the final atomic identification (ID) of a feature with an input feature, once it has passed through all the instrumentation (effective areas, re-distribution matrix, etc). This is why we aim at assessing this problem, generating a spectrum, over which we have absolute control of every single feature (absorption lines and edges). For that purpose we have created an artificial first dispersion order $\pm 1$ HETG+MEG 900 ks (HETGS with medium energy grating \[MEG\]), where we know exactly where every feature is located, and described in more detail as follows.

We begin by building a physical model [(pow)\*wabs1\*wabs2]{} constituted by an UV+X-rays source, redshifted by $z=0.00976$, emitting a powerlaw ([pow]{}), modified by two constant-density ($\log[n_H]=12$ \[\]) absorbers ([wabs1]{}, with ionization parameter $\log \xi=2.08$ \[1\], column densities $N_H=10^{22}$ , and [wabs2]{}, with ionization parameter $\log \xi=1.15$ \[1\], column densities $N_H=10^{21}$ ) [^1]. We call this model A. We then use [xspec]{} 12 to generate a 900 ks MEG spectrum, using appropriate +1 and -1 dispersion orders of MEG effective areas and 1$^{\rm st}$ order response matrix (see science threads in the [ciao]{} main page, [http://cxc.harvard.edu/ciao/]{}).

Table \[tbl1\] gives the physical parameters of model A to produce one of the spectra we use 