Abstract
We present our first attempt to model a new metallicity indicator for old and intermediate aged stellar populations on the basis of the Mg[b] absorption index at λ ∼ 5175 Å. Model spectra of stellar populations are convolved with the responses of the Tunable Filters of OSIRIS-GTC to provide a photometric index matching the spectroscopic Mg[b]. The new index allows us to obtain galaxy images of this feature on the basis of a photometric approach, without the very high signal-to-noise requirements of the spectroscopic methods.
