[en] We have developed two different optical systems in order to position detectors with respect to a reference laser beam. The first system, a telescope, permits the absolute positioning of an element with respect to a reference laser beam. The resolution is of the order of 10 μm in translation and 50 μrad in rotation. It is highly transparent (∼90%) permitting several elements to be aligned. A calibration procedure has also been studied and is currently being tested in order to obtain an absolute alignment information. The second system is a highly transparent (95%) two-dimensional position sensor which allows the accurate positioning (below 20 μm) of several (up to 10) elements to which each sensor is attached, transversally to a laser beam used as a reference straight line. The present useful area of the first sensor is 20x20 mm² and is 15x15 mm² for the second. In both cases it can be further increased to meet the experiment's requirement