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Recent work has used spectra of about 5000 stars in NGC 6397 that wereextracted from a MUSE mosaic to determine the atmospheric parameters for thesestars by fitting the spectra against the Goettingen Spectral Library. Asignificant change in metallicity between the turn off and the red giant branchwas found and was discussed as a possible manifestation of predicted effects ofatomic diffusion. However, the small amplitude of the effect and inconsistencywith earlier measurements call for more attention before this result isinterpreted. Systematic effects due to the interpolation or to the syntheticspectra cannot be ruled out at this level of precision. We reanalyze the datawith the ELODIE and MILES reference libraries in order to assess the robustnessof the result. These empirical libraries have a finer metallicity coverage downto approximately the cluster metalicity turn-off. We used the ULySSfull-spectrum fitting package, together with the library interpolators toremeasure the three atmospheric parameters effective temperature, surfacegravity, and [Fe/H] metallicity. We find a very low [Fe/H] dispersion along theisochrone (0.07 dex), consistent with our error estimate (0.05 dex). However,the [Fe/H] trend is not reproducible. This shows that the data have thepotential to reveal patterns of the magnitude of the expected physical effects,but the analysis methods need to be refined to cull systematic effects thatcurrently dominate the patterns.

NGC 6397: The metallicity trend along the isochrone revisited