comments on multiple diffracted light in the czerny-turner spectrometer
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Comments on Multiple Diffracted Light in the Czerny-Turner Spectrometer Arthur B. Shafer and Donald 0. Landon
Spex Industries, Inc., 3880 Park Avenue, Metuchen, New Jersey 08840. Received 26 December 1968.
Hawes is quite correct in his comment,1 but we feel that a
May 1969 / Vol. 8, No. 5 / APPLIED OPTICS 1063
novice in the field could be led to believe that things are simpler than reality proves. The solution given will certainly work in instruments having low to medium resolution. Unfortunately, this solution to multiple diffracted light introduces other problems for moderate to high resolution instruments.
By tilting the collimator to larger off-axis angles, the optical aberrations (particularly coma and astigmatism) are increased.
The effect of these aberrations cannot be absolutely corrected, in any case, by the camera or focusing mirror.
A grating intervenes between the two mirrors and the wave-front transformation (unless the wavefront is plane) is nonlinear due to diffraction.2
Hence, any increase in the aberrations introduced by the collimating mirror will decrease the resolution (frequency response) of the spectroscopic system.
If the numerical aperture of the spectrometer optics is small (equivalent //15 to >//30), then the tilting of the two Czerny-Turner mirrors may be acceptable in some cases of moderate and high resolution.
Even in this situation there are problems. By using a small numerical aperture, coma is reduced in the aberrations introduced by the collimating mirror; but, astigmatism is transformed non-linearly by the grating. Adding the effect of coma and astigmatism will produce effective coma at the exit slit.3
So far, the discussions have assumed the use of spherical mirrors, as used in the Czerny-Turner design. One does not make things much better by switching to off-axis parabolas. Although these will correct the aberrations that the off-axis use of the spheres cause, they introduce a new series of difficulties; the worst probably being the rapid growth of coma in the image as any significant height of slit is illuminated. Designers will still generally prefer spherical mirrors because of the over-all control that can be exercised by the use of spheres.
We are not familiar with the design details of the various instruments mentioned by Hawes; but they must be of smaller than optimum numerical aperture and/or of modest resolution for the tilted mirror solution to remove multiple diffraction without producing noticeable aberrations in the signal.
References 1. R. Hawes, Appl. Opt. 8, 1063 (1969). 2. A. Shafer, L. Megill, and L. Droppleman, J. Opt. Soc. Amer.
54, 879 (1964). 3. G. Chandler, J. Opt. Soc. Amer. 58, 895 (1968).
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