MANUFACTURING METHOD FOR LIGHTWEIGHT LARGE FORMAT TELESCOPIC MIRROR BLANKS AND MIRROR BLANKS MANUFACTURED ACCORDING TO THE SAME METHOD

Abstract

A method for producing a mirror blank, comprising: providing a primary piece of glass comprising a primary planar surface and a supporting piece of glass comprising a supporting planar surface; and assembling a mirror blank assembly, the assembly of the mirror blank assembly comprising arranging a plurality of glass wedges between the primary glass and the supporting glass, the arranging of the plurality of glass wedges comprising: abutting a first surface of the glass wedge and the primary planar surface of the primary glass, and abutting a second surface of the glass wedge and the supporting planar surface of the supporting glass, the second surface being opposite to the first surface, for each glass wedge;and arranging the plurality of glass wedges such that they are circularly symmetric about an axis of symmetry orthogonal to the primary planar surface and the supporting planar surface, and extend in directions radial to the axis of symmetry; wherein for each glass wedge, a dimension l1 of the radial extent of the first surface abutting the primary planar surface of the primary glass is greater than a dimension l2 of the radial extent of the second surface abutting the supporting planar surface of the support glass; and heating the mirror blank assembly to fuse the interposed glass wedges with the primary glass and the support glass, while the primary glass and the secondary glass remain spaced apart from each other by the interposed glass wedges, thus providing the mirror blank.

Description

Related registrations This application claims priority over US patent application No. 62 / 055592, filed on September 25, 2014, which is incorporated herein by reference. field of expertise The invention relates to mirror blanks and their manufacture. Particular non-restrictive embodiments provide lightweight, large-format telescope mirror blanks and methods for their manufacture. background Some optical instruments, such as telescopes and the like, use large-format mirrors (e.g., with a diameter of 18 inches or larger). Such large-format mirrors can be used as the primary (high-aperture) mirrors of such optical instruments. Mirrors for optical instruments are typically formed by coating a reflective material (such as silver) onto the surface of a mirror blank, which is usually made of glass. Large-format mirror blanks (e.g., with a diameter of 18 inches and larger) are typically manufactured from a single piece of solid glass to provide sufficient structural rigidity. The solid piece of glass is preheated until it is soft and then thermoformed into the desired shape. After thermoforming, the glass is cooled and drawn. After cooling and drawing, a significant amount of residual stress typically remains within the glass that forms the blank. To reduce this residual stress, the blank is usually annealed. Annealing involves the controlled and slow heating of the glass to a stress-relieving temperature, followed by the slow and controlled cooling of the glass. Annealing requires a considerable amount of time (e.g., hundreds of hours). It is not uncommon for the glass to crack during the heating and cooling processes associated with annealing. To prevent optical distortion, such as that caused by the deformation of a rigid glass mirror under its own weight, telescope frames must be strong and rigid. This requirement increases the manufacturing costs of telescopes and makes their assembly, disassembly, and transport more difficult. US Patent 6,045,231 A discloses a lightweight telescope mirror with a core positioned between an optical plate and a support plate. US Patent 7,766,494 B1 discloses a lightweight mirror blank assembly with at least one layer of corrugated material in close contact with at least one cover layer. There is a general desire for improved mirror blanks and improved processes for manufacturing mirror blanks. The preceding examples of prior art and their associated limitations are intended to be illustrative and not exclusive. Other limitations of the prior art will be apparent to those skilled in the art upon reading the description and examining the drawings. Brief description of the drawings Exemplary embodiments are illustrated in the referenced figures of the drawings. It is intended that the embodiments and figures disclosed herein are to be considered illustrative rather than restrictive. Fig. 1 shows a mirror blank arrangement according to one particular embodiment. Fig. 2 shows the primary glass of the mirror blank arrangement from Fig. 1. Fig. 3 shows the support glass of the mirror blank arrangement from Fig. 1. Fig. 4 shows a glass wedge of the mirror blank arrangement from Fig. 1. Fig. 5 shows a mirror blank arrangement according to another embodiment. Fig. 6 shows a mirror blank arrangement according to another embodiment. Fig. 7 shows an exploded view of one of the mirror blank arrangements from Fig. 5 or Fig. 6. Fig. 8 shows a mirror blank arrangement according to another embodiment. Fig. 9 shows an exploded view of the mirror blank arrangement from Fig. 8.Figure 10 shows a mirror blank assembly according to another embodiment. Figure 11 shows an exploded view of the mirror blank assembly from Figure 10. Figure 12 shows a connecting element that can be used with the mirror blank assembly from Figure 10 or any other mirror blank assembly described herein. Figure 13 shows a mirror blank assembly according to another embodiment. Figure 14 shows a mirror blank assembly according to another embodiment. Figure 15 shows a pair of glass layers used to provide a primary glass according to a particular embodiment. Figure 16 shows an exploded view of the pair of glass layers from Figure 15. Figure 17 shows an enlarged partial view of a surface of one glass layer of the pair of glass layers from Figure 15. Figure 18 shows a spherical convex shape according to a particular embodiment. Description The following description provides details to enable those skilled in the art to gain a more precise understanding of the invention. However, well-known elements may not be shown or described in detail to avoid unnecessarily complicating the disclosure. Accordingly, the description and drawings should be considered illustrative rather than restrictive. Aspects of the invention provide mirror blank assemblies. In some embodiments, the mirror blank assembly is large (e.g., with a diameter of 18 inches or more) and lightweight. In some embodiments, a mirror blank assembly comprises a plurality of separate components that are assembled relative to one another but are not yet fused or otherwise joined together. A mirror blank assembly can be assembled as part of a manufacturing process for a mirror blank. Aspects of the invention provide methods for manufacturing a mirror blank, which include assembling a mirror blank assembly and subsequently heating the mirror blank assembly to fuse its components together, thereby providing a monolithic (i.e., one-piece) mirror blank. Aspects of the invention provide mirror blanks manufactured by such methods. Fig. 1 shows a mirror blank arrangement 10 according to a particular embodiment. The mirror blank arrangement 10 of the embodiment shown in Fig. 1 comprises a primary piece of glass 12 (which may be referred to herein as the primary glass 12), a supporting piece of glass 14 (which may be referred to herein as the support glass 14), and a plurality of glass wedges 16 which may be arranged between the primary glass 12 and the support glass 14. As shown in Fig. 2, the primary glass 12 comprises a primary planar (e.g., flat) surface 12b. By grinding, polishing, shaping, or otherwise making the planar surface 12b relatively smooth (e.g., relatively free of protrusions or indentations), the prevention of bubble formation between the primary planar surface 12b and adjacent surfaces (e.g., interface surfaces 16a) of the glass wedges 16 during heating (as described below) can be aided. In the illustrated embodiment of Fig. 2, the primary glass 12 is generally disk-shaped with a generally planar opposite surface 12c (i.e., opposite the primary planar surface 12b), a circular circumference with a diameter d1, and a circumferential edge 12a.The perimeter edge 12a can be located between the primary planar surface 12b and the opposite surface 12c with a uniform thickness t1 and can be orthogonal to both the primary planar surface 12b and the opposite surface 12c. The thickness t1 of the primary glass 12 may depend on the diameter d1 of the primary glass 12 (e.g., a greater thickness t1 may be desirable to support a larger diameter d1), but this is not necessarily the case. In some embodiments, the primary glass 12 can assume other shapes. As a non-limiting example: the opposite surface 12c need not be flat or planar and may have other surface shapes; the perimeter shape of the primary glass 12 may be other than circular; the perimeter edge 12a may be angled, beveled, or rounded; the thickness t1 may vary along the perimeter of the primary glass 12; and / or the like. The support glass 14 is best shown in Fig. 3. In the illustrated embodiment, the support glass 14 is generally disk-shaped with a generally planar opposite surface 14c (i.e., opposite to the supporting planar surface 14b), a circular circumference with a diameter d2, and a circumferential edge 14a. The circumferential edge 14a may extend between the supporting planar surface 14b and the opposite surface 14c with a uniform thickness t2 and may extend orthogonally to both the supporting planar surface 14b and the opposite surface 14c. The thickness t2 of the support glass 14 may depend on the diameter d2 of the support glass 14 (e.g., a greater thickness t2 may be desirable to support a larger diameter d2), but this is not necessarily the case. In some embodiments, the support glass 14 may assume other shapes.As a non-restrictive example: the opposite surface 14c need not be flat or planar and may have other surface shapes; the circumferential shape of the support glass 14 may be other than circular; the circumferential edge 14a may be angled, beveled, or rounded; the thickness t2 may vary along the perimeter of the support glass 14; and / or the like. In some embodiments, such as the illustrated embodiment, the diameter d2 of the support glass 14 is smaller than the diameter d1 of the primary glass 12 (i.e., d2 <d1). In manchen Ausführungsformen macht der Durchmesser d2etwa die Hälfte des Durchmessers d1aus. As shown in Fig. 1, the mirror blank assembly 10 comprises a plurality of glass wedges 16. The number of glass wedges 16 can vary for different embodiments. In some cases, as the diameter of the mirror blank assembly 10 increases (e.g., the diameter d1 of the primary glass 12 and / or the diameter d2 of the support glass 14), it may be advantageous to increase the number of glass wedges 16. In particular, some embodiments include between 3 and 20 glass wedges 16; and some embodiments include between 8 and 16 glass wedges. Fig. 1 shows an embodiment with 12 glass wedges 16. In the illustrated embodiment, all glass wedges 16 have substantially the same size and shape, although this is not necessary. In some embodiments, some glass wedges 16 may have different sizes and / or shapes than other glass wedges 16. The glass wedges 16 from the mirror blank assembly 10 of Fig. 1 are best shown in Fig. 4. The glass wedge 16 of the embodiment shown in Fig. 3 comprises a substantially planar (e.g., flat) interface 16a, which (as explained in detail below) may abut the primary planar surface 12b of the primary glass 12 in the mirror blank assembly 10. The opposite sides 16b and 16c extend from opposite ends of the interface 16a. The interface 16a may have a dimension 12 between the opposite sides 16b and 16c. In some embodiments, the opposite sides 16b and 16c extend parallel to each other and generally orthogonal to the interface 16a. In other embodiments, the opposite sides 16b and 16c can extend in different directions (e.g. at different angles) from each other and / or relative to the interface surface 16a.In the illustrated embodiment, side 16b has a dimension x (in a direction orthogonal to the planar interface 16a) and side 16c has a dimension y (in a direction orthogonal to the planar interface 16a). Without loss of generality, these dimensions x and y may herein be referred to as height dimensions, it being understood that, unless the context otherwise requires, the word "height" is merely a suitable term for these dimensions to distinguish them from other dimensions of interest. The glass wedge 16 of the illustrated embodiment also comprises a substantially planar (e.g., flat) opposite interface 16d. The opposite interface 16d generally extends parallel and in the opposite direction to the interface 16a, from side 16b to side 16c.The opposite interface can have a dimension l2 in a direction aligned with the dimension l1 of the interface 16a. In the illustrated embodiment, y <x und l2<l1. In Ausführungsformen, in denen l2<l1gilt, kann der Glaskeil 16 eine zusätzliche Oberfläche 16e umfassen, die sich von der entgegengesetzten Grenzoberfläche 16d zur Seite 16c erstreckt. In manchen Ausführungsformen ist es möglich, l2<l1zu haben, da der Durchmesser d2des Stützglases 14 (und seine planare Stützoberfläche 14b) geringer ist als der Durchmesser d1des Primärglases 12 (und seine planare Primäroberfläche 12b). Die Bedingungen l2<l1und / oder d2<d1zu haben, reduziert das Gewicht der Spiegelrohlinganordnung 10 und daraus ausgebildeter Spiegelrohlinge vorteilhafterweise. Jedoch das Bereitstellen von l2<l1ist nicht nötig.In some embodiments, the height x is equal to the height y, the opposite interface 16d extends over the entire distance from side 16b to side 16c, and the dimensions l1 = l2. If the dimensions x = y and l1 = l2, the glass wedge 16 is a right-angled prism. In some embodiments, the height x is approximately ½ to 1 / 20 of the diameter d1 of the primary glass 12. In other embodiments, the height x is 1 / 6 to 1 / 10 of the diameter d1 of the primary glass 12. As shown in Fig. 1, the mirror blank assembly 10 can be constructed by means of glass wedges 16 arranged between the primary glass 12 and the support glass 14. The arrangement of glass wedges 16 between the primary glass 12 and the support glass 14 can include the abutting of the interface surfaces 16a, 16d of the glass wedges 16 and the planar surfaces 12b, 14b of the primary glass 12 and the support glass 14. In this way, the glass wedges 16 can support the support glass 14 relative to the primary glass 12 (or vice versa) and can maintain the spaced relationship between the support glass 14 and the primary glass 12. In the mirror blank assembly 10 of the embodiment shown in Fig. 1, the corresponding planar surfaces 12b, 14b are opposite each other, parallel to each other, and spaced apart from each other. The mirror blank arrangement 10 of the embodiment shown in Fig. 1 is circularly symmetrical about an axis of symmetry 20. The axis of symmetry 20 can be orthogonal to the planar surfaces 12b, 14b of the primary glass 12 and the secondary glass 14, although this is not necessary. The axis of symmetry 20 can extend through a center point of the primary glass 12 and a center point of the support glass 14 (e.g., through the centers of their respective circular circumferences), although this is not necessary. In some embodiments, the glass wedges 16 can be arranged relative to each other and / or relative to the primary glass 12 and the support glass 14, such that the glass wedges 16 are circularly symmetrical about the axis of symmetry 20. In some embodiments, the glass wedges 16 are arranged to extend in radial directions from the axis of symmetry 20 (although, as shown in Fig. 1, not every glass wedge 16 needs to extend over the entire distance to the axis of symmetry 20).In some embodiments, the glass wedges 16 are arranged to extend in radial directions between a circular circumference of the primary glass 12 and the axis of symmetry 20. In some embodiments, each pair of adjacent (e.g., circumferentially adjacent) glass wedges 16 can be spaced apart from each other at an angle. In some embodiments, each pair of adjacent glass wedges 16 can have an angular separation of at least approximately the same amount. In the embodiment shown in Fig. 1, the glass wedges 16 between the primary glass 12 and the support glass 14 are arranged such that the sections of the circumferentially adjacent glass wedges 16 closest to the axis of symmetry 20 (e.g., the edges of the sides 16b) touch each other. These sections of circumferentially adjacent glass wedges 16 closest to the axis of symmetry 20 may touch each other at points spaced away from the axis of symmetry 20. This contact between circumferentially adjacent glass wedges 16 is not necessary. In some embodiments, the glass wedges 16 are arranged such that the sections of the glass wedges 16 closest to the axis of symmetry 20 are spaced away from the axis of symmetry 20 and / or from each other. In some embodiments, the sections of the glass wedges closest to the axis of symmetry 20 can be spaced away from the axis of symmetry 20 by a radial dimension that is the same for each glass wedge 16. In the mirror blank arrangement 10 of the embodiment shown in Fig. 1, the support glass 14 is generally spaced away from the primary glass 12 by interposed glass wedges 16 at a distance x equal to the height of the side 16b closest to the axis of symmetry, or equal to the distance between the parallel interface surfaces 16a, 16d of the glass wedges 16. The support glass 14 can project beyond the planar surfaces 16d of the glass wedges 16, can be flush with the outer ends of the planar surfaces 16d (as is the case in the embodiment shown in Fig. 1), or can not reach the radially outer edges of the interface surfaces 16d. Fig. 5 shows a mirror blank assembly 110 according to a particular embodiment. Fig. 6 shows a mirror blank assembly 110' according to a particular embodiment. Fig. 7 shows an exploded view of one of the mirror blank assemblies 110, 110'. The mirror blank assemblies 110, 110' are generally similar to the mirror blank assembly 10 described elsewhere herein, and the same reference numerals have been used to describe features of the mirror blank assemblies 110, 110' that are similar to those of the mirror blank assembly 10, except that the reference numerals for the mirror blank assemblies 110, 110' are preceded by the number "1". The mirror blank arrangements 110, 110' differ from the mirror blank arrangement 10 mainly in that the support glass 114 of both mirror blank arrangements 110, 110' is shaped to define a support opening 114d.In some embodiments, the support opening 114d has a circular circumference. In other embodiments, the support opening 114d may have a differently shaped circumference. The support opening 114d may be shaped and / or positioned such that the axis of symmetry 120 passes through a center point of the support opening 114d. The support opening 114d may have any diameter that allows sufficient contact between the interface surfaces 116d of each of the plurality of glass wedges 116 and the planar surface 114b of the support glass 114. The support opening 114d reduces the weight of the mirror blank assemblies 110, 110' and provides a fluid communication channel that can provide ventilation and / or improve heat transfer during the heating of the mirror blank assemblies 110, 110'. The components of the mirror blank assemblies 110, 110' (e.g., primary glass 112, support glass 114, and glass wedges 116) are essentially similar to one another. The mirror blank assemblies 110, 110' differ from each other mainly in that, when arranged between the primary glass 112 and the support glass 114, the glass wedges 116 of the mirror blank assembly 110 (Fig. 5) are arranged such that the sections of the glass wedges 116 closest to the axis of symmetry 120 are spaced apart from one another, whereas the glass wedges 116 of the mirror blank assembly 110' (Fig. 6) are arranged such that the sections of the glass wedges 116 closest to the axis of symmetry 120 touch their circumferentially adjacent wedges 116. In both the case of the mirror blank arrangement 110 and the case of the mirror blank arrangement 110', the sections of the glass wedges 116 closest to the axis of symmetry 120 are located at a generally constant radial distance from the axis of symmetry 120. Figures 8 and 9 show a mirror blank assembly 210 according to a particular embodiment. The mirror blank assembly 210 is generally similar to the mirror blank assemblies 10, 110, 110' described elsewhere herein, and the same reference numerals have been used to describe features of the mirror blank assemblies 210 that are similar to those of the mirror blank assemblies 10, 110, 110', except that the reference numerals for the mirror blank assemblies 210 are preceded by the number "2". The mirror blank assembly 210 differs from the mirror blank assembly 110' in that the primary glass 212 is shaped to define a primary aperture 212d. In some embodiments, the primary aperture 212d has a circular circumference. In other embodiments, the primary aperture 212d may have a differently shaped circumference.The primary aperture 212d can be shaped and / or positioned such that the axis of symmetry 220 passes through a center point of the primary aperture 212d. The primary aperture 212d can have any diameter that allows sufficient contact between the interface surfaces 216d of each of the plurality of glass wedges 216 and the planar surface 214b of the support glass 214, thus meeting the requirements for an optical instrument for which the mirror blank assembly 210 is manufactured. The primary aperture 212d reduces the weight of the mirror blank assembly 210 and provides a fluid communication channel that can provide ventilation and / or improve heat transfer during the heating of the mirror blank assembly 210. Like the mirror blank assemblies 110, 110', the mirror blank assembly 210 of the illustrated embodiment also includes a support glass 214 shaped to form a support opening 214d, which may have features substantially similar to the support opening 114d of the mirror blank assemblies 110, 110' described elsewhere herein. However, the support opening 214d is not necessary, and in some embodiments, the mirror blank assembly 210 may be provided without the support opening 214d. In the illustrated embodiment, the mirror blank assembly 210 has its glass wedges 216 arranged such that the sections of the glass wedges closest to the axis of symmetry 220 touch each other (similar to the mirror blank assembly 110' (Fig. 6)). This is not necessary.In some embodiments, the mirror blank arrangement 210 can be mounted such that its glass wedges 216 are arranged such that the sections of the glass wedges 216 closest to the axis of symmetry 220 are spaced apart from each other (in a similar way to mirror blank arrangement 110 ( Fig. 5)). Figures 10 and 11 show a mirror blank assembly 310 according to a particular embodiment. The mirror blank assembly 310 is generally similar to the mirror blank assemblies 10, 110, 110', and 210 described elsewhere herein, and the same reference numerals have been used to describe features of the mirror blank assembly 310 that are similar to those of the mirror blank assemblies 10, 110, 110', and 210, except that the reference numerals for the mirror blank assembly 310 are preceded by the number "3". The mirror blank assembly 310 differs from the mirror blank assembly 210 in that the mirror blank assembly comprises a connecting element 318 (e.g., a generally tubular connecting element 318) that is arranged between the primary glass 312 and the support glass 314.The arrangement of the connecting element 318 between the primary glass 312 and the support glass 314 can include the abutting of the first and second interface surfaces 318b, 318c and the planar surfaces 312b, 314b of the primary glass 312 and the support glass 314. In the illustrated embodiment, the connecting element 318 comprises a cylindrical outer surface 318a. In some embodiments, the cylindrical outer surface 318a of the connecting element 318 can have a cylindrical axis that is coaxial with (or otherwise aligned with) the axis of symmetry 320. In the illustrated embodiment, the connecting element 318 is shaped to define a hole 318d extending through it. In some embodiments, at least part of the hole 318d extends between the planar surfaces 312b, 314b of the primary glass 312 and the support glass 314. In the embodiment shown in Fig.10. The sections of the glass wedges 316 closest to the axis of symmetry 320 (e.g., sides 316b) border the outer surface 318a of the connecting element 318. This is not necessary. In some embodiments, the glass wedges 316 can be positioned at a distance from the connecting element 318. The connecting element 318 can provide additional stability to the mirror blank assembly 310 by providing an enlarged surface area in the connections between the primary glass 312, the glass wedges 316, and the support glass 314. Like the mirror blank assembly 210, the mirror blank assembly 310 of the illustrated embodiment also comprises a support glass 314 shaped to define a support opening 314d and a primary glass 312 shaped to define a primary opening 312d. The features of the support opening 314d and the primary opening 312d may be substantially similar to the support opening 214d and the primary opening 312d of the mirror blank assembly 210 described elsewhere herein. However, neither the support opening 314d nor the primary opening 312d is necessary. In some embodiments, the mirror blank assembly 310 may be provided without the support opening 314d and / or without the primary opening 312d. In some embodiments of the mirror blank assembly 310, particularly where the primary glass 312 does not include a primary opening 312d and / or the support glass 314 does not include a support opening 314d, a modified connecting element 322 can replace the connecting element 318. The connecting element 322, as shown in Fig. 12, includes one or more vent openings 322e to allow fluid communication between the opening 322d and a region outside the opening 322d when the connecting element 322 is arranged between the primary glass 312 and the support glass 314. In the illustrated embodiment, the vent openings are partially defined by the concave feature 322e in the connecting element 322 and partially by one or both planar surfaces 312b, 314b. This is not necessary. In some embodiments, vent openings can additionally or alternatively be completely defined by the connecting element 322.The connecting element 322 of the embodiment shown in Fig. 12 can also be used in embodiments in which the mirror blank arrangement 310 includes a primary opening 312d or a support opening 314d (as in the illustrated embodiment shown in Figs. 10 and 11). Figures 13 and 14 show schematic views of mirror blank assemblies 410, 410' according to particular embodiments. The mirror blank assemblies 410, 410' are generally similar to the mirror blank assemblies 10, 110, 110', 210, 310 described elsewhere herein, and the same reference numerals have been used to describe features of the mirror blank assemblies 410, 410' that are similar to those of the mirror blank assemblies 10, 110, 110', 210, 310, except that the reference numerals for the mirror blank assemblies 410, 410' are preceded by the number "4". The mirror blank arrangements 410, 410' differ from the mirror blank arrangements 10, 110, 110', 210, 310 mainly in that the mirror blank arrangements 410, 410' each comprise glass struts 422, 422' which are arranged between and adjacent to circumferentially adjacent pairs of glass wedges 416.In the illustrated embodiment, the glass struts 422, 422' are arranged such that they adjoin each circumferentially adjacent pair of glass struts 416, although this is not necessary, and in some embodiments the glass struts 422, 422' can be arranged such that they adjoin one or more selected pairs of circumferentially adjacent glass wedges 416. The glass struts 422, 422' can be straight (with a pair of opposite planar surfaces) with opposite edge surfaces chamfered with respect to the planar surfaces to abut circumferentially adjacent glass wedges 16. Additionally or alternatively, the glass struts 422, 422' can be curved with opposite edge surfaces that are chamfered or unchamfered to abut circumferentially adjacent glass wedges 416, as illustrated in Fig. 13. The glass struts 422, 422' can be arranged between the primary glass 412 and the supporting glass 414. The glass struts 422, 422' can abut either one or both of the primary planar surface 412b and the supporting planar surface 414c, although this is not necessary. The glass struts 422' can be arranged relatively close to the axis of symmetry 420 in order to space out adjacent glass wedges 416 around the circumference, as illustrated in Fig. 14.Since the glass struts 422' are positioned at a distance d2 / 2 from the axis of symmetry 420, they can abut (and subsequently fuse with) the planar surfaces 412b, 414b of both the primary glass 412 and the support glass 414. Alternatively, or in addition, the glass struts 422 can be arranged further away from the axis of symmetry 420, as illustrated in Fig. 13. Since the struts 422 are positioned at distances greater than d2 / 2 from the axis of symmetry 420, they can only abut (and subsequently fuse with) the planar surface 412b of the primary glass 412. In some embodiments, the glass struts 422, 422' may have ventilation grooves (e.g. similar to the groove 322e of the connecting element 322), openings or the like to allow air to escape from the center of the mirror blank assembly 410.In some embodiments of the mirror blank assembly 410, different glass struts 422, 422' can be positioned at different radial distances from the axis of symmetry. In others, all glass struts 422, 422' can have the same distance from the axis 420 of the mirror blank assembly 410. In some situations, the primary glass 12 may only be available in a limited selection of thicknesses for suitable purposes (e.g., for manufacturing, transport, and / or purchase). To achieve a desired thickness t1, two or more layers 512d, 512e of primary glass can be bonded together, as illustrated in Figs. 15 and 16, to form the primary glass 512. For example, if the primary glass layers 512d, 512e are only available in suitable thicknesses of 6 mm and 8 mm, while the desired thickness of the primary glass 512 is 14 mm, a 6 mm layer 512d can be fused with an 8 mm layer 512e. Layers 512d and 512e can be fused by bringing their complementary surfaces 513d, 513e together and then applying heat to fuse the layers 512d, 512e. In some embodiments, when layers 512d, 512e are part of a glass blank assembly, they can be heated and fused in the same step as the rest of the glass blank assembly. In other embodiments, layers 512d, 512e can be fused in a separate step. In the illustrated embodiment, the complementary surfaces 513d, 513e of layers 512d, 512e are generally planar (e.g., flat), although this is not necessary. During the heating process, air bubbles may form between layers 512d, 512e (e.g. due to air trapped by irregularities in the smoothness of the complementary surfaces 513d, 513e).Such air bubbles can negatively affect the strength, hardness, shape, and other properties of the primary glass 512. To minimize the likelihood of air bubble formation, the complementary surfaces 513d, 513e of one or both layers 512d, 512e can include one or more channels or grooves 512f to allow air to escape from between the layers 512d, 512e. In the illustrated embodiment of Figures 15-17, such channels 512f are provided in the complementary surface 513d of layer 512d. The channels 512f can include inward bulges in their corresponding complementary surface (e.g., in the complementary surface 513d in the illustrated embodiment). The channels 512f can include inward bulges that open toward the other layer (e.g., toward layer 512e in the illustrated embodiment).In some embodiments, the channels 512f are arranged in a grid pattern with rows perpendicular to columns, the rows and columns extending to a perimeter of their corresponding complementary surface (e.g., to the perimeter of the complementary surface 513d in the illustrated embodiment). In some embodiments, the channels 512f can be arranged in any pattern suitable for allowing air to escape to the periphery edges of the primary glass 512. The channels 512f can assume any cross-sectional shape. In some embodiments, the channels 512f have a square cross-section, while in other embodiments, the channels 512f have a rounded cross-section, as shown in Fig. 17. The components of the mirror blank assemblies according to various embodiments (e.g., primary glass 12, support glass 14, glass wedges 16, connecting elements 318 and 322, and glass struts 422) can be made of the same type of glass or a variety of glass types, provided the components can be fused together. In currently preferred embodiments, the glass used for manufacturing the components of mirror blank assemblies can have the same or similar coefficients of thermal expansion. In particular, in some embodiments, the primary glass 12, the support glass 14, the glass wedges 16, the connecting elements 318 and 322, and the glass struts 422 can have coefficients of thermal expansion within ±5% of each other. In some embodiments, these coefficients of thermal expansion are within ±15% of each other. In some embodiments, these coefficients of thermal expansion are within ±1% of each other.In some embodiments, the glass 12, the support glass 14, the glass wedges 16, the connecting elements 318 and 322 and the glass struts 422 have the same coefficient of thermal expansion. One aspect of the invention provides a method for manufacturing mirror blanks. In some embodiments, the method serves to manufacture large (e.g., diameters of 18 inches and larger), lightweight mirror blanks. In some embodiments, a first step consists of assembling a mirror blank assembly. A mirror blank assembly can comprise a variety of components, as described above with respect to mirror blank assemblies 10, 110, 210, 310, and 410. The following procedure can be practically carried out with any of the mirror blank assemblies described above or with any variation of the mirror blank assemblies comprising a combination of the features described above. After a mirror blank assembly (for example, 10, 110, 210, 310, or 410) has been assembled, the assembled mirror blank is heated to fuse the components together. In some embodiments, a kiln or similar device is used to heat the mirror blank assembly, although other heating devices and / or methods may be used to heat the mirror blank assembly. In some embodiments, the mirror blank assembly is placed in or onto a mold, such as mold 50, as illustrated in Fig. 18. Specifically, in some embodiments, when the mirror blank assembly is placed in or onto a mold 50, the opposite surface of the primary glass (for example, the opposite surface 12c of primary glass 12) is placed in contact with a mold surface 51 of mold 50. In some embodiments, mold 50 may comprise a round, convex mold surface 51.In some embodiments, mold 50 may comprise a parabolic mold surface with a convex surface 51. Mold 50 (or at least mold surface 51) may be uniformly coated with a mold release agent before the mirror blank assembly is placed in or on mold 50. The mold release agent may be sprayed or applied such that the mold surface 51 is coated with the mold release agent. In some embodiments, the mold release agent may comprise aluminum powder or the like. If aluminum powder is applied, the aluminum powder may be allowed to harden. Mold 50 coated with aluminum powder may be dried in a kiln. Mold surface 51 (with or without the mold release agent) should be relatively smooth (e.g., without significant depressions or protrusions). A mold, together with the mirror blank assembly located on or within the mold, can be placed together in a kiln or similar appliance for heating. The kiln temperature can be raised just below the melting point of glass (or another suitable temperature), and the glass can be allowed to heat through at this temperature. In some embodiments, this temperature is approximately 750 °C (+ / - 5%), and the mirror blank assembly is allowed to heat through at this temperature for 1–12 hours (e.g., approximately two hours). In some embodiments, the heating time and temperature may vary. During heating, the contact surfaces of the individual components of the mirror blank assembly melt together.Specifically, the interface surfaces 16a, 16d of each of the plurality of glass wedges 16 fuse with the primary planar surface 12b of the primary glass 12 and the supporting planar surface 14b of the support glass 14. In some embodiments, the weight of at least one of the primary glass 12, support glass 14, and glass wedges 16 contributes to the fusing of the glass wedges 16 with the primary glass 12 and support glass 14. In some embodiments, interface surfaces 318b, 318c of a connecting element 318 can fuse with the primary planar surface 12b of the primary glass 12 and the supporting planar surface 14b of the support glass 14. In some embodiments, surface 318a of the connecting element 318 can fuse with glass wedges. In some embodiments, surfaces of struts 422, 422' can also be fused with glass wedges, with primary glass and / or with support glass.During heating, the primary glass can adapt to the mold surface 51 of mold 50 to assume a shape complementary to the shape of mold surface 51. In certain embodiments where mold surface 51 is spherically convex, the primary glass assumes a shape with a round concave curvature. In some embodiments, Form 50 can have a parabolic shape and can produce parabolic mirror blanks. This can minimize the amount of grinding and polishing required to finish the mirror blank. After heating, the mirror blank (which is now monolithic, i.e., in one piece) is allowed to cool naturally. Natural cooling may involve allowing the mirror blank to cool in an environment without temperature control or in an environment where the temperature is regulated to a constant (e.g., non-varying) temperature (e.g., room temperature in a thermostatically controlled room). Natural cooling is not necessary. In some embodiments, the mirror blank may be stress-relieved after heating in a process in which the mirror blank is in an environment where the temperature is actively regulated to meet a specified temperature profile. In some embodiments, the mirror blank is arranged during cooling within a vessel that has insulating properties (e.g., low thermal conductivity and / or high specific heat capacity), such as a kiln. In such embodiments, the interior of the vessel is at or near the heating temperature (e.g., 750 °C) after heating, while the ambient temperature outside the vessel can range between 5 °C and 50 °C. In some embodiments, the ambient temperature outside the vessel is not regulated. In other embodiments, the ambient temperature outside the vessel can be regulated to a constant temperature (e.g., room temperature in a thermostatically controlled room). In certain embodiments, the temperature outside the vessel can be regulated to a constant temperature of approximately 20 °C to 25 °C.Due to its insulating nature, the container can slow down or otherwise delay the rate at which the mirror blank cools. In some embodiments, the mirror blank is allowed to cool to the ambient temperature outside the vessel while remaining inside the vessel. In other embodiments, natural cooling may involve multiple cooling steps. In one particular embodiment, a first cooling step involves allowing the mirror blank to cool while remaining in the closed vessel until a transition temperature is reached. A second cooling step may involve opening the interior of the vessel so that the mirror blank is in fluid communication with the ambient environment on the outside of the vessel after the transition temperature has been reached. In some embodiments, the transition temperature is approximately 110 °C (+ / - 5%), although this may vary in other embodiments. Cooling the mirror blank using multiple steps may reduce the cooling time.Specifically, the first step may take approximately 24 hours, while the second step may take approximately 4 hours. After cooling, the resulting mirror blank can be removed from the mold 50. If the mold surface 51 is spherically convex, the mirror blank has an approximately round concave surface and negligible internal stress. In some embodiments, the concave surface of the mirror blank may require a grinding and / or polishing step to further shape the mirror blank. In some embodiments, the concave surface of the mirror blank may first be ground and / or polished into a substantially spherical shape, although this step is not necessary. In some embodiments, the concave surface of the mirror blanks may be ground and / or polished into a parabolic shape (either by first grinding and / or polishing the concave surface into a round shape or by not doing so). The grinding and polishing steps may require several iterations and tests to achieve the desired shape. The finished mirror blank has a structure similar to the mirror blank assembly, except that the mirror blank is monolithic (e.g., one-piece). For example, when the mirror blank assembly 10 is placed in mold 50 and heated to fuse its components together, the finished mirror blank comprises a primary glass section 12, a plurality of glass wedge sections 16, and a support glass section 14. Similarly, when an assembled mirror blank assembly 110 is used, the resulting mirror blank comprises a primary glass section 112, a plurality of glass wedge sections 116, and a support glass section 114. The mirror blank additionally includes a support aperture 114d. The support aperture may have a circular perimeter having a center point through which the axis of symmetry extends. When an assembled mirror blank assembly 210 is used, the resulting mirror blank comprises a primary glass section 212, a plurality of glass wedge sections 216, and a support glass section 214. The mirror blank additionally includes the primary aperture 212d and may or may not include a support aperture 214d. The primary aperture 212d may have a circular perimeter with a center point through which the axis of symmetry extends. When an assembled mirror blank assembly 310 is used, the resulting mirror blank comprises a primary glass section 312, a plurality of glass wedge sections 316, and a support glass section 314. The mirror blank may include either one or both of the primary aperture 312d and the support aperture 314d and additionally includes a connecting element section 318 or 322. When an assembled mirror blank assembly 410 is used, the resulting mirror blank comprises a primary glass section 412, a plurality of glass wedge sections 416, and a support glass section 414. The mirror blank may include any or all of the primary aperture 412d, the support aperture 414d, and the connecting element section 318 (or 322). The mirror blank also includes one or more strut sections 422. The resulting mirror blank can share many other properties of the assembled mirror blank. The resulting mirror blank can have a multitude of wedge sections of the same shape and size, extending radially from an axis of symmetry of the resulting mirror blank. Wedge sections can have an equal angular spacing between circumferentially adjacent pairs of wedge sections. The resulting mirror blank can have a primary circular circumference with a diameter d1, the center point of which intersects the axis of symmetry of the resulting mirror blank. The support section can have a circular support circumference, the center point of which intersects the axis of symmetry of the resulting mirror blank. The primary section and the support section of the resulting mirror blank may be spaced approximately x apart, corresponding to a first height of the wedge sections closest to the axis of symmetry. In some embodiments, the wedge sections most distal to the axis of symmetry may have a height of approximately y equal to or less than the first height x. For each wedge section, the first height of the wedge section may be in a range between 1 / 2 and 1 / 20 of the diameter d1 of the primary section of the resulting mirror blank. In further embodiments, the first height of the wedge section for each wedge section is in a range between 1 / 6 and 1 / 10 of the diameter d1 of the primary section of the resulting mirror blank. The resulting mirror blank, or any mirror blank based on assembled mirror blanks 10, 110, 210, 310, or 410, or a variation thereof, has a reduced weight compared to a solid glass mirror blank. Furthermore, in some embodiments, the entire process described above, including manufacturing time (such as cutting, grinding, and shaping the individual components), assembly of the mirror blank assembly, fusing, and cooling, can take only about 72 hours. Because the internal stresses are reduced, there is a lower risk of the mirror blanks breaking. Accordingly, the process is less expensive and produces a product that is easier to assemble, enabling significant weight savings in telescope manufacturing. Although a number of exemplary aspects and embodiments are discussed herein, those skilled in the art will recognize certain modifications, permutations, additions, and subcombinations thereof. For example: - Although only a few embodiments are shown in the drawings, a mirror blank arrangement may comprise any combination of one or more connecting elements, glass struts, multilayer round glass segments, primary openings, and support openings in the primary and support glass, respectively. - Although the above method is described for producing a concave mirror blank, a similar method may be used to form a convex mirror blank. Such a method would require a concave shape and essentially similar mirror blank arrangements. Although a number of exemplary aspects and embodiments have been discussed herein, those skilled in the art in the field of the invention will recognize certain modifications, permutations, additions and subcombinations thereof.

Claims

A method for producing a mirror blank, comprising: providing a primary piece of glass comprising a primary planar surface and a supporting piece of glass comprising a supporting planar surface; and assembling a mirror blank assembly, the assembly of the mirror blank assembly comprising arranging a plurality of glass wedges between the primary glass and the supporting glass, the arranging of the plurality of glass wedges comprising: abutting a first surface of the glass wedge and the primary planar surface of the primary glass, and abutting a second surface of the glass wedge and the supporting planar surface of the supporting glass, the second surface being opposite to the first surface, for each glass wedge;and arranging the plurality of glass wedges such that they are circularly symmetric about an axis of symmetry orthogonal to the primary planar surface and the supporting planar surface, and extend in directions radial to the axis of symmetry; wherein for each glass wedge, a dimension l1 of the radial extent of the first surface abutting the primary planar surface of the primary glass is greater than a dimension l2 of the radial extent of the second surface abutting the supporting planar surface of the support glass; and heating the mirror blank assembly to fuse the interposed glass wedges with the primary glass and the support glass, while the primary glass and the secondary glass remain spaced apart from each other by the interposed glass wedges, thus providing the mirror blank. Method according to claim 1, wherein the primary glass, the support glass and the glass wedges have coefficients of thermal expansion that are equal, + / - 5%. Method according to one of claims 1 to 2, wherein the primary glass, the support glass and the glass wedges have the same coefficients of thermal expansion. Method according to any one of claims 1 to 3, wherein the primary glass has a primary circular circumference having a primary diameter d1, and the support glass has a circular support circumference having a support diameter d2, wherein d1>d2· The method of claim 4, comprising aligning the primary glass and the support glass such that the axis of symmetry intersects the center point of the primary circular circumference and the circular support circumference. A method according to any one of claims 1 to 5, wherein each of the glass wedges has a first height dimension x in a direction extending between the primary planar surface and the supporting planar surface on a side of the glass wedge closest to the axis of symmetry, and a second height dimension y in the direction extending between the primary planar surface and the supporting planar surface on a side of the glass wedge most distal to the axis of symmetry, wherein y <x. Method according to claim 6, wherein the primary glass has a primary circular circumference having a primary diameter d1, and wherein the first height dimension x for each glass wedge is in a range between ½ and 1 / 20 of the primary diameter d1. Method according to one of claims 6 to 7, wherein the primary glass has a primary circular circumference having a primary diameter d1, and wherein the first height dimension x for each glass wedge is in a range between 1 / 6 and 1 / 10 of the primary diameter d1. Method according to any one of claims 1 to 8, comprising shaping the primary glass to provide a primary opening therein. The method of claim 9, comprising arranging the primary glass such that the axis of symmetry extends through a center point of the primary opening. Method according to one of claims 9 to 10, comprising forming the primary opening so that it has a circular circumference. Method according to any one of claims 1 to 11, comprising forming the support glass to provide a support opening therein. The method of claim 12, comprising arranging the support glass such that the axis of symmetry extends through a center point of the support opening. Method according to one of claims 12 to 13, comprising forming the support opening so that it has a circular circumference. Method according to any one of claims 1 to 14, wherein the arrangement of the plurality of glass wedges between the primary glass and the support glass comprises spacing the glass wedges around the axis of symmetry with equal angular spacing between circumferentially adjacent pairs of glass wedges. Method according to any one of claims 1 to 15, wherein the arrangement of the plurality of glass wedges between the primary glass and the support glass comprises arranging the plurality of glass wedges such that sections of circumferentially adjacent glass wedges nearest to the axis of symmetry touch each other at points spaced apart from the axis of symmetry. Method according to any one of claims 1 to 16, wherein the arrangement of the plurality of glass wedges between the primary glass and the support glass comprises arranging the plurality of glass wedges such that sections of the glass wedges nearest to the axis of symmetry are spaced apart from the axis of symmetry and from each other. The method of claim 17, wherein arranging the plurality of glass wedges such that the sections of the glass wedges closest to the axis of symmetry are spaced apart from the axis of symmetry, comprises arranging the plurality of glass wedges such that the sections of the glass wedges closest to the axis of symmetry are spaced apart from the axis of symmetry by a radial dimension which is the same for each glass wedge. A method according to one of claims 17 to 18, wherein the assembly of the mirror blank assembly comprises arranging a glass connecting element between the primary glass and the support glass, wherein the arranging of the glass connecting element between the primary glass and the support glass comprises abutting a first surface of the glass connecting element and the primary planar surface of the primary glass and abutting a second surface of the glass connecting element and the supporting planar surface of the support glass, and wherein the heating of the mirror blank assembly comprises heating the glass connecting element arranged between them and the resulting fusion of the glass connecting element with the primary glass and the support glass. The method of claim 19, comprising forming the glass connecting element to define a hole therein, and wherein arranging the glass connecting element between the primary glass and the support glass comprises widening at least part of the hole between the primary planar surface of the primary glass and the supporting planar surface of the support glass. The method of claim 20, comprising shaping the glass connecting element to define at least a part of one or more ventilation openings that allow fluid communication between the hole of the glass connecting element and a region outside the hole of the glass connecting element when the glass connecting element is arranged between the primary glass and the support glass. Method according to claim 21, wherein the one or more ventilation openings are partially defined by at least one of the primary planar surface and the supporting planar surface. Method according to one of claims 19 to 22, wherein the glass joining element comprises a cylindrical outer surface and wherein arranging the glass joining element between the primary glass and the support glass comprises aligning the cylindrical axis with the axis of symmetry and, for each glass wedge, bringing a surface of the glass wedge closest to the axis of symmetry and the cylindrical outer surface of the glass joining element into contact. Method according to claim 23, wherein the heating of the interposed glass connecting element comprises fusing the cylindrical outer surface of the glass connecting element with the glass wedges. Method according to any one of claims 1 to 24, wherein the assembly of the mirror blank arrangement comprises arranging a plurality of glass struts between the plurality of glass wedges, wherein the arranging of the plurality of glass struts between the plurality of glass wedges for each glass strut comprises causing opposite surfaces of the glass strut and each of a corresponding pair of circumferentially adjacent glass wedges to abut each other. Method according to claim 25, wherein the glass struts are curved. Method according to claim 25, wherein the glass struts are generally straight and for each glass strut the opposite surfaces of the glass strut are chamfered with respect to the straight dimension of the glass strut. Method according to one of claims 25 to 27, wherein the heating of the mirror blank arrangement comprises the heating of the glass struts arranged between them and the resulting fusion of the opposite surfaces of the glass strut with the corresponding pair of circumferentially adjacent glass wedges for each glass strut. Method according to one of claims 25 to 28, wherein the heating of the mirror blank arrangement for each glass strut comprises fusing the glass strut with the primary glass. Method according to one of claims 25 to 29, wherein the heating of the mirror blank arrangement for each glass strut comprises fusing the glass strut with the supporting glass. Method according to any one of claims 1 to 30, wherein the provision of the primary piece of glass comprises bringing complementary surfaces of a glass layer pair into contact with each other, wherein at least one of the complementary surfaces of the glass layer pair comprises a plurality of channels, and wherein the heating of the mirror blank arrangement comprises heating the glass layer pair in order to fuse the complementary surfaces of the glass layer pair together. Method according to claim 31, wherein at least one of the plurality of channels extends between edge edges of at least one of the complementary surfaces of the glass layer pair. A method according to any one of claims 1 to 32, comprising, after heating the mirror blank assembly to fuse the interposed glass wedges with the primary glass and the support glass, cooling the mirror blank using a plurality of cooling steps, wherein the plurality of cooling steps comprises: a first cooling step comprising allowing the mirror blank to cool in a first environment until the mirror blank reaches a target temperature; a second cooling step comprising allowing the mirror blank to cool in a second environment until the mirror blank reaches an ambient temperature; wherein the first environment is regulated to have a decreasing temperature, and the second environment is regulated to have a substantially constant temperature. Method according to any one of claims 1 to 33, wherein the heating of the mirror blank arrangement for fusing the intervening glass wedges with the primary glass and the support glass comprises heating the mirror blank arrangement to a temperature just below its melting point and heating the primary glass, the support glass and the intervening glass wedges at this temperature. Method according to any one of claims 1 to 34, wherein the heating of the mirror blank arrangement to fuse the interposed glass wedges with the primary glass and the support glass comprises, for each glass wedge, the fusing of the first surface of the glass wedge with the primary planar surface of the primary glass and the fusing of the second surface of the glass wedge with the supporting planar surface of the support glass. Method according to any one of claims 1 to 35, wherein the heating of the mirror blank arrangement for fusing the intervening glass wedges with the primary glass and the support glass comprises allowing the weight of at least one of the primary glass, the support glass and the intervening glass wedges to contribute to fusing the intervening glass wedges with the primary glass and the support glass. Mirror blank, manufactured according to the method of any one of claims 1 to 36. A monolithic glass mirror blank comprising: a primary section and a support section spaced apart from each other; and a plurality of wedge sections extending between the primary section and the support section; wherein for each wedge section, a first surface of the wedge section is fused to the primary section and a second surface of the wedge section is fused to the support section, the second surface being opposite to the first surface; wherein the plurality of wedge sections are arranged to be circularly symmetric about an axis of symmetry and extending in directions radial to the axis of symmetry; and wherein for each wedge section, a dimension l1 of the radial extent of the first surface abutting the primary section is greater than a dimension l2 of the radial extent of the second surface abutting the support section. Mirror blank according to claim 38, wherein the primary section, the support section and the wedge sections have coefficients of thermal expansion that are equal, + / - 5%. Mirror blank according to one of claims 38 to 39, wherein the primary section, the support section and the wedge sections have the same coefficients of thermal expansion. Mirror blank according to one of claims 38 to 40, wherein the primary section has a primary circular circumference having a primary diameter d1, and the support section has a circular support circumference having a support diameter d2, wherein d1>d2. Mirror blank according to claim 41, wherein the primary section and the support section are aligned such that the axis of symmetry intersects the center point of the primary circular circumference and the circular support circumference. Mirror blank according to one of claims 38 to 41, wherein each of the wedge sections has a first height dimension x in a direction extending between the primary section and the support section on a side of the wedge section closest to the axis of symmetry, and a second height dimension y in the direction extending between the primary section and the support section on a side of the wedge section most distal to the axis of symmetry, wherein y <x.Mirror blank according to claim 43, wherein the primary section has a primary circular circumference having a primary diameter d1, and wherein for each wedge section the first height dimension x lies in a range between ½ and 1 / 20 of the primary diameter d1. Mirror blank according to one of claims 43 to 44, wherein the primary section has a primary circular circumference having a primary diameter d1, and wherein for each wedge section the first height dimension x lies in a range between 1 / 6 and 1 / 10 of the primary diameter d1. Mirror blank according to one of claims 38 to 45, wherein the primary section is shaped such that it provides a primary opening. Mirror blank according to claim 46, wherein the primary section is oriented such that the axis of symmetry extends through a center point of the primary opening. Mirror blank according to one of claims 46 to 47, wherein the primary opening has a circular circumference. Mirror blank according to one of claims 38 to 48, wherein the support section is shaped in such a way as to provide a support opening. Mirror blank according to claim 49, wherein the support section is oriented such that the axis of symmetry extends through a center point of the support opening. Mirror blank according to one of claims 49 to 50, wherein the support opening has a circular circumference. Mirror blank according to one of claims 38 to 51, wherein the glass wedge sections are arranged around the axis of symmetry such that they have an equal angular distance between circumferentially adjacent pairs of wedge sections. Mirror blank according to one of claims 38 to 52, wherein the plurality of wedge sections are arranged such that sections of circumferentially adjacent wedge sections which are closest to the axis of symmetry touch each other at points which are spaced apart from the axis of symmetry. Mirror blank according to one of claims 38 to 53, wherein the plurality of wedge sections are arranged such that sections of the wedge sections which are closest to the axis of symmetry are spaced apart from the axis of symmetry and from each other. Mirror blank according to claim 54, wherein the plurality of wedge sections are arranged such that the sections of the wedge sections that are closest to the axis of symmetry are spaced from the axis of symmetry by a radial dimension that is the same for each wedge section. Mirror blank according to one of claims 54 to 55, comprising a connecting element section between the primary section and the support section, wherein a first surface of the connecting element section is fused with the primary section and a second surface of the connecting element section is fused with the support section. Mirror blank according to claim 56, wherein the connecting element section is shaped such that it defines a hole therein, and at least part of the hole extends between the primary section and the support section. Mirror blank according to claim 57, wherein the connecting element section is shaped such that it defines at least a part of one or more ventilation openings that allow fluid communication between the hole of the connecting element section and a region outside the hole of the connecting element section. Mirror blank according to claim 58, wherein the one or more ventilation openings are partially defined by at least one of the primary section and the support section. Mirror blank according to one of claims 56 to 59, wherein the connecting element section comprises a cylindrical outer surface having a cylindrical axis aligned with the axis of symmetry, and for each wedge section, a surface of the wedge section closest to the axis of symmetry is fused with the cylindrical outer surface of the connecting element section. Mirror blank according to one of claims 38 to 60, comprising a plurality of strut sections arranged between the plurality of wedge sections, wherein for each glass strut opposite surfaces of the glass strut are fused with each of a corresponding pair of circumferentially adjacent glass wedges. Mirror blank according to claim 61, wherein the strut sections are curved. Mirror blank according to claim 61, wherein the strut sections are generally straight and for each strut section the opposite surfaces of the strut section are chamfered with respect to the straight dimension of the strut section. Mirror blank according to one of claims 61 to 63, wherein each strut section is fused with the primary section. Mirror blank according to one of claims 61 to 64, wherein each strut section is fused with the support section. Mirror blank according to one of claims 38 to 65, wherein the primary section comprises a pair of glass layers having complementary surfaces fused together, wherein at least one of the complementary surfaces comprises a plurality of channels. Mirror blank according to claim 66, wherein at least one of the plurality of channels extends between edge edges of at least one of the complementary surfaces of the glass layer pair.

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