The Imaging MAgnetograph eXperiment, IMaX, is one of the three postfocal instruments of the Sunrise mission. The Sunrise project consists of a stratospheric balloon with a 1 m aperture telescope, which will fly from the Antarctica within the NASA Long Duration Balloon Program. IMaX should work as a diffraction limited imager and it should be capable to carry out polarization measurements and spectroscopic analysis with high resolution (50.000-100.000 range). The spectral resolution required will be achieved by using a LiNbO3 (z-cut) Fabry-Perot etalon in double pass configuration as spectral filter. Up to our knowledge, few works in the literature describe the associated problems of using these devices in an imager instrument (roughness, off-normal incidence, polarization sensitivity...). Because of that, an extensive and detailed analysis of etalon has been carried out. Special attention has been taken in order to determine the wavefront transmission error produced by the imperfections of a real etalon in double pass configuration working in collimated beam. Different theoretical models, numeric simulations and experimental data are analysed and compared obtaining a complete description of the etalon response.