A concave grating (also known as Rowling grating) combines the functions of optical imaging and diffraction into one optical element. Made by spacing straight grooves equally along the chord of a concave spherical or parabolic mirror surface, both collimates and disperses the light falling upon it. The grating equation is the same as for a flat grating. The important part is that the angles AMC and BMC, incidend and defraction angles respectively, is does not vary with the point 'M' choosen on the grating surface.
Traditional mountings for the concave spherical grating mounting use a slit source oriented to maximal resolution. The spectral lines obtained with a concave grating show the same aberrations as images obtained with a concave mirror, primarily astigmatism. Concave grating are typicallw used in polychromators based on the Rowland circle.
First published on the web: 8 December 2007.