We use the RASS-SDSS galaxy cluster sample to compare the quality of optical and X-ray luminosities as predictors of other cluster properties such as their masses , temperatures , and velocity dispersions .
We use the SDSS spectroscopic data to estimate the velocity dispersions and the virial masses of a subsample of 69 clusters within r _ { 500 } and r _ { 200 } .
The ASCA temperature of the intra-cluster medium , T _ { X } , is retrieved from the literature for a subsample of 49 clusters .
For this subsample we estimate the cluster masses also by using the mass-temperature relation .
We show that the optical luminosity , L _ { op } , correlates with the cluster mass much better than the X-ray luminosity , L _ { X } . L _ { op } can be used to estimate the cluster mass with an accuracy of 40 % while L _ { X } can predict the mass only with a 55 % accuracy .
We show that correcting L _ { X } for the effect of a cool core at the center of a cluster , lowers the scatter of the L _ { X } - M relation only by 3 % .
We find that the scatter observed in the L _ { op } - L _ { X } relation is determined by the scatter of the L _ { X } - M relation .
The mass-to-light ratio in the SDSS i band clearly increases with the cluster mass with a slope 0.2 \pm 0.08 .
The optical and X-ray luminosities correlate in excellent way with both T _ { X } and \sigma _ { V } with an orthogonal scatter of 20 % in both relations .
Moreover , L _ { op } and L _ { X } can predict with the same accuracy both variables .
We conclude that the cluster optical luminosity is a key cluster parameter since it can give important information about fundamental cluster properties such as the mass , the velocity dispersion , and the temperature of the intra-cluster medium .