Focalization process of spherical concave diffraction gratings

The method of making a monochromator using a concave, reflective holographic grating, which comprises the steps of:

• a. determining the physical parameters of the monochromator by using a concave classical reflective diffraction grating having straight, parallel lines which are equidistantly spaced to provide a line density 1/n, a height H.sub.0 and a width W.sub.0, the monochromator having fixed entrance and exit slits with the grating being mounted for rotation only with respect to said slits whereby to scan a spectral range of interest, said physical parameters being the positions of said entrance and exit slits with respect to the intersection of their optical axes at the center of the reflective surface of said grating, and said physical parameters being selected to provide a defect of focus which balances second order aberration terms excepting astigmatism whereby the partial derivative .delta.F.sub.1 /.delta.w is equal to zero, F.sub.1 being the optical path associated with said classical grating and w and h being Cartesian coordinates with respect to said center of the grating extending respectively perpendicular and parallel to said grating lines;
• b. holographically forming a grating by locating two point sources of monochromatic light of wavelength .lambda..sub.o in the w plane passing through the center of a holographic blank having the concavity of said classical grating, such that said point sources are located by four position parameters which are the respective distances rc and rd of said point sources from said center of the holographic blank and the angles .gamma. and .eta. which lines from such point sources to said center respectively make with a line passing through such center and normal to said holographic blank, said four position parameters being established by four simultaneous equations the first of which is sin.eta.-sin.gamma. = n.gamma..sub.0 and the remaining three of which are determined on the basis of ##EQU34## and ##EQU35## and by cancelation of the two principal coma terms associated with the coma contribution of said point sources, F.sub.1 as before being the optical path associated with said classical grating and F.sub.2 being the optical path relative to said entrance and exit slits associated with said holographically formed grating;
• and
• substituting the holographically formed grating for said classical grating.