Firstly, a telescope system is made up with a concave lens and a long-focus convex lens, by which the angle of the parallel beam from the collimator can be change.
该方法借助一个焦距较长的凸透镜与被测凹透镜组成无焦系统，可以使准直管出射光束的张角发生改变；
2
Among the reshaping systems converting a Gaussian beam to a flattop beam, the system composed of a plane-concave and a plane-convex lens, is a relatively simple structure.