Rowland circle gratings represent the basic form of imaging concave gratings. In the Rowland circle configuration the entrance slit, grating and detector are located on a circle, the diameter of which equals the radius of the grating substrate. 

This configuration ensures that the imaging of the entrance slit is free from defocusing as well as primary coma and displays only minimal spherical aberrations. Although pronounced astigmatism is present, the resulting elongation of the slit images does not significantly reduce the resolving power.