Path length differences monochromators

Pulse dispersion and pulse shift can be avoided by using a double monochromator of the subtractive dispersion type. In this design the second monochromator is turned 180 °, and the gratings are moving in opposite directions. Thus the path length differences for both sides of the gratings and for different wavelength cancel. 

Figure 4.24. Path length difference across a monochromator grating. /V i ttiB total numb of facets in the ttng, and Ax is the path length difference between adjacent reOcctioiis. From Ref. 120.

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