Device grade semiconductor epitaxial growth is usually performed on rotating substrates in order to achieve uniform layer thickness and composition. This often complicates in-situ optical measurements for growth control. Therefore, in this paper we present a method that significantly reduces the signal distortions imposed on RAS (reflectance anisotropy spectroscopy) and ellipsometry spectra by rotating and wobbling samples. Several cases (phase and frequency of the sample rotation either known or unknown, wobbling and ideally rotating samples) are investigated and the respective optimized experimental set-ups are presented for both RAS and ellipsometry.