
396 15. Plane Grating Spectrometers
15.5.f.
CONCLUDING REMARKS
Although our discussion of echelle formats and spectrometer configurations is
given in terms of echelles with tan
5
= 2 or larger, the relations used apply to any
blaze angle. An echellette grating can also be used with a cross disperser to
separate orders. Consider, for example, a grating with 300 grooves/mm and
tan
6
= 0.75. From the grating equation in Table 15.4 we get
m^Q
(^m) = 4 cos 9,
hence most of the visible spectrum is covered in four orders, m = 6 through
m = 9. These orders are easily separated with a crossed prism in the configuration
shown in Fig.
15.11.
Because o ...