code: def vel_disp_one(self, gamma, rho0_r0_gamma, r_eff, r_ani, R_slit, dR_slit, FWHM):
          
          a = 0.551 * r_eff
          while True:
              r = self.P_r(a)  # draw r
              R, x, y = self.R_r(r)  # draw projected R
              x_, y_ = self.displace_PSF(x, y, FWHM)  # displace via PSF
              bool = self.check_in_slit(x_, y_, R_slit, dR_slit)
              if bool is True:
                  break
          sigma_s2 = self.sigma_s2(r, R, r_ani, a, gamma, rho0_r0_gamma)
          return sigma_s2
docs: computes one realisation of the velocity dispersion realized in the slit
  
          :param gamma: power-law slope of the mass profile (isothermal = 2)
          :param rho0_r0_gamma: combination of Einstein radius and power-law slope as equation (14) in Suyu+ 2010
          :param r_eff: half light radius of the Hernquist profile (or as an approximation of any other profile to be described as a Hernquist profile
          :param r_ani: anisotropy radius
          :param R_slit: length of the slit/box
          :param dR_slit: width of the slit/box
          :param FWHM: full width at half maximum of the seeing conditions, described as a Gaussian
          :return: projected velocity dispersion of a single drawn position in the potential [km/s]