In this paper the critical area using imaging optics is discussed, i. e. the ground based astronomical telescopes, observing systems from the space to the Earth and the projecting lithographic systems. According to the trend of technical progress, it is unavoidable to use reflective optical systems, and consequently as-pherical surfaces. For optical system design, the main problem is how to construct an effective reflecting system which may be optimized by the optical CAD. The need of unobscurating entrance pupil leads using off-axis reflecting system and tilted mirror system. The key problem in fabrication is to make high accuracy, large relative aperture, high-order, large asphericity/aspherical slope and section symmetric surfaces. The null method is the principal way for testing aspheric surfaces up to now. The design of compensator for highorder, off-axis aspheric mirrors is worth studying yet.