GENERATION, CONTROL, ^ND MEASUREMENT 



207 



LENS 



//SLIT 



(b) PARABOLIC MIRROR 



\ 



SLIT 

 j~^/7 AXIS O F O RI GINA L MIRR OR 



selected for the shortest wave length to be used. Then the decreasing 

 efficiency with wave length will in part compensate for the increase in 

 spectral intensity of the source. 



It is evident from Eq. (3-19) that, in the grating monochromator, 

 energy of wave length X emerging 

 from the exit slit for the first order, 

 m = 1, will be mixed with energy 

 of the second order, m = 2 of 1/2X; 

 for the third order, m = 3 of 1/3X, 

 etc. When using the first-order 

 spectrum of a grating, it is relatively 

 simple to eliminate the higher orders 

 in the visible and ultraviolet with 

 long-wave-pass glass or gelatin fil- 

 ters. Quartz, which absorbs all 

 wave lengths beyond 180 m^u, will 

 eliminate orders higher than the 

 first to about 350 m/x ; window glass 

 (300 m/u), to 600 m/u; and yellow 

 glass or gelatin filters (600 van), to 

 1200 m)u. It is more difficult to 

 make use of the second-order spec- 

 trum and eliminate the first, third, 

 and higher orders because band- 

 pass filters are required. 



Collimator and Focusing Systems. 

 Both the collimator and focusing 

 systems can employ either positive 

 lenses or concave mirrors. Small 

 spectroscopes for the visible usually 

 employ compound lenses corrected 

 for spherical and chromatic aberra- 

 tion. In the ultraviolet, achro- 

 matic lenses are less commonly 

 employed, since they are difficult 

 to make and expensive. 



For instruments of large aperture, 

 mirror optics offer many advantages 

 over lenses. Mirrors introduce no 

 parabolic mirrors (Fig. 3-23) introduce little astigmatism and spherical 

 aberration. Mirrors are, as a rule, less expensive than lenses for the same 

 size and degree of correction. The two principal disadvantages of mirrors 

 are that, since it is usually necessary that the beam be doubled back on 

 itself, the optical arrangement sometimes presents difficult problems of 



('c) OFF-AXIS PARABOLIC MIRROR 



Fig. 3-23. CoUimating systems employ- 

 ing lenses and mirrors. The slit is at the 

 focus of the lens or mirror. The lens 

 collimator (a) can be made approximately 

 achromatic by the use of compound 

 lenses; concave mirrors are achromatic. 

 The parabolic mirror at b produces only 

 approximately parallel rays, since the 

 slit is not on the geometrical axis. The 

 off-axis parabolic mirror of c is cut from a 

 larger mirror, as indicated, and elimi- 

 nates the astigmatism produced by the b 

 arrangement. 



chromatic aberration, and off-axis 



