The globular cluster NGC 6388 is among the most massive clusters in our Milky Way and has been the subject of many studies .
Recently , two independent groups found very different results when measuring its central velocity-dispersion profile with different methods .
While Lützgendorf et al .
( 2011 ) found a rising profile and a high central velocity dispersion ( 23.3 \mathrm { km s ^ { -1 } } ) , measurements obtained by Lanzoni et al .
( 2013 ) showed a value 40 % lower .
The value of the central velocity dispersion has a serious impact on the mass and possible presence of an intermediate-mass black hole at the center of NGC 6388 .
The goal of this paper is to quantify the biases arising from measuring velocity dispersions from individual extracted stellar velocities versus the line broadening measurements of the integrated light using new tools to simulate realistic IFU observations .
We use a photometric catalog of NGC 6388 to extract the positions and magnitudes from the brightest stars in the central three arcseconds of NGC 6388 , and create a simulated SINFONI and ARGUS dataset .
The construction of the IFU data cube is done with different observing conditions ( i.e. , Strehl ratios and seeing ) reproducing the conditions reported for the original observations as closely as possible .
In addition , we produce an N-body realization of a \sim 10 ^ { 6 } M _ { \odot } stellar cluster with the same photometric properties as NGC 6388 to account for unresolved stars .
We find that the individual radial velocities , i.e .
the measurements from the simulated SINFONI data , are systematically biased towards lower velocity dispersions .
The reason is that due to the wings in the point spread function ( PSF ) of adaptive optics ( AO ) corrected data sets , the velocities get biased towards the mean cluster velocity .
This study shows that even with AO supported observations , individual radial velocities in crowded fields do not reproduce the true velocity distribution .
The ARGUS observations do not show this kind of bias but were found to have larger uncertainties than previously obtained .
We find a bias towards higher velocity dispersions in the ARGUS pointing when fixing the extreme velocities of the three brightest stars but find those variations are within the determined uncertainties .
We rerun Jeans models and fit them to the kinematic profile with the new uncertainties .
This yields a black-hole mass of M _ { \bullet } = ( 2.8 \pm 0.4 ) \times 10 ^ { 4 } M _ { \odot } and M/L ratio M / L = ( 1.6 \pm 0.1 ) M _ { \odot } / L _ { \odot } , consistent with our previous results .