What is the difference between Rowland circle and polychromator mounts for gratings?

Concave grating mounts can be monochromatic or spectrographs (polychromatic). Two common concave grating mount configurations are Rowland circle spectrographs and polychromator mounts (also known as flat-field spectrographs). Edmund Optics^® offers Zeiss Concave Diffraction Gratings in both configurations.

In a spectrometer using a Rowland circle mount, the entrance slit, illumination source, and the center of the concave grating all lie on the Rowland circle, which has a diameter equal to the tangential radius of the grating curvature. This mount was designed in 1883 by Henry A. Rowland, who was also the first to rule concave gratings of spectroscopy grade quality. Spectra recorded using this mounting configuration are generally free from defocus and primary coma across the wavelength range and spherical aberration is negligible. However, astigmatism can cause issues with the efficiency of the instrument, as a significant fraction of the diffracted light is lost. Therefore, Rowland circle configurations are best suited when a light source with high intensity is used.

Spectrometers with polychromator mounts (flat-field spectrographs) use a fixed arrangement of the entrance slit, concave grating, and flat image plane, which is a linear detector array covering wavelengths of interest. The flat image plane is made possible by changing the groove pattern of the holographic grating by varied line-spacing (VLS) or interferometric methods, which change the focus plane of the grating from a curved shape to a flat surface. This maintains resolution while reducing aberrations such as astigmatism. Instruments with polychromator mounts have a higher light collection efficiency and offer high resolution on flat imaging sensors.

One of the main advantages of using a concave grating in both Rowland circle and polychromator configurations is that, unlike instruments that use plane gratings, no additional focusing optics, such as spherical concave mirrors, are required.