Metrology requirements for optical components for third‐generation synchrotron sources are taxing the state of the art in manufacturing technology. We have investigated a number of error sources in a commercial figure measurement instrument, the Long‐Trace‐Profiler II, and have demonstrated that, with some simple modifications, we can significantly reduce the effect of error sources and improve the accuracy and reliability of the measurement. By keeping the optical head stationary and moving a penta prism along the translation stage, as in the original pencil‐beam interferometer design of von Bieren, the stability of the optical system is greatly improved, and the remaining error signals can be corrected by a simple reference beam subtraction. We illustrate the performance of the modified system by investigating the distortion produced by gravity on a typical synchrotron mirror and demonstrate the repeatability of the instrument despite relaxed tolerances on the translation stage. ©1995 American Institute of Physics.