Astigmatism corrected gratings for plane grating and spherical mirror spectrographs

摘要

A modified diffraction grating having a predetermined amount of astigmatic power correction is substituted for the plane diffraction grating in an Ebert type spectrograph having a single concave collimating and focusing mirror for directing a light beam onto the surface of the modified diffraction grating. The surface of the modified diffraction grating is curved to introduce the proper amount of astigmatic power correction to converge the sagittal focus and the tangential focus of the diffracted beam to produce a stigmatic image. The modified diffraction grating may have a pure cylindrical surface or a toroidal surface. Another embodiment replaces the plane grating in a two mirror Czerny-Turner type of instrument with a modified grating according to this invention to reduce or eliminate astigmatism in that type of instrument as well.