Method for manufacturing an oblique incidence optical element and optical element obtained thereby - Patents.com

Abstract

A method for manufacturing an oblique incidence optical element (1) includes the steps of providing a mandrel (11) having a surface (12) with a contour complementary to the contour of the optical surface (5) of the optical element (1) to be made, bonding at least one glass sheet (31, 32) to the mandrel (11) to define the optical surface (5), and depositing a carbon fiber reinforcement material on the outer surfaces (51, 52) of the glass sheets (31, 32) to create a shell (3) that is firmly connected to the glass sheets (31, 32).

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This patent application claims priority from Italian Patent Application No. 102022000001904, filed February 3, 2022, the disclosure of which is incorporated herein by reference in its entirety.

[0002] The present invention relates to a method for manufacturing a grazing incidence optical element. [Background technology]

[0003] As is known, grazing incidence optics comprise optical surfaces which are usually surfaces of revolution, for example paraboloids, hyperboloids or ellipsoids, that may be combined together to obtain a double reflection system.

[0004] Grazing incidence optics are used for X-ray optical devices because, unlike what typically occurs for visible light, X-rays can only be efficiently reflected at angles of incidence relative to the normal to the reflecting surface that are close to 90° (e.g., at angles of incidence greater than 88°).

[0005] Grazing incidence optics with a modular structure are known, whose optical surface consists of a number of individual modules, usually made from glass, having, for example, a substantially rectangular or hexagonal shape. Obviously, this individualization entails a deterioration of the optical properties, such as the angular resolution, and the need to integrate a number of modules, which makes the production process long, expensive and difficult to manage.

[0006] Grazing incidence optics are also known that comprise a structure consisting of a metal or glass shell, for example made of nickel and replicated from a mandrel by electroforming, or of a single glass-ceramic material, and a continuous optical surface formed by a reflective coating, for example of gold. However, to manufacture such optics, a substrate (or mandrel, if replication techniques are used) with high dimensional accuracy and extremely low roughness is required, since the deposition of the metal involves replicating the roughness of the substrate or mandrel on the optical surface of the reflective cover. These requirements make the production process for a single grazing incidence optic very costly. Summary of the Invention [Problem to be solved by the invention]

[0007] SUMMARY OF THE PRESENT EMBODIMENT It is an object of the present invention to provide an optical element which is able to overcome the above problems. [Means for solving the problem]

[0008] The above mentioned object is achieved by the method according to claim 1.

[0009] The invention further relates to an optical element according to claim 13 and to a mirror according to claim 16.

[0010] In order that the invention may be better understood, preferred embodiments will now be described, by way of non-limiting example, with reference to the accompanying drawings, in which: [Brief description of the drawings]

[0011] [Figure 1] FIG. 1 is a perspective view of an optical element according to the present invention. [Diagram 2] 2 is a perspective view of a mirror according to the present invention comprising a plurality of the optical elements of FIG. 1; [Diagram 3] 2A-2D show schematic diagrams of successive steps of a method for manufacturing the optical element of FIG. 1; [Figure 4]2A-2D show schematic diagrams of successive steps of a method for manufacturing the optical element of FIG. 1; [Diagram 5] 2A-2D show schematic diagrams of successive steps of a method for manufacturing the optical element of FIG. 1; [Figure 6] 2A-2D show schematic diagrams of successive steps of a method for manufacturing the optical element of FIG. 1; [Figure 7] 1 shows a schematic representation of a plant for carrying out the steps of the manufacturing method according to the invention; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] 1, a grazing incidence optical element according to the present invention is indicated generally by the numeral 1. The optical element 1 comprises a glass layer 2 and a shell 3 of reinforcing material comprising a plurality of carbon fiber layers deposited on the glass layer 2.

[0013] The optical element 1 may constitute a module of a mirror 4, made by integrating several optical elements 1 nested within one another (FIG. 2).

[0014] The glass layer 2 consists of one or more thin sheets having a thickness of 100 μm or less, preferably 50 μm or less, and a flexibility that can be compared to a sheet of paper. For example, SCHOTT AF32® ECO glass can be used for this purpose. The use of one or more sheets depends on the geometry of the optical surface 5 of the optical element 1, as will be described in detail below. The glass layer 2 essentially has a negligible roughness of less than 0.5 nm.

[0015] In the following, without loss of generality, an optical element 1 for a Wolter telescope of type I is considered to have an optical surface 5 consisting of a first part 6, which is a paraboloid of axis A, and a second part 7, which is a hyperboloid coaxial with the paraboloid and joined to it along a plane perpendicular to axis A. Such a telescope has a focal plane 8, on which the incident light reaches after double reflection. In particular, the incident light is reflected from the inner surface of the first part 6 and then from the inner surface of the second part 7, which is interposed between the first part 6 and the focal plane 8.

[0016] The method for making the optical element 1 comprises several steps and is described in detail below.

[0017] First (FIG. 3), a mandrel 11 is provided having a side surface 12 with a complementary contour to the contour of the optical surface 5 of the optical element 1 .

[0018] Thus, mandrel 11 is a B-axis solid, and side 12 of mandrel 11 has a first surface 21 that is a B-axis paraboloid and a second surface 22 that is a hyperboloid that corresponds to the paraboloid along a plane coaxial with and perpendicular to axis B.

[0019] The mandrel 11 is machined by diamond turning, i.e. it is turned with a diamond tool so that the side surface 12 is exactly equal to the desired shape of the optical surface 5. Thus, the first surface 21 and the second surface 22 of the mandrel 11 correspond respectively to the first portion 6 and the second portion 7 of the optical surface 5. It is noted that in the process shown, the application of a polishing step is not necessarily required.

[0020] The mandrel 11 is provided with a number of suction ducts inside which are connected at one end to respective suction holes 23 arranged along the generatrix of at least one of the cones of the mandrel 11 and at their opposite ends to a rotary connector 24 which is connected to a vacuum source (not shown).

[0021] Thereafter (FIG. 4), in the example described for an optical element 1 for a Type I Wolter telescope, a first glass sheet 31 is bonded to the first surface 21 of the mandrel 11 and a second glass sheet 32 ​​is bonded to the second surface 22 of the mandrel 11.

[0022] Specifically, each of the glass sheets 31, 32 is wrapped around a respective surface 21, 22 of the mandrel 11 and held in place by suction through the holes 23. Thus, the glass sheets 31, 32 adhere to the side 12 of the mandrel 11 and replicate its shape. After being wrapped around the mandrel 11, the first glass sheet 31 and the second glass sheet 32 ​​correspond to each other along a plane orthogonal to the axis A, in particular along each adjacent circular edge, and are connected to each other via a first adhesive tape 41; The first glass sheet 31 and the second glass sheet 32 ​​each have opposing corresponding edges along the conical generatrices of the mandrel 11, which edges are connected together via a second adhesive tape 42.

[0023] Thereafter (FIG. 5), carbon fiber reinforcement material is deposited on the outer faces 51 , 52 of the glass sheets 31 , 32 to create the shell 3 .

[0024] In particular, the reinforcing material is carbon fibre reinforced polymer, also known as CFRP, which has low density, high stiffness and a low coefficient of thermal expansion, properties which are necessary for creating the shell 3 .

[0025] However, carbon fiber has anisotropic properties, including a modulus of elasticity and a coefficient of thermal expansion that change when the ply orientation is changed.

[0026] Thus, according to a preferred embodiment of the invention, to make the shell 3 out of CFRP, a filament winding process (FIG. 7) is carried out, in which unidirectional carbon fibres, i.e. in the form of filaments, are impregnated with resin and wound onto the glass sheets 31, 31 so as to form a distribution as isotropic as possible.

[0027] Preferably, carbon fibres with a standard modulus of elasticity are used. Such fibres, unwound from at least one reel 61, pass through an impregnation tank 62 so as to be impregnated with resin. Advantageously, a Huntsman® system is used, in which the epoxy resin Araldite® LY5052 is used together with the epoxy hardener Aradur® 5052. During the filament winding process, the fibres and the resin are combined and wound onto the glass sheets 31, 32 by a deposition head 63 programmed to deposit CFRP filaments forming successive layers until a predetermined thickness is achieved. In order for the shell 3 to have isotropic properties, the filaments of one layer are deposited in a different direction than those of the successive layers.

[0028] After depositing the CFRP, the resin is cured, in particular at room temperature in order to avoid differential thermal expansion between the CFRP and the glass sheets 31, 32.

[0029] Afterwards (FIG. 6), the optical element 1 is separated from the mandrel 11 by cooling the mandrel itself.

[0030] The optical element 1 may finally be subjected to an internal optical coating step designed to optimize the reflectivity of the mirror at a desired energy or wavelength. The optical coating may be performed by a variety of techniques, such as vacuum evaporation, deposition by a sputtering process, or deposition by chemical reduction of a thin film. The optical coating may consist of one or more metallic or non-metallic thin films.

[0031] An examination of the properties of the optical element 1 reveals the advantages of the present invention.

[0032] In particular, the optical element 1 is of monolithic construction, which, compared to optical elements having distributed optical surfaces, provides improved optical properties such as angular resolution, and also reduces the time, cost and management complexity of manufacturing the optical element 1.

[0033] The contour of the glass layer 2 has a high dimensional accuracy, since it is complementary to the contour of the mandrel 11, which can be achieved by high-precision machining, such as diamond turning. In contrast to optical elements with a metallic structure (e.g. in nickel) and gold optical surfaces, the roughness of the side surface 12 of the mandrel 11, produced by diamond turning of the mandrel 11, is essentially inconsequential for the purposes of the optical element 1, since the roughness of the glass layer 2 of the optical element 1 is negligible and the micro-roughness of the mandrel is not transferred to the glass by elastic or inelastic deformation due to its hardness.

[0034] The properties of the CFRP are made suitable for the shell 3 of the optical element 1 .

[0035] Avoiding the use of woven carbon fiber structures, the filament winding process, in which resin and unidirectional carbon fibers are used, can prevent surface irregularities of the carbon fibers from being reproduced on the optical surface 5 (print-through effect).

[0036] As an alternative to the filament winding process, it is also possible to use a fiber placement process, i.e., robotic deposition of fibers in the form of a prepreg tape unwound from a reel, or to manually deposit the fibers in the form of a prepreg.

[0037] The characteristics of the production process make it possible to manufacture optical elements 1 with high rigidity, thermal and mechanical stability, diameters of up to 1 m, and angular resolutions of less than 5 arcsec.

[0038] Since no polishing operation is required, the manufacturing time and cost of the optical element 1 are reduced.

[0039] Finally, it is obvious that modifications and variations can be made to the optical element 1 without departing from the scope of protection defined by the claims.

[0040] For example, the glass layer 2 may have a conical shape or a geometric shape defined by a polynomial. [Explanation of symbols]

[0041] 1 Optical elements 2 Glass Layer 3. Shell 4. Mirror 5 Optical surface 6 First Part 7. Second Part 8 focal plane 11 Mandrel 12 Side 21 First Surface 22 Second Surface 23 Suction hole 24 Rotation Connector 31 First Glass Sheet 32 Second Glass Sheet 41 First adhesive tape 42 Second adhesive tape 51 Exterior 52 Exterior 61 Reels 62 Impregnation tank 63 Deposition Head A-axis B-axis

Claims

1. A method for manufacturing a grazing incidence optical element (1), comprising the steps of: - providing a mandrel (11) having a surface (12) with a contour complementary to the contour of the optical surface (5) of the optical element (1) to be produced; - bonding at least one glass sheet (31, 32) to said mandrel (11) in order to define said optical surface (5); - depositing a carbon fibre reinforcing material on the outer faces (51, 52) of said glass sheets (31, 32) in order to create a shell (3) which is rigidly connected to said glass sheets (31, 32); The method includes:

2. The method of claim 1, wherein the mandrel (11) is machined by diamond turning.

3. 3. The method according to claim 1 or 2, wherein the glass sheets (31, 32) have a thickness of less than 100 μm.

4. The method according to claim 1 , wherein the carbon fibre reinforced material is CFRP (carbon fibre reinforced polymer).

5. 5. The method according to claim 1, wherein the step of adhering the at least one glass sheet (31, 32) to the mandrel (11) comprises wrapping the glass sheet (31, 32) around the mandrel (11) and holding the glass sheet (31, 32) in place by creating a vacuum between the mandrel (11) and the at least one glass sheet (31, 32).

6. 6. The method according to claim 1, wherein the step of adhering the at least one glass sheet (31, 32) to the mandrel (11) comprises the step of applying adhesive tapes (41, 42) along opposing, mutually corresponding edges of the glass sheets (31, 32).

7. 7. The method according to any one of claims 1 to 6, wherein the mandrel (11) comprises at least two portions (21, 22) of corresponding different geometric shapes along a plane perpendicular to the axis (B) of the mandrel (11), and the optical surface (5) is defined by at least two glass sheets (31, 32) deposited on each portion (21, 22) of the mandrel (11).

8. 8. The method of claim 7, wherein the glass sheets (31, 32) are joined together via adhesive tapes (41, 42) applied to each adjacent edge of the glass sheets (31, 32) that correspond along the plane perpendicular to the axis (B) of the mandrel (11).

9. 9. The method of claim 1, wherein the carbon fiber reinforcement material is deposited in the form of filaments.

10. The method of claim 9 , wherein the filaments are impregnated with a resin before being deposited.

11. The method of claim 10 including the step of curing at ambient temperature.

12. The method according to any one of the preceding claims, comprising the step of cooling the mandrel (11) to separate the optical element (1) from the mandrel (11).

13. An optical element comprising at least one glass sheet (31, 32) and a shell (3) of carbon fibre reinforced material firmly connected to an outer surface (51, 52) of said glass sheet (31, 32).

14. 14. Optical element according to claim 13, wherein the glass sheets (31, 32) have a thickness of less than 100 μm.

15. The optical element according to claim 13 or 14, wherein the carbon fiber reinforced material is CFRP (carbon fiber reinforced polymer).

16. Mirror for a telescope, comprising a plurality of optical elements (1) according to any one of claims 13 to 15, coaxial and nested with respect to one another.

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