Compared with plane gratings, concave gratings offer one important advantage: they act as both the dispersing and focusing element for spectrographs as well as monochromators. This leads to a system with fewer optical components.
A "classical" concave grating (one whose grooves are straight, parallel and equally spaced) can be generated on a ruling engine or by recording a laser interference pattern in photoresist (to form a holographic grating). If the object (e.g., entrance slit or fiber) is placed on the Rowland circle (whose diameter equals the radius of curvature of the grating blank), high spectral imaging resolution is obtained. Although narrow in the spectral direction, the images suffer from a large amount of astigmatism (the extent of the image in the direction perpendicular to the dispersion direction), which is inherent in this type of grating system. This leads to relatively "tall" images. If this astigmatism cannot be tolerated, the optical designer should consider an aberration-corrected, concave holographic (or flat-field) grating.
Please see Spectra-Physics' Diffraction Grating Handbook for a review of concave gratings and their mounts. |