A grating may be employed to fold a laser resonator, thereby enabling the simultaneous extraction of several frequencies from the laser. This limits the range of useful grating constants as compared to the Littrow case. The astigmatism introduced by the grating is examined through the use of sigma-circle diagrams. It will be seen that the grating performs an angle-dependent transformation on the sigma-circle representative of the plane of diffraction to make the mode astigmatic. Some consequences of this related to the design of similar resonators are discussed.