Grating echellette

One of the advantages of a monochromator with an echellette grating is that in contrast to a prism monochromator, the dispersion of radiation along the focal plane is, for all practical purposes, linear. Figure 25-9 demonstrates this property. The linear dispersion of a grating greatly simplifies the design of monochromators. 

Figure 25-9 Dispersion of radiation along the focal plane AB of a typical prism (a) and echellette grating (b). The positions of 4 and B in the scale in (c) are shown in Figure 25-6.

A monochromator with a focal length of 0.78 m was equipped with an echellette grating of 2.5IK) blazes per millimeter. 

Figure 25-8 Mechanism of diffraction from an echellette-type grating. Angle i from the grating normal is the angle of the incident beam angle r is the angle of the reflected beam. The distance between successive rulings is d.

HGURE 7-21 Mechanisms of diffraction from an echellette-type grating. 

Both spectrometers are equipped with a self-controlling wavelength calibration routine using the possibility to avoid the order pre-selection in fl-ont of the echelle monochromator. This takes up the well-known method from classical VUV spectrometry, where VUV lines diffracted from echellettes or concave gratings in higher orders were calibrated... 

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