def return_params(islitlet, csu_bar_slit_center, params, parmodel):
    """Return individual model parameters from object of type Parameters.

    Parameters
    ----------
    islitlet : int
        Number of slitlet.
    csu_bar_slit_center : float
        CSU bar slit center, in mm.
    params : :class:`~lmfit.parameter.Parameters`
        Parameters to be employed in the prediction of the distorted
        boundaries.
    parmodel : str
        Model to be assumed. Allowed values are 'longslit' and
        'multislit'.


    Returns
    -------
    c2 : float
        Coefficient corresponding to the term r**2 in distortion
        equation.
    c4 : float
        Coefficient corresponding to the term r**4 in distortion
        equation.
    ff : float
        Scaling factor to be applied to the Y axis.
    slit_gap : float
        Slit gap.
    slit_height : float
        Slit height.
    theta0 : float
        Additional rotation angle (radians).
    x0 : float
        X coordinate of reference pixel.
    y0 : float
        Y coordinate of reference pixel.
    y_baseline : float
        Y coordinate employed as baseline.

    """

    if parmodel == "longslit":
        # set each variable in EXPECTED_PARAMETER_LIST to the value
        # transferred through 'params'
        c2 = params['c2'].value
        c4 = params['c4'].value
        ff = params['ff'].value
        slit_gap = params['slit_gap'].value
        slit_height = params['slit_height'].value
        theta0_origin = params['theta0_origin'].value
        theta0_slope = params['theta0_slope'].value
        x0 = params['x0'].value
        y0 = params['y0'].value
        y_baseline = params['y_baseline'].value
    else:
        # set each variable in EXPECTED_PARAMETER_LIST_EXTENDED to the value
        # transferred through 'params'
        c2_a0s = params['c2_a0s'].value
        c2_a1s = params['c2_a1s'].value / 1E3
        c2_a2s = params['c2_a2s'].value / 1E6
        c2 = c2_a0s + \
             c2_a1s * csu_bar_slit_center + \
             c2_a2s * csu_bar_slit_center ** 2
        # ---
        c4_a0s = params['c4_a0s'].value
        c4_a1s = params['c4_a1s'].value / 1E3
        c4_a2s = params['c4_a2s'].value / 1E6
        c4 = c4_a0s + \
             c4_a1s * csu_bar_slit_center + \
             c4_a2s * csu_bar_slit_center ** 2
        # ---
        ff_a0s = params['ff_a0s'].value
        ff_a1s = params['ff_a1s'].value / 1E3
        ff_a2s = params['ff_a2s'].value / 1E6
        ff = ff_a0s + \
             ff_a1s * csu_bar_slit_center + \
             ff_a2s * csu_bar_slit_center ** 2
        # ---
        slit_gap_a0s = params['slit_gap_a0s'].value
        slit_gap_a1s = params['slit_gap_a1s'].value / 1E3
        slit_gap_a2s = params['slit_gap_a2s'].value / 1E6
        slit_gap = slit_gap_a0s + \
                   slit_gap_a1s * csu_bar_slit_center + \
                   slit_gap_a2s * csu_bar_slit_center ** 2
        # ---
        slit_height_a0s = params['slit_height_a0s'].value
        slit_height_a1s = params['slit_height_a1s'].value / 1E3
        slit_height_a2s = params['slit_height_a2s'].value / 1E6
        slit_height = slit_height_a0s + \
                      slit_height_a1s * csu_bar_slit_center + \
                      slit_height_a2s * csu_bar_slit_center ** 2
        # ---
        theta0_origin_a0s = params['theta0_origin_a0s'].value
        theta0_origin_a1s = params['theta0_origin_a1s'].value / 1E3
        theta0_origin_a2s = params['theta0_origin_a2s'].value / 1E6
        theta0_origin = theta0_origin_a0s + \
                        theta0_origin_a1s * csu_bar_slit_center + \
                        theta0_origin_a2s * csu_bar_slit_center ** 2
        # ---
        theta0_slope_a0s = params['theta0_slope_a0s'].value
        theta0_slope_a1s = params['theta0_slope_a1s'].value / 1E3
        theta0_slope_a2s = params['theta0_slope_a2s'].value / 1E6
        theta0_slope = theta0_slope_a0s + \
                       theta0_slope_a1s * csu_bar_slit_center + \
                       theta0_slope_a2s * csu_bar_slit_center ** 2
        # ---
        x0_a0s = params['x0_a0s'].value
        x0_a1s = params['x0_a1s'].value / 1E3
        x0_a2s = params['x0_a2s'].value / 1E6
        x0 = x0_a0s + \
             x0_a1s * csu_bar_slit_center + \
             x0_a2s * csu_bar_slit_center ** 2
        # ---
        y0_a0s = params['y0_a0s'].value
        y0_a1s = params['y0_a1s'].value / 1E3
        y0_a2s = params['y0_a2s'].value / 1E6
        y0 = y0_a0s + \
             y0_a1s * csu_bar_slit_center + \
             y0_a2s * csu_bar_slit_center ** 2
        # ---
        y_baseline_a0s = params['y_baseline_a0s'].value
        y_baseline_a1s = params['y_baseline_a1s'].value / 1E3
        y_baseline_a2s = params['y_baseline_a2s'].value / 1E6
        y_baseline = y_baseline_a0s + \
                     y_baseline_a1s * csu_bar_slit_center + \
                     y_baseline_a2s * csu_bar_slit_center ** 2

    theta0 = theta0_origin / 1E3 + theta0_slope / 1E4 * islitlet

    return c2, c4, ff, slit_gap, slit_height, theta0, x0, y0, y_baseline