  A spectroscopic analysis of stellar spectra can be carried out using
different approaches; different methods, line lists, atomic parameters, solar
abundances etc. The resulting atmospheric parameters from these choices can
vary beyond quoted uncertainties in the literature. Here we characterize these
differences by systematically comparing some of the commonly adopted
ingredients; line lists, equivalent width measurements and atomic parameters.
High resolution and high signal-to-noise spectroscopic data of one
helium-core-burning red giant star in each of the three open clusters, NGC6819,
M67 and NGC188 have been obtained with the FIES spectrograph at the Nordic
Optical Telescope. The M67 target has been used to benchmark the analysis, as
it is a well studied cluster with asteroseismic data from the K2 mission. For
the other two clusters we have obtained higher quality data than previously
analyzed, which allows us to establish their chemical composition more
securely. Using a line by line analysis, we tested several different
combinations of line lists and programs to measure equivalent widths of
absorption lines to characterize systematic differences within the same
spectroscopic method. The obtained parameters for the benchmark star in M67
vary up to ~170 K in T_eff, ~0.4 dex in log g and ~0.25 dex in [Fe/H] between
the tested setups. Using the combination of equivalent width measurement
program and line list that best reproduce the inferred log g from
asteroseismology, we determined the atmospheric parameters for the three stars
and securely established the chemical composition of NGC6819 to be ~solar,
[Fe/H]=-0.02+-0.01 dex. We have highlighted the significantly different results
obtained with different combinations of line lists, programs and atomic
parameters, which emphasize the importance of benchmark stars studied with
several different methods to anchor spectroscopic analyses.
