A variable fixed-focus constant optimization method for a variable-included-angle varied-line-spacing plane-grating monochromator

Variable-included-angle varied-line-spacing plane-grating monochromators (VIA-VPGM) have been applied to many beamlines because of the self-focusing and aberration-correction function of the varied-line-spacing grating. Unfortunately, to optimize the variable-line-spacing coefficient of the grating, the fixed-focus constant (C_ff) has to be fixed first in the VIA-VPGM. In this way, some of the advantages of these monochromators are lost, such as the flexibility of choosing a different energy-resolving power by varying the C_ff. In this work, a variable C_ff optimization method is introduced for a VIA-VPGM. By adopting this method, the C_ff could be arbitrarily selected in the VIA-VPGM.