alignment technique for calibrating a phase - shifting point diffraction interferometer involves three independent steps where the first two steps independently align the image points and pinholes in rotation and separation to a fixed reference coordinate system , e . g , ccd . once the two sub - elements have been properly aligned to the reference in two parameters , the third step is to align the two sub - element coordinate systems to each other in the two remaining parameters using standard methods of locating the pinholes relative to some easy to find reference point .