an optical system and method comprising a diffraction grating which consists of diffracting elements spaced from one another by unequal distances . correction of residual defocusing in the image produced by such a grating is accomplished by translating it along its surface . as one embodiment , a monochromator is constructed on which a self - focusing grating scans the value in wavelength which is transmitted between fixed slits by rotation of the grating about an axis fixed in space . combined with a translation of the grating along its surface , such a monochromator produces a symmetrical image exactly in focus at the exit slit for all scanned wavelengths .