When multi-beam optical stress sensor (MOSS) system is mounted at a large incident angle (alpha), despite an improvement of the resolution in the measurements, it also induces optical astigmatism in the spot images on a charge-coupled device. During epitaxial growth, as the film stress increases, the astigmatism may result in the beam deflection changing at different rates in the directions parallel and perpendicular to the incident plane, if the alpha is large. In this paper, the system error due to the astigmatism is analyzed by the ray-tracing method and its predictions are compared with the experimental results. It is demonstrated here, how the spot spacing changes along the above mentioned orthogonal directions can be considered separately to minimize the error due to astigmatism in the MOSS measurements at any large alpha. (C) 2003 Elsevier B.V. All rights reserved.