I-A.

Considerations on the deformations during machining and metrology of thin glass shells

As already mentioned, the wall thickness of WFXT shells is very small, in order to meet the effective area and mass requirements; the thickness is a factor of 5-10 less with respect to past experiences such as ROSAT or CHANDRA. The wall thickness is a function of the shell diameter, ranging from 1.2 mm (for the innermost shell) until 2.2 mm (for the outermost shell), to have constant stiffness behaviour across the whole series of mirror shells. In the last few years, new machines and polishing techniques have been developed with performances very promising, offering the possibility to perform the direct optical machining of thin substrates. The WFXT mirror shells are not only thin, but they are also characterized by a very small Length-to-Diameter ratio (L/D), 3 times smaller than XMM-Newton mirrors. For this reason, their realization is more difficult because short mirror shells are more sensitive to perturbing effects related to edge loads. Under the same perturbing edge loads, short mirror shells show degradation 6-16 times larger with respect to long mirror shells and this ratio becomes even bigger in case of perturbing loads producing local deformed shapes. Moreover the angular resolution is strongly affected by the slope errors caused by out-of-phase azimuthal errors, which are inversely proportional to the mirror shell length. It is worth noting that the consideration of concentrated loads at the free edges of the shells is really representative of the actual conditions. Indeed normally the mirror shells are fixed to the mirror module structure by spot connections at the one or both end sections and, during the metrology and integration operations, they rest on concentrated astatic supports at one-end section. An astatic support (see Fig. 2-A) is needed to measure the intrinsic shape of the thin shells, since relevant deformations would be otherwise caused by gravity. Two astatic supports are already available at INAF-OAB, one with 12 and the other with 16 sustaining points. It was verified by FEM and ray-tracing analyses that with the available astatic supports, the deformations introduced on the shells are negligible, in the order of 0.5 arcsec HEW.

Fig. 2.

Side view of the astatic support system used during OOR metrology. B) Raw quartz glass tube mounted on the grinding machine at the Heraeus Quarzglas facility. C) First shell integrated in its SSS.

I-B.

Considerations on the resistance to launch conditions of thin glass shells

Preliminary FEM analyses have also been carried out in order to estimate a value for stress peaks in the quartz glass mirrors during launch, even if at this stage of the project, exact load levels, and operative and survival temperatures are not yet available and need to be evaluated in near future. A complete check of quartz components will require some more information, relevant to the characterization of the specific material, which are not known at the moment. Nevertheless, simple preliminary considerations can be performed, based on engineering evaluations and past experiences. Assuming a traditional X-ray telescope, each single mirror shell is connected to the telescope structure just at the end sections (one or both) through spokes wheel elements. Point connections between each mirror shell and the spokes wheel are realized by using adhesive contacts. If such a configuration is adopted for the FEM analyses, the maximum stresses in the glass might exceed the tensile stress limit in points of stress concentration. Local refinement of the analyses and the design are needed considering the use of a brittle material like quartz. Revised design criteria with respect to the traditional approach will allow obtaining surviving loads conditions. In addition to that, the use of chemical etching treatment of the shell surfaces in order to reduce the micro-cracks cause by the grinding is under investigation to improve the intrinsic stiffness of the quartz.

II-A.

Grinding results

The grinding of the raw quartz tubes has been pursued by Heraeus Quarzglas GmbH & Co KG. The requirement was to obtain a thin quartz shell with double cone profile and an Out-Of-Roundness (OOR) error not larger than 60 micron. The raw material is represented by the high purity HSQ 300 quartz glass, produced by the same company. Several options have been tested to optimize the grinding set-up, studying parameters like the grains type and size of the abrasive slurry, the tool diameter and speed, and the quartz tube supporting configuration during the grinding process. In particular, three runs have been performed to study those aspects. During the first run the raw parameters of the process have been selected. The other two runs have been pursued to optimize the support system of the quartz tube during the machining (see Fig. 2-B). In fact, during the first and the second runs, high OOR errors were present in the produced shells (see Table 6) and a thorough analysis of the problem confirmed that the deformations were originated by a wrong supporting method adopted for fixing the quartz tube on the grinding machine, that has been afterwards corrected. The selected approach foresees a two-step grinding process: first the external and internal surfaces of the quartz glass tube are machined, by means of a rough grinding machining; then the internal surface undergoes a finer grinding step. The purpose of this second step is to reduce the surface damage depth left by the initial grinding. Even if the polishing removes the subsurface damage on the internal surface, however residuals still remain on the external surface and this make the material quite brittle. To address this problem a treatment of chemical etching has been investigated, as already mentioned. The chemical etching of the whole surface has been performed by Heraeus on three of the produced shells after the grinding, to demonstrate the feasibility and to test the compliance with the requirements in terms of shape. The tuning of the process will be addressed by further studies to be carried out in near future. Heraeus Quarzglas performed the metrology with 3D Zeiss Coordinate Measuring Systems, supporting the shell on three points, providing absolute values for diameters and measurement of profiles. INAF-OAB afterwards performed the shells metrology with an optical sensor mounted on a rotational table, supporting the shell with an astatic system. The measured values are reported in Table 3. The capability to grind the thin shells with the required double-cone profile and accuracy in terms of OOR of a few tens of microns has been demonstrate during this first phase of the study.

Table 3.

Out-Of-Roundness and longitudinal P-V errors for the 8 pre-machined prototype shells.

1

Average of 4 to 6 OOR values from roundness profiles at different height.

2

Average of 8 P-V values (4 parabolas, 4 hyperbolas) at azimuthal distances of 90°.

II-B.

Development of suitable jig for polishing and metrology phases

A suitable jig, called Shell Supporting Structure (SSS), has been designed and manufactured to allow the handling of the shell during all the production steps. The main requirement for the jig was to have a system able to maintain the shell intrinsic shape in both the vertical (intended with respect to the optical-axis) and horizontal positions without introducing deformations due to gravity or thermal loads. The horizontal position is required during metrology and X-ray characterization phases; the vertical position is used during polishing, coating deposition and assembly into the final structure steps. The SSS is the result of an optimization process carried out through FEM analyses aimed at the evaluation of stresses, deformations and related optical performances degradation introduced in the thin shell by the supporting method. A direct connection between the mirror shell and the supporting structure through bonding would introduce important degradations of the optical performances in the case of large CTE and adhesive cure shrinkage. It would also cause deformations of the supporting structure to propagate into the shell. The phenomenon can be mitigated to an acceptable level (HEW degradation less than 0.5 arcsec) by introducing a radial flexure (realized by thin metal foil) at the connection between the mirror shell and SSS. The final SSS is composed of two rings made of glass that allocate metallic inserts for interface purposes with the polishing machine. The two rings are joined through metallic bars. The shell is connected only to one ring through the metallic comb flexure that allows a radial flexure at the connection between the mirror shells and SSS. Removable space-qualified glue with low shrinkage and low CTE has been selected to glue the shell to the metallic comb in order to avoid problems in the vacuum chambers during coating deposition or X-ray tests. The first shell integrated in the SSS has been broken after the delivery at OAB due to an incorrect handling. However, it was used as a dummy shell to test and optimize the SSS design and integration process, even if the OOR value resulted to be strongly degraded (see Table 4). With the other two integrated shells, it has been verified that the integration process in the jig is able to maintain or even improve the original shape.

Table 4.

Comparison of the Out-Of-Roundness measurement performed before and after integration into SSS.

Fig. 3.

A) FEM simulation carried out to verify the effect on the shell due to SSS deformations in the case of direct connection between the shell and the SSS. Deformations of the SSS are transmitted to the shell. B) Detail of radial displacement isocontours at the connection between the mirror shells and the SSS ring. The effect of the adhesive expansion due to thermal effects is evident. C) The use of a metal foil introduces a radial flexure, implemented in order to avoid the transmission of possible thermo-mechanical deformations into the shell.

II-C.

Figuring and polishing results

The polishing process is being performed by the Zeeko firm (UK), using a IRP600 machine, a CNC polishing systems, controlled on 7-axis, that uses a patented process for the shells polishing, named “Bonnet polishing” [7]. The Zeeko IRP Serie machines have the capability of machining shells with diameter up to 1100 m and height up to 440 mm, as required for WFXT telescope. By acting on the process parameters, such as the tool pressure, precession angle, compression offset and head speed, it is possible to select the desired influence function, i.e. the 3D depression left by the spinning precessing tool as it is pressed onto a piece of the material to be polished. The so called “influence” or “removal” function is generated, in order to determine the best volumetric removal rate in dependence of the specific materials and requirements. Several tests have been performed during the study, in order to evaluate the problems caused by the deformations of the thin walls by effect of the tool pressure. The shell used for these preliminary tests was the first ground dummy shell used to test integration into SSS that presents high OOR values (800 µm). The possibility of performing an intermediate coarser step (named “grolishing”) before the polishing was investigated. Grolishing is needed to correct possible residual errors coming from the grinding step and to remove the external layer of surface damaged at a microscopic level. Different process parameters were investigated in order to define the best influence (or removal) function. Faster influence functions are used for bulk material removal. While slow removal functions provide better results in manipulating smaller scale errors. At first, selected segments have been raster “grolished”: the machined area was 225 mm long x 100 mm wide (from top edge to transition zone). Three different radial segments have been treated in such a way to compare different tool speed and cloth. An entire half of the shell surface has been machined and important information has been obtained regarding the effects of vibration and deformation introduced during the Bonnet machining. It should be noted that the results achieved so far are very promising and useful parameters were inferred to carry out future activities. In particular, it has been demonstrate that the OOR errors have effects on the radius of curvature of the Bonnet tool to be used and on the removal coefficient. For OOR errors less than 10 μm P-V the grolishing process can be skipped ad only the polishing phase should be performed. On the contrary, if the OOR error ranges between 10 and 60 μm P-V a pre-phase of grolishing should be considered, in order to remove defects coming from the precedent phase.

Fig. 4.

A) Shell mounted on the Zeeko IRP600 machine. B) Example of the influence function of the grolishing process. The circle indicates the grolishing starting point. C) Particular of the Bonnet polishing tool during one of the first grolishing tests to evaluate the suitability of the SSS as interface to the polishing machine.

The polishing phase will start very soon on the second and third shells integrated in the SSS. Those new tests will be important to evaluate the possibility of obtain proper polynomial profiles starting from the double-cone shape. It should be mentioned that it has been also necessary to define and partially realize the metrological tools needed to monitor the surface during the polishing phase. Profile measurements have been performed with a PGI 1200 Thomson system, while the surface texture and roughness have been checked with a CCI Taylor and Hobson instrument. Custom interfaces and jigs for the connection to the polishing machine and to the metrology tools have been specifically developed at the Brera Observatory.