AbstractWith the large improvement in effective area of Astro-H's micro-calorimeter soft X-ray spectrometer (SXS) over grating spectrometers, high-resolution X-ray spectroscopy with good signal to noise will become more commonly available, also for faint and extended sources. This will result in a range of spectral lines being resolved for the first time in celestial sources, especially in the Fe region. However, a large number of X-ray line energies in the atomic databases are known to a lesser accuracy than that expected for Astro-H/SXS, or have no known uncertainty at all. To benchmark the available calculations, we have therefore started to measure reference energies of K-shell transition in L-shell ions for astrophysically relevant elements in the range 11≤ Z ≤ 28 (Na to Ni), using the Lawrence Livermore National Laboratory's EBIT-I electron beam ion trap coupled with the NASA/GSFC EBIT calorimeter spectrometer (ECS). The ECS has a resolution of ~5 e V, i.e., similar to Astro-H/SXS and Chandra/HETG. A comparison to crystal spectra of lower charge states of sulfur with ~0.6 e V resolution shows that the analysis of spectra taken at ECS resolution allows to determine the transition energies of the strongest components.