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New telescopes, using acive optics features and high quality tracking capabilities, require an high precision movement of the secondary mirror M2 during exposures. Moving this mirror to introduce an amount of decentering coma is one of the tasks of active optics. This paper shows that this target is accomplished with high accuracy rotating the mirror around a point located near, but not exactly at the center of curvature of M2. Basic equations of the displacement of the barycenter of the geometrial blur, to the third order, are here developed, and a final relation to get the correct position around which the rotation is to be performed, is obtained. Such position is the harmonic mean of the radius of curvature and of the distance from the vertex of the neutral point, properly weighted. A brief discussion is given about the application of such results to typical two-mirror telescopes. Ray tracing results are compared to analytical ones in the case of the Italian National Galileo telescope, that will be equipped with an high precision M2 driving device; the close matching with the analytical calculations is demonstrated.