The elements are suspended by means of metallic wires of different material and dimensions. We report their dimensions in the following table. We notice that the mirror is suspended using copper strips of rectangular cross-section. 
 
 
 

    The strips suspending the mirror are hooked to the inner core of marionette , while the steel wires of the mirror reference mass are clamped to the marionette outer corona made of diamagnetic stainless steel AISI 316 LN. The actuators for steering the marionette are composed by 8 coils hosted  by the marionette reference mass.

    We report in figure 1 and 2 the mechanical drawings of the reference mass of the marionette and the marionette itself.  
 
 

    Fig.1 -  The reference mass of the marionette 
 
 

Fig.2 -  The marionette

    The two elements were suspended each by a mechanical coupling widely used in the VIRGo super attenuator , called “junction box”. In figure 3 we show this coupling system for the two elements of the payload. 
 

    Figure 3. - The suspension element between the reference mass of the marionette and the marionette itself. In the center of the drawing the junction box is shown. 
 

    The refrigeration power is transmitted to the marionette and its reference mass independently, using  24 strips of rectangular cross section ( 0.5 mm  x 2 mm) made of pure aluminium (5N degree). To minimize the thermal resistance in the contact area  the strips are pressed against the element bulk by means of  copper washers of suitable form.

    The reference mass of the mirror is suspended from the marionette using C85 steel. The reference mass shape permits to surrounds the mirror so that its center of mass coincides with that of the mirror. The mass includes the 4 coils of the actuators by means of which we can steer the mirror. In figure 4 we show on the right the mechanical drawing of this element and on the left its three-dimensional model.  
 
 
 

    Figure 4. - The reference mass of the mirror. 
 

On the model we notice  the  steel ring  bolt on the front side of the reference mass. This extra  element is used to insure the horizontal balance of the suspended mass.

In the following page we show the six steps of the assembly procedure. 

The first picture shows the thermal links for cooling the marionette. Once the marionette is in place, we suspend the mirror .  Then the reference mass of the marionette is moved down to cover the marionette. In picture number 4 we show a detail of the suspension point of the marionette on top of the marionette reference mass.

Finally the reference mass of the mirror is suspended and in the picture number 6 we show the back of the reference mass hosting the actuator coils. 
 
 

Figure 5. - The payload assembly procedure 
 

As we said before, we can apply forces on the mirror using 4 e.m. actuators, while the marionette is controlled using 8 actuators The electromagnetic actuators are similar to those used in Virgo: the coil support is made of macor ceramic and the coil wire is kapton insulated for insuring the high vacuum compatibility at room temperature and the stability at low temperature. In the table we summarize the actuator characteristics.  
 
 

The mirror actuators and the marionette have been used to drive the mode frequencies of the system. At room temperature an optical lever sensor based on a bi-dimensional Position Sensing Device (PSD) and  an optical fiber bundle readout have been set up to detect the mechanical modes. These have been identified also by comparing the spectral measurements with the predictions based on a finite element model (FEM) of the payload. An accurate prediction of the dynamic behaviour of the payload is obtained by a fine meshing of the mechanical structure based on a high number of nodes (~3 10⁵ ) and a suitable choice of the basic element. In the figure we show the meshing on the base of which we simulated the payload dynamics. 
 
 

Figure 6. - The payload meshing of the finite element mode  developed using the ANSYS software package.

This simulation permits to identify the oscillation mode of the structure which we measured at room and at low temperature using the optical sensors. In the following figures we show the displacement spectrum of the mirror of the payload obtained by using the PSD readout. The PSD provides two signal outputs for the vertical and horizontal displacements. The two spectra are compared with that obtained using the optical fiber bundle when the payload was cooled at low temperature. 
 
 
 

Figure 7. - Displacement spectrum of the mirror measured with the PSD and fiber bundle readout in the low frequency range. 
 

Here we show the ANSYS simulation output of the spatial mode shape for the torsion and pendulum modes of the payload, corresponding to the peak of the blue curve at 0.6 Hz and the peak of the red curve at 1Hz respectively

Pendulum mode  1 Hz

Torsional mode  0.6  Hz

Figure 8. – ANSYS identification of the pendulum and torsional oscillation modes. 
 

Then, we report also the ANSYS graphic output on the base of which we the identify the triplet in the centre of the spectrum of figure 4.  
 
 
 

Figure 8. – The oscillation modes corresponding to the triplet which appears in the plot of the mirror displacement spectrum. 
 

Similar results have been obtained for the other modes shown in the higher frequency spectrum reported below.

3.3 Hz

Figure 9. -   Displacement spectrum of the mirror measured in the range 0-20 Hz  with the PSD and fibre bundle readout.