Method for manufacturing optical elements for telescope optics usable in space missions

Abstract

A process for manufacturing an optical element (1), comprising a first step of rotating a circular sheet (2) of a first metallic material to adhere the circular sheet (2) of the first metallic material to a rotating matrix (3) and form a shell (8), a second step of assembling the shell (8) on a temporary support (10), and at least a third step of diamond spinning the shell (8) using a diamond tool (13) to form an optical surface (14).

Description

Technical Field

[0001] Cross - reference to related applications This patent application claims priority from Italian Patent Application No. 102020000017086, filed on July 14, 2020, the entire disclosure of which is incorporated herein by reference.

[0002] The present invention relates to an optical element, particularly to a process for manufacturing a telescopic optical system that can be used in space missions.

[0003] The present invention is not exclusive, but preferably, without loss of generality, is applied particularly to the manufacture of grazing - incidence optical systems for X - ray telescopes.

Background Art

[0004] A grazing - incidence optical system consists of a plurality of aspherical annular optical elements that are concentric with each other. For example, in a Wolter - I configuration, each element has a parabolic - shaped portion and a hyperbolic - shaped portion. 3]

[0005] These optical components can be manufactured by a modular process, especially in the case of particularly large mirrors (on the order of 3 m in diameter). That is, a series of modules are manufactured by "stacking" a plurality of segments spaced apart from each other. The segments are typically on the order of 100 mm × 100 mm and are typically composed of a sheet of glass (fused silica, borosilicate glass or other types of glass) or a silicon wafer with a thickness of less than 1 mm. The optical component is composed of up to tens of thousands of segments. 7]

[0006] The problem with this manufacturing method is that it takes time and cost to create each module and to integrate the modules to form a complete mirror, and it is difficult to integrate a large number of modules while maintaining high precision, so there is a limit to the accuracy.

[0007] Therefore, there is a method for manufacturing mirrors from a limited number of monolithic annular glass shells (for example, fewer than 200 shells for an optical system with an outer diameter of 3m).

[0008] This eliminates the need for module assembly and integration, and is expected to improve performance due to the shell's azimuthal symmetry.

[0009] However, glass shells are expensive, fragile, and difficult to procure. Furthermore, the minimum shell thickness is limited not by the material's mechanical strength, but by a safety factor for fragile materials during the manufacturing process. In particular, surface treatment of the outer surface is necessary to increase mechanical strength. While aesthetically pleasing, the use of a monolithic glass shell in this technology has significant limitations in its practical application.

[0010] Furthermore, optical elements are manufactured by machining metal pieces obtained by casting to remove material and obtain thin substrates. This technique has several major drawbacks, including the difficulty in manufacturing thin substrates, the high cost of the first single piece obtained by casting, the long working time, the very low probability of confirming material uniformity until the end of the process, the presence of non-uniform residual stress in the material, and the risk of breakage during the process. Another very broad technique for manufacturing mirrors in one piece is nickel electroforming replication, which starts with a metal mandrel having a negative profile of the mirror to be manufactured. However, this method also has several contraindications, including: i) the difficulty in manufacturing large-diameter mirrors, in particular the difficulty in manufacturing large mandrels with optical-grade lap surfaces (the largest diameter mirror manufactured so far using this method is 70 cm in diameter), ii) the high density of nickel, which is a disadvantage for manufacturing devices that will fly in space, and iii) the difficulty in obtaining very high angular resolution with replication techniques. [Overview of the project] [Problems that the invention aims to solve]

[0011] An object of the present invention is to make available an alternative process for the manufacture of optical elements, which solves the problems associated with the known methods described above. [Means for solving the problem]

[0012] The above objective is achieved by the process according to claim 1.

[0013] For a better understanding of the present invention, preferred embodiments are described below as non-limiting examples, with reference to the accompanying drawings. [Brief explanation of the drawing]

[0014] [Figure 1] Figure 1 is a schematic perspective view of a semi-finished product for shell manufacturing. [Figure 2] Figure 2 is a schematic diagram of the first work phase according to the process of the present invention. [Figure 3] Figure 3 is a schematic perspective view of the semi-finished product at the end of the stage shown in Figure 2. [Figure 4] Figure 4 shows the semi-finished product from Figure 3 in a support structure for subsequent work stages. [Figure 5] Figure 5 is a schematic diagram of the subsequent work steps of the process of the present invention. [Figure 6] Figure 6 is a schematic diagram of the subsequent work steps of the process of the present invention. [Figure 7] Figure 7 is a schematic diagram of the subsequent work steps of the process of the present invention. [Figure 8] Figure 8 is a schematic diagram of the subsequent work steps of the process of the present invention. [Figure 9] Figure 9 shows the completed optical element in a support structure for integration with other optical elements. [Modes for carrying out the invention]

[0015] According to the present invention, the optical element 1 (FIG. 9) is manufactured starting from a circular metal sheet 2 (FIG. 1). The material is preferably an aluminum alloy, such as an aluminum alloy having magnesium (0.4 to 1.7%), manganese (0.5 to 1%) and silicon (1 to 5%), such as Anticorodal. The thickness of the sheet ranges from 0.5 to 15 mm, preferably about 2 mm.

[0016] According to the present invention, the sheet 2 first undergoes a step known as metal spinning, in which the sheet 2 is axially blocked between a forming matrix 3 defining the shape to be obtained, attached to the mandrel of a lathe (not shown), and a counter head 4.

[0017] The matrix 3, the counter head 4 and the sheet 2 are arranged coaxially with each other.

[0018] The mandrel is rotated, and as a result, the assembly of the matrix 3 / sheet 2 / counter head 4 is rotated.

[0019] During rotation, the sheet 2 is deformed by a tool 5 acting on the face of the sheet facing the matrix 3 and is moved parallel to the generatrix of the matrix, so that the sheet 2 can be deformed and gradually attached to the outer surface 6 of the matrix 3 to reproduce its shape.

[0020] At the end of this stage of the process, the central part 7 of the sheet 2 and, optionally, the outer edge 8 facing the central part 7 of the sheet 2 are removed by any known cutting process, and thus a semi-finished product 8 in the form of an axially symmetric shell (hereinafter, for the sake of brevity, "shell 8") similar to that of the optical element to be obtained (FIG. 3) is obtained.

[0021] The shell 8 is then preferably mounted within a support 10 (Figure 4) having a vertical axis, which blocks it axially and radially so as to induce a minimum stress, and thus internal tension, in the material. For this purpose, the support can comprise a plurality of support ribs 11 extending along the equidistant outer shape of the shell 8.

[0022] Alternatively, the rib 11 can be fixed to the shell, for example by adhesion or welding, before removing the shell 8 from the matrix 3. Thus, the rib 11 can constitute a positioning and fixing element in the support 10 provided with a fixing element 12 cooperating with the rib 11.

[0023] Once positioned in the support 10, the shell 8 undergoes a series of successive process steps (Figures 5 - 8) until the finished optical element 1 is manufactured (Figure 9).

[0024] Referring to Figure 5, the shell 8 undergoes a diamond spinning step (precision turning with a diamond tool 13). The support 10 is mounted on and rotated about a mandrel having a vertical axis not shown. The diamond tool 13 is brought into contact with the optical surface 14 (inner surface in the example shown) of the shell 8 and machined with very high precision.

[0025] Thereafter (Figure 6), the optical surface 14 of the shell is coated with electroless nickel (nickel - phosphorus alloy, e.g., Kanigen R) by a deposition process by immersion in an autocatalytic chemical bath in an aqueous solution. In this way, a coating layer in the range of 10 - 100 μm, preferably about 20 μm, is obtained. The immersion in the bath is preferably carried out without removing the shell 8 from the support 10 and can be preceded by inversion. In this way, the shell does not undergo unwanted tension. Alternatively, the shell 8 can be removed from the support 10 and mounted on another dedicated support for immersion in the bath. <​Subsequently, the shell 8 may undergo a second diamond spinning process (optional) - Figure 7 - in which any deformation caused by nickel plating is corrected and the design shape is restored to the optical surface 14.

[0027] Finally, (Figure 8) the optical surface 14 undergoes a final step (optional) of grinding the azimuthal surface with a precision milling cutter, adjusting the profile by bonnet polishing, and superpolishing with a pitch pad or other abrasive material, resulting in a finished optical element 1 with a minute roughness of less than a fraction of a nanometer on the optical surface 14.

[0028] Once completed, the optical element 1 is conveniently assembled on an integrated support 20 (Figure 9), which is a circular structure in which different shells forming the optical system are coaxially fixed by fixing devices 21 that can cooperate with the ribs 11 of the shell 8, if present.

[0029] The integrated support 20 is conveniently made of the same material as the shell 8, for example, aluminum, which eliminates thermally induced tension and allows for the manufacture of a mechanically stable and lightweight optical system.

[0030] Next, this process is repeated to manufacture other optical elements 1, which are then coaxially assembled on the integrated support 20 to form a complete optical system.

[0031] Considering the process described, the advantages offered by the present invention are clear.

[0032] By using spinning technology, the drawbacks associated with known techniques for manufacturing glass shells and for casting and machining metal shells can be eliminated. Therefore, it is possible to manufacture optical components at a lower cost and without the risk of shell breakage during the process. The time required for prototyping and production of optical components is significantly reduced. Because it is a replication process, different prototypes and backup optical components can be manufactured at a low cost.

[0033] Finally, it is clear that modifications and variations can be made to the described process without departing from the scope of protection of the present invention.

[0034] In particular, depending on the requirements for shape accuracy and surface roughness, nickel plating and diamond spinning can be omitted.

[0035] On the surface opposite the optical surface, electrochemical treatment can be performed to prevent oxidation, and / or machining or electrical discharge machining can be performed to lighten the material.

[0036] While the shell is still supported on the matrix, it is also possible to provide or apply support elements or patterns to the surface (e.g., by bonding, welding, or 3D printing). The total thickness of the optical system required to meet structural requirements can be achieved by inserting lightweight materials such as a honeycomb structure and forming the overall structure with two metal shells.

[0037] The aluminum alloy used to manufacture the shell can be selected to harmonize its coefficient of thermal expansion with respect to the support, mitigate the bimetallic effect with nickel, and optimize structural resistance.

[0038] Although the example of a low-light incidence optical system was used, the process of the present invention can also be used in the manufacture of a normal incidence optical system. In this case, the shell can be made as a cup-shaped body that integrally defines the primary mirror and the cylindrical side wall of the telescope.

[0039] This process can also be used to manufacture non-axisymmetric optical components, provided that the deviation of the matrix's parallel line profiles from the circumference is sufficiently small and that the diamond tool can follow those profiles as it rotates.

[0040] According to an optional variation of this process, the matrix can be treated with a silicone oil designed to "fill" the micro-grooves that inevitably exist in the matrix due to machining during the manufacturing stage, such as Bluestar Silicones' Rhodorsil® 47 V 50 oil. This prevents the micro-grooves from being reproduced on the sheet during the metal spinning stage, thus preventing a loss of mirror accuracy.

[0041] According to any further modifications of the process, the shell may undergo stress-relieving annealing heating during machining and / or before the deposition of the coating layer onto the optical surface, thereby eliminating or at least reducing internal tension resulting from machining. [Explanation of symbols]

[0042] 1 Optical element 2 sheets 3 Matrix 5 Tools 8 shells 10 Support 11 Support elements 13 Diamond Tools 14 Optical surface 20 Integrated support

Claims

1. A process for manufacturing optical elements (1) for telescope optics usable in space missions, A first step involves attaching a circular sheet (2) of a first metal material to a rotating matrix (3) and rotating the circular sheet (2) of the first metal material in order to form a shell (8), The second step is to assemble the shell (8) on the temporary support (10), The process includes at least a third step of using a diamond tool (13) to diamond spin the shell (8) to form an optical surface (14), A process for manufacturing an optical element (1), wherein the diamond spinning process is a precision turning process using the diamond tool (13).

2. The process according to claim 1, further comprising a fourth step of depositing a coating layer of a second metallic material onto an optical surface (14).

3. The process according to claim 1 or 2, wherein the first metal material is an aluminum alloy.

4. The process according to claim 2, wherein the second metallic material is a nickel alloy.

5. The process according to claim 4, wherein the second metal material is electroless nickel.

6. The process according to any one of claims 2 to 5, further comprising a fifth step of diamond spinning the coating layer using a diamond tool (13).

7. The process according to claim 5 or 6, further comprising a sixth step of superpolishing the coating layer.

8. The process according to any one of claims 1 to 7, further comprising at least one step of machining the surface of the shell (8) opposite to the optical surface (14) while the semi-finished product is on the matrix.

9. The process according to claim 8, wherein the processing step includes mechanical machining.

10. The process according to claim 8, wherein the processing step includes electrical discharge machining.

11. The process according to any one of claims 1 to 10, wherein the surface of the matrix to which the sheets are bonded is treated with silicone oil.

12. The process according to any one of claims 1 to 7, comprising at least one step of fixing at least one support element (11) on the surface of a semi-finished product opposite to the optical surface (14) while the shell (8) is on the matrix (3).

13. The process according to claim 12, wherein the fixing step includes a welding operation.

14. The process according to claim 12, wherein the fixing step includes an adhesive operation.

15. The process according to any one of claims 1 to 14, further comprising the step of mounting a completed optical element (1) to an integrated support.

16. The process according to claim 15, wherein the above step is repeated periodically to manufacture a plurality of optical elements (1) coaxially with each other and mount them on an integrated support.

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