Effect of a poor mirror drive

The quality of the drive mechanism of the moving mirror ultimately determines whether a certain interferometer can be used to measure a spectmm with a resolution that corresponds to the reciprocal of the maximum retardation of the interferometer. The resolution of a Fourier transform spectrometer is determined by the maximum retardation only if the planes of the mirrors remain in good alignment throughout the entire scan and if the beam that passes through the interferometer is sufficiently collimated. 

The effect of a drive mechanism that does not allow the plane of the moving mirror of the interferometer to maintain its angle relative to the plane of the beamsplitter is somewhat analogous to the effect of beam divergence discussed in Section 2.6. In the case of the poor mirror drive, however, an optical path difference is generated between the two extreme rays of the beam passing through the interferometer rather than a path difference between the extreme rays and the central ray. 

Loss of fringe modulation will start to become apparent when x O.R. Thus, to ensure that no degradation of resolution occurs during the scan. 

If the resolution is found to have been degraded because of mirror tilt, the effect may be minimized by reduction of the diameter of the beam. For example, if the desired resolution is not attained above 2000 cm in an interferometer with D = 5 cm, the beam may be apertured down to a diameter of 2.5 cm. When the area of the beam is so reduced, however, the amount of energy that reaches the detector is reduced in proportion to the area of the beam, that is, by a factor of 4, and the SNR of the spectrum would also be reduced by this amount. 

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