We present the wavelength solution derived for the G800L grism with the Wide Field Channel from the spectra of two Galactic Wolf-Rayet stars, WR 45 and WR 96. The data were acquired in-orbit during the SMOV tests and the early INTERIM calibration program. We have obtained an average dispersion of 39.2 A/pix in the first order, 20.5 A/pix in the second and -42.5 A/pix in the negative first order. We show that the wavelength solution is strongly field-dependent, with an amplitude of the variation of about 11% from the center of the WFC aperture to the corners. The direction of the field-dependence is the diagonal from the image left top corner (amplifier A) to the bottom right corner (amplifier D). These trends are observed for all grism orders. We also describe the calibration files derived from the SMOV and INTERIM data which are used by the ST-ECF slitless extraction code aXe.
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