An observation system, usable with a projection optical system for optically projecting a first object upon a second object by use of a light of a first wavelength, for observing the second object by way of the projection optical system and by use of a light of a second wavelength different from the first wavelength. The observation system includes an observation optical system having a lens element and a parallel-surface plate which is inclined with respect to an optical axis of the observation optical system, wherein the observation optical system is arranged to form an image of the second object on a predetermined image surface and wherein the parallel-surface plate is arranged to substantially correct coma caused by the projection optical system.