New plane grating monochromator with off-axis parabolical mirrors

The design of a new plane grating monochromator is described. The system employs off-axis parabolical mirrors both in the collimator and the camera. The entrance and exit slits lie on the same side of the grating and are curved to obtain the invariance of the curvature of the spectral lines with respect to wavelength variations. The grating is oriented to avoid multiple dispersions. The system is free from aberrations in the center of the field, and since the dependence of the aberrations on any other point of the field is very weak, it is possible to use long slits. The images obtained with this design are of excellent quality and may be improved even further using mirrors with slightly different off-axis angles and focal lengths for the collimator and the camera. Analytical expressions of the aberrations are obtained by the plate diagram method; the results are checked by finite ray tracing for a particular case.