Abstract
The effect of imperfect collimation on the fringe separation of a Fizeau wavemeter is calculated. It is shown that illumination in a plane normal to the wedge face and parallel to the wedge axis enables the detector array to be positioned such that the wavelength error caused by spherical wave-front curvature introduced by the beam expander becomes insignificant to 1 part in 108, and no dispersion correction for the wedge plate is required. It is also shown that a sealed air wedge is capable of a wavelength precision approaching 1 part in 108.

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