1.1

Opto-Mechanical Groups (OMGs) design load

Each lens, together with the metallic support where it is bonded, forms the six Opto-Mechanical Groups (OMGs) that are integrated in the telescope barrel, made in Aluminium - Beryllium alloy. Such OMGs are shown in Figure 4.

Figure 4.

Optical-Mechanical groups (OMGs) of the Telescope

Considering that the telescope will operate in the orbital Lagrangian L2 point and that it will be oriented always toward the deep space, the operative thermal environment is very stringent. The operative temperature of the TOU is -80 °C, and it will be adjusted between -70 °C and -90 °C to better adjust the focus on the FPA. However, the OMGs shall withstand to the not-operative temperature, that reaches -115 °C, including also the qualification margins.

The Coefficient of Thermal Expansion (CTE) of each lens material matches exactly the CTE of the metallic alloy chosen for the mount. In this way the thermo-elastic load induced by the mount on the lens will be minimized. On the other hand, the lens is bonded to its mount by means of many epoxy adhesive pads. This structural adhesive, even if it is characterized by a strong strength to maintain the lens in position despite the dynamic load at launch, it has also a high CTE, that usually is ten times higher than the glass ones. The CTE mismatching between the lens material and the adhesive induces on the glasses a peak of strength that becomes a design limit for the OMG design.

The best way to proceed to limit the strength on the glass induced by this CTE mismatching is to reduce as much as possible the dimension of the adhesive pad. The limit of the pad area shall be determined by the dynamic load at launch to which the OMG is subjected. In particular, all OMGs have been designed to withstand a design limit load of 55 g.

This means that the total number of adhesive pads that sustains the lens – each pad has a diameter of 3,5 mm – has been determined in order to sustain a shear load equivalent to the mass of the lens multiplied for the 55 g of design limit load.

The bonding procedure has been setup foresees the use of a centering machine, in order to have the best control of the adhesive pad diameter and of the position of the lens with respect to its mount, in terms of centering, axial position and tilt. Figure 5 shows respectively the alignment and the bonding phases for the OMG 5.

Figure 5.

Alignment and bonding of Optical-Mechanical Groups (OMGs) under centering machine.

1.2

Issues in OMG Margin of Safety determination

The strong thermal load to be considered, the very small diameter of the bonding pads and in general the assumption used in a Finite Element Model (mesh size, bonding modeling, material properties definition at cryogenic temperature, etc) make the determination of the strength on the lens hard to be calculated with good accuracy. Moreover, the applicable ECSS defines strong safety factors to be used for brittle materials in the Margin of Safety calculation, and on the same time the allowable strength for glasses at cold temperature are hard to find.

The lens materials used on the TOU are listed here below, together with the mount material to which it is coupled.

Table 1.

TOU OMG materials

In particular, two of the materials used on the TOU, the CaF2 of OMG-3 and the S-FPL51 used on OMG-1 and OMG-4, are characterized by a very low value of allowable strength, therefore, for these materials was very hard to choose a design confirmed by an analytical approach that includes all the safety factors defined by the space normative.

Another issue that cannot be solved with a simple analytical approach is the definition of the failure mode in a bonded junction. In general, three different failure modes can be observed on a bonded junction:

Sometimes a combination of different failure modes can be observed on a dedicated test campaign. In addition, the failure load and the failure mode are strongly dependent from the bonding procedure and operator, therefore it is very hard to determine with a FEM model the system allowable.

1.3

BreadBoards (OMGs BB) Test Campaign

To solve this issue, a dedicated test campaign on OMGs BB have been setup to qualify the design of the six OMGs and to validate the procedures used for their bonding and alignment.

For each OMG, two identical BBs have been produced. To reduce cost, each lens was polished but uncoated, the L1 was a spherical lens, in place of the aspherical one adopted for the nominal design, and the L6 glass was in not rad-hard version. All the mounts were representative of the design and the coating, with particular attention to the bonding interface. With these assumptions all the bonded interfaces between each lens and its mount were flight-like.

All the OMGs BB have been subjected to thermal cycles at cryogenic temperature before doing the vibration test. After thermal cycles the first set of BBs (BB1) have been subjected to random vibration in order to validate the bonding design against the 55 g design limit load, while the second set of BBs (BB2) have been used to check the bonding design when subjected to shock environment.

Next chapters present the results obtained in particular for the OMG-3 and OMG-4.

The OMG-3 (Figure 6) has a lens made by CaF2 and bonded on a RSA-905 mount. The STOP, always made by RSA-905, is mounted on the mount through six countersunk screws. A section view of the OMG-3 is shown below.

Figure 6.

OMG-3 design.

The OMG-4 instead is composed by a mount in RSA-443 and a lens bonded on it in S-FPL51 (Figure 7).

Figure 7.

OMG-4 design.

2.1

TVC Test setup

The TVC test has been performed in the CRV thermal vacuum chamber facility at Leonardo premises in Campi Bisenzio (Figure 8),

Figure 8.

CRV front view (left) and internal view (right).

All the OMG BB1 and BB2 has been subjected to 8 cycles between -115 °C and 45 °C, i.e. the qualification not-operative temperatures. The maximum temperature has been covered also during the bake out cycle done on each OMG after adhesive curing at 65 °C.

As shown in Figure 9 (left side), two thermistors were mounted directly on the glasses, on the center of the lens and near the edge, and one thermistor was mounted on the OMG mount. The temperature was monitored and controlled in order to avoid overstressing the glasses and possible failure for thermal shocks. For this reason, the OMGs were simply leaning on the cold plate without any preload that could increase the conductivity, limiting the temperature gradient on the lenses, CaF2 of OMG-3 in particular, to 0.2 °C/min maximum. The duration of each complete thermal test has been about 11 days.

Figure 9.

OMG TVC test setup.

OMG4 BBs have been tested with the same TVC, whilst OMG3 BBs have been tested with two different TVCs : OMG3 BB1 has been cycled together with the OMGs-4 (Figure 9– left side) while OMG-3 BB2 has been cycled together with OMGs-6 OMGs-5 and OMGs-2 (Figure 9 – right side).

2.2

TVC temperature profiles

The different boundary condition given by the position inside the chamber and the different interface on the cold plate of the OMGs lead some variation on the minimum temperature reached by each OMG. In particular, the minimum temperature measured by OMG-4 thermistors on the lens is -115.3 °C, whilst the temperature on the OMG-3 was higher than the one measured on the other OMGs. After some adjustments and optimizations of the thermal cycle, the minimum temperature reached by the OMG3 was -105.2 °C on the BB1 and -109.1 °C on the BB2. For this reason, only for OMG-3 BB2, an additional thermal cycle has been done to verify the bonding design against the qualification minimum temperature of -115 °C.

The following figures show the plots of the temperatures vs time measured by means the reference thermistors mounted on the shroud, on the cold plate and on the OMGs.

Figure 10.

OMG-3 BB1 and BB2 TVC temperature profile.

Figure 11.

OMG-3 BB2 additional TVC temperature profile.

Figure 12.

OMG-4 BB1 and BB2 TVC temperature profile.

The visual inspection of the OMGs performed at completion of each TVC confirmed that the OMG passed successfully the thermal cycling without any degradation.

3.1

Vibration Test setup

The vibration test performed on the OMG-3 and 4 BB1 has been subdivided in two phases:

The random tests were performed on the shaker LDS V964LS/DPA110-140K DC located in Florence at the site of Leonardo. Figure 13 shows the shaker in the Out-Plane configuration (left side) and in the In-Plane configuration with the slip table mounted (right side).

Figure 13.

Shaker used for vibration test on BBs.

For each run the OMG3 was rigidly attached on a reference fixture and two mono-axial accelerometers were attached on it to control the vibration input. The control has been done using the mean value of these two accelerometers during the resonance search and the maximum value during the random vibration.

Other two tri-axial accelerometers have been used for the control response and the resonance research on the lens and on the mount. The positions of the accelerometers are reported in Figure 14. The position on the lens was chosen in correspondence of the point at maximum acceleration in order to guarantee that the notching based on the response of this accelerometer leads to a load on the lens always under the 55g limit load in the first phase.

Figure 14.

OMG-3 (left) and OMG-4 (right) test setup

3.2

Qualification test and resonance verification

A sine vibration in the frequency range from 20 to 2000 Hz with a sweep rate of 2 oct/min (one sweep up) has been performed before and after each random run. Resonance frequencies have been measured during the whole resonance search test to verify that they are higher than 140 Hz and that no frequency shift greater than 5 % is observed between first and last modal survey. The level of the resonance search was chosen considering the amplification factor, in order to avoid to exceed the qualification load.

Table 2 shows the summary of the resonance search, in terms of main frequency of each OMG and relative amplification factors. The Transfer Function measured with the initial resonance search has been used to evaluate the notching profile.

Table 2.

OMG-3 and OMG-4 resonance search summary

The initial TF for all axes are plotted in the following figures together with the TF measured during the last run after random test with qualification level on the center of each lens. When comparing the resonance search at the start and at the end of the qualification campaign, one can observe the absence of any dynamic changes in the whole frequency range.

Figure 15.

OMG-3 (left) and OMG-4 (right) resonance search comparison

3.3

Random test till failure

Starting from the full level random load given to each OMG at the end of the qualification campaign along the Z axis, the plateau of the notching has been increased by +3dB for each run, until failure was reached. Before and after each random run, a resonance search and a visual inspection was done to verify the status of the lens and of the adhesive pads.

For OMG-3 and OMG-4 the failure of the lens happened respectively with an input load of 15.6 gRMS and 22.43 gRMS, which corresponded to a load measured on the lens of 190 gRMS and 199 gRMS. Instead, the last load sustained by the lens without any failure was 137 gRMS, given by an input of 14.3 gRMS, for OMG-3 and 163.5 gRMS, given by an input of 18.44 gRMS, for the OMG-4.

The following pictures show the input given to the OMG-3 and OMG-4 for each run (black curve), together with the response on the lens (red curve),for the last successful run without failures and damages. On each picture it is also indicated the gRMS reached (input at the I/F written in black and output on the lens written in red) and the maximum PSD (y value written in red at the frequency x) reached on the lens.

Figure 16.

OMG-3 and OMG-4 successful random run before failure

3.4

Visual inspection after random test

The following pictures show the OMG at the failure of the lens. The crystalline structure of the CaF2 leads a mode of breaking into multiple parts. In particular, it can be noted two main planes along which the glass is broken. The adhesion of the adhesive on the lens and on the mount do not present any sign of degradation or detachment.

Figure 17.

OMG-3 visual inspection after failure.

The failure of the OMG-4 shows the breakage of the lens in correspondence of the edge. The adhesion of the adhesive on the lens and on the mount do not present any sign of degradation. The following figures show the photo of each area after the failure of the lens.

Figure 18.

OMG-3 visual inspection after failure.