematic representation of the LAR (click on the image to enlarge).
    Fig. 2: Artistic representation of the LAR (click on the image to enlarge).


    We are specifically interested in the third element, since it divides into sub-systems which act together, namely the principle manipulator (MP) and the confluence point mechanism (CPM). These two sub-systems are cable actuated parallel mechanisms. The MP is gigantic (at least 600 metres large) and maintain in tension with the help of a helium balloon. This manipulator roughly positions and orients the focal platform. The CPM, which is smaller (about 20 metres large) and much more precise, serves to correct the imperfections in positioning and orientation of the MP so as to achieve a precision of the order of a centimetre and less than one degree. This mechanism links the focal platform to the radio receptor.