A two-beams interference technique developed to test the wedge of a lens (Ref. 1) has been applied to precision mounting of optical systems. When an optical system requires very high centration and tolerates very little tilt for each of its optical elements, one way of achieving that goal is to mount each individual lens well-aligned in a sub-cell, and the sub-cells are made press-fit to the main lens housing. The main housing and sub-cells are made with high mechanical precision. A twelve lenses optical system for a laser scanner has been assembled that way, the whole optical system achieves diffraction limited quality.