The three stellar calibration sources cover most of the ionization states from collisionally ionized plasmas. The question arises as to the usefulness of stellar spectra for atomic physics benchmarks, given their intrinsic complexity.
Abundances then affect the relevant contributions from each element.
All other physics drops out, isolating the atomic physics. The LETG spectral region above 150 eV is rich in diagnostics for plasmas at this temperature. The LETG will observe L shell emission for O, Ne, Mg, Si, S, Ar and Ca. In addition to benchmarking the relative line intensities from a given ion, these data will for the first time ever give us constraints on the ionization balance calculations. By constructing separate emission measure distributions from each element one can tell by the relative shapes how well the ionization balance calculations agree.