Qualitative evaluation of a parabolic mirror with substructured Ronchi gratings

In this work we propose to qualitatively test the quality of a concave parabolic mirror by means of the Ronchi interferometric test using substructured gratings to increase the sharpness of the fringes. The substructured gratings are designed considering that the grating period is divided into several stripes of equal width that can be transparent or opaque; the transmission coefficients of the stripes along the grating period are not periodic, but a previously chosen binary sequence. Equations were derived to obtain the corresponding intensity profiles and an analysis of these profiles was performed for different sequences and positions of the substructured gratings with respect to the test mirror. It was experimentally verified that the fringes are sharper at the Rayleigh distance inside and outside the mirror focus.