In the low-k1 lithography age such as today, reducing wavefront aberrations of projection optics is of crucial importance. Whereas the NA of an exposure tool becomes close to 1.0 or more, its residual aberration has attained 10 milli-lambda or less. In order to guarantee such small aberrations, the development of its on-machine measurement system with high accuracy is an urgent need. Many methods have been proposed in the literature, but their applicability is questionable in this hyper-NA region because of the emerging non-linearity of the measurement. In order to meet the severe requirements from the projection optics, we have reviewed various measurement methods, in particular from the viewpoint of their applicability to the high NA including immersion. The usage of the exposure light source, the control of coherence, and hyper-NA light detection were the basic items of the development. In addition, as a projection optics is very sensitive to the circumstances in the region of less than 10 milli-lambda, its dynamic control is necessary to keep its performance. High-speed measurement is indispensable to control the aberration in the dynamic sense. With all these things into consideration, we have built a new on-machine measurement system to cope with the hyper-NA specific issues. In this paper, we will show the basic principle and the experimental data of our new system using a projection optics. In addition, its applicability to the immersion system will also be discussed.