An instrument for moving and positioning of optical elements with nanometric mechanical stability and resolution in synchrotron beamlines, comprising: - a reference structure; - a mounting structure; - an optical element mounted to the mounting structure; - moving means configured to move the mounting structure in relation to the reference structure; - at least one reaction mass attached in a movable manner to the mounting structure, configured to receive the reaction force resulting from the positioning of the mounting structure in relation to the reference structure, through an actuation of the moving means; - a means of position measurement configured to measure a position of the mounting structure in relation to the reference structure; - a main feedback loop including the means of position measurement and the moving means, configured to use information from the means of position measurement to control the position of the mounting structure through the moving means, characterized by including: - a complementary mounting structure rigidly mounted to the reference structure; - a complementary optical element rigidly mounted to the complementary mounting structure, - a complementary structure attached in a movable manner to the reference structure, - a complementary actuation means configured to move the complementary structure in relation to the reference structure, - a first set of spring means (l 1) by which the mounting structure is attached in a movable manner to the complementary structure thereby allowing relative translation or rotation movements along or about one or more axes, - a second set of spring means (l 2) by which the reaction mass is attached in a movable manner to the complementary structure thereby allowing relative translation and rotation movements along or about one or more axes, and a third set of spring means (l 3) by which the complementary structure is connected to the reference structure. - a reference structure; - a mounting structure; - an optical element mounted to the mounting structure; - moving means configured to move the mounting structure in relation to the reference structure; - at least one reaction mass attached in a movable manner to the mounting structure, configured to receive the reaction force resulting from the positioning of the mounting structure in relation to the reference structure, through an actuation of the moving means; - a means of position measurement configured to measure a position of the mounting structure in relation to the reference structure; - a main feedback loop including the means of position measurement and the moving means, configured to use information from the means of position measurement to control the position of the mounting structure through the moving means, - a complementary mounting structure rigidly mounted to the reference structure; - a complementary optical element rigidly mounted to the complementary mounting structure, - a complementary structure attached in a movable manner to the reference structure, - a complementary actuation means configured to move the complementary structure in relation to the reference structure, - a first set of spring means (l 1) by which the mounting structure is attached in a movable manner to the complementary structure thereby allowing relative translation or rotation movements along or about one or more axes, - a second set of spring means (l 2) by which the reaction mass is attached in a movable manner to the complementary structure thereby allowing relative translation and rotation movements along or about one or more axes, and