It is very important to analyse phase ingomogenity of high-power laser beam. Usually Shack-Hartmann wavefront sensor or interferometer is used to test both laser beam aberrations and quality of the optical element surface. But we found out that in case of strong distortions the measured aberrations differ from the real ones. To investigate this problem we used bimorph deformable mirror to introduce strong aberrations to the laser beam. The results of our experiments are discussed.© (2008) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.