  We study the bright Seyfert 1 galaxy Mrk~509 with the Reflection Grating
Spectrometers (RGS) of XMM-Newton using the RGS multi-pointing mode of
XMM-Newton for the first time in order to constrain the properties of the
outflow in this object. We want to obtain the most accurate spectral properties
from the 600 ks spectrum of Mrk 509 which has excellent statistical quality. We
derive an accurate relative calibration for the effective area of the RGS,
derive an accurate absolute wavelength calibration, improve the method for
adding time-dependent spectra and enhance the efficiency of the spectral
fitting by two orders of magnitude. We show the major improvement of the
spectral data quality due to the use of the new RGS multi-pointing mode of
XMM-Newton. We illustrate the gain in accuracy by showing that with the
improved wavelength calibration the two velocity troughs observed in UV spectra
are resolved.
