  The design of modern instruments does not only imply thorough studies of
instrumental effects but also a good understanding of the scientific analysis
planned for the data. We investigate the reliability of Milne-Eddington (ME)
inversions of high-resolution magnetograph measurements such as those to be
obtained with the Imaging Magnetograph eXperiment (IMaX) aboard the Sunrise
balloon. We also provide arguments to choose either Fe I 525.02 or 525.06 nm as
the most suitable line for IMaX. We reproduce an IMaX observation using
magnetoconvection simulations of the quiet Sun and synthesizing the four Stokes
profiles emerging from them. The profiles are degraded by spatial and spectral
resolution, noise, and limited wavelength sampling, just as real IMaX
measurements. We invert these data and estimate the uncertainties in the
retrieved physical parameters caused by the ME approximation and the spectral
sampling.It is possible to infer the magnetic field strength, inclination,
azimuth, and line-of-sight velocity from standard IMaX measurements (4 Stokes
parameters, 5 wavelength points, and a signal-to-noise ratio of 1000) applying
ME inversions to any of the Fe I lines at 525 nm. We also find that telescope
diffraction has important effects on the spectra coming from very high
resolution observations of inhomogeneous atmospheres. Diffration reduces the
amplitude of the polarization signals and changes the asymmetry of the Stokes
profiles. The two Fe I lines at 525 nm meet the scientific requirements of
IMaX, but Fe I 525.02 nm is to be preferred because it leads to smaller
uncertainties in the retrieved parameters and offers a better detectability of
the weakest (linear) polarization signals prevailing in the quiet Sun.
