  We present the X-ray luminosity (L) versus dynamical mass (M) relation for 63
nearby clusters in the HIFLUGCS. The luminosity measurements are obtained based
on ~1.3 Ms of clean XMM data and ROSAT pointed observations. The masses are
estimated using optical spectroscopic redshifts of 13647 cluster galaxies in
total. Given sufficient numbers of member galaxies in computing the dynamical
masses, the L-M relations agree between the disturbed and undisturbed clusters.
The cool-core clusters still dominate the scatter in the L-M relation even when
a core corrected X-ray luminosity is used, which indicates that the scatter
mainly reflects the structure formation history of the clusters. As shown by
the clusters with a small number of redshifts, the dynamical masses can be
underestimated leading to a biased scaling relation. To investigate the
potential of spectroscopic surveys to follow up high-redshift galaxy
clusters/groups observed in X-ray surveys for the identifications and mass
calibrations, we carried out Monte-Carlo re-sampling of the cluster galaxy
redshifts and calibrated the uncertainties of the redshift and dynamical mass
estimates when only reduced numbers of galaxy redshifts per cluster are
available. The re-sampling considers the SPIDERS and 4MOST configurations,
designed for the follow-up of the eROSITA clusters, and was carried out for
each cluster at the actual cluster redshift as well as at z=0.2, 0.4, 0.6, and
0.8. For following up very distant cluster/groups, we carried out the mass
calibration based on the re-sampling with only 10zs/cluster, and redshift
calibration based on the re-sampling with only 5zs/cluster and 10zs/cluster,
respectively. Our results demonstrate the power of combining upcoming X-ray and
optical spectroscopic surveys for mass calibration. The scatter in the
dynamical mass estimates for the clusters with at least ten members is within
50%.
