Aspheric concave grating spectrographs

Modifications to the surface of ellipsoidal gratings with straight rulings have been investigated. We find that with the addition of a term proportional to wl2 the stigmatic Rowland circle geometry can be greatly improved. We also show that with the addition of a term proportional to w3, comatic aberration can be corrected, and geometries outside the Rowland circle become well behaved. The modifications require only a few microns of figuring polish away from the nominal ellipsoid. Fabrication of these gratings is well within the capabilities of modern optics.