code: def _vel_disp_one(self, kwargs_profile, kwargs_aperture, kwargs_light, kwargs_anisotropy):
          
  
          while True:
              r = self.lightProfile.draw_light(kwargs_light)  # draw r
              R, x, y = util.R_r(r)  # draw projected R
              x_, y_ = util.displace_PSF(x, y, self.FWHM)  # displace via PSF
              bool = self.aperture.aperture_select(x_, y_, kwargs_aperture)
              if bool is True:
                  break
          sigma_s2 = self.sigma_s2(r, R, kwargs_profile, kwargs_anisotropy, kwargs_light)
          return sigma_s2
docs: computes one realisation of the velocity dispersion realized in the slit
          :param gamma:
          :param rho0_r0_gamma:
          :param r_eff:
          :param r_ani:
          :param R_slit:
          :param dR_slit:
          :param FWHM:
          :return: