PROBLEM TO BE SOLVED: To provide a far ultraviolet rays catadioptric anamorphic telescope to be used for narrowing the band of the beam width of a pulse laser comprising a pulse electric discharge laser. SOLUTION: The telescope is provided with an off-axis projected spherical surface reflector and a reflection/refraction optical element combined by off-axis, and is generally known as a Mangin mirror 206, a first refraction surface and a reflection rear surface are arranged and an aberration derived by the off-axis deflection of a beam 202 is cancelled. The telescope 205 is especially useful for far ultraviolet rays (DUV) by wave length being shorter than about 250 nm. In a certain application, diffraction grating or another wave length scattering element arrayed to reversely reflecting a selection wave length is irradiated in the telescope. In another application, the telescope combined with the diffraction grating is incorporated inside a laser optical cavity so that the band of the output wave length of the laser is narrowed. Besides, an improved general purpose DUV beam expander is provided.