A doublegrating interferometer capable of determining the spherical aberration of the optics is described. Two Ronchi rulings of 19.69 lines/mm have been used in the setup to generate large shear among the diffracted wavefronts. Three zero-order diffracted wavefronts will then be collected to interfere by means of spatial filtering. The modulation fringes, appeared in the three-beam interferogram, may be used to determine the coefficient of the primary spherical aberration of the optics under test.
