AU2011349053A1

AU2011349053A1 - Heliostat construction

Info

Publication number: AU2011349053A1
Application number: AU2011349053A
Authority: AU
Inventors: Glenn Andrews; Alexander Ian BURTON
Original assignee: Commonwealth Scientific and Industrial Research Organization CSIRO
Current assignee: Commonwealth Scientific and Industrial Research Organization CSIRO
Priority date: 2010-12-22
Filing date: 2011-12-22
Publication date: 2013-07-04
Anticipated expiration: 2031-12-22
Also published as: WO2012083374A1; AU2011349053B2

Abstract

A parabolic mirror assembly for a heliostat comprising a mirror (14) and a backing support frame (20) for the mirror having elongate frame elements (22, 22a) arranged so that the mirror exhibits a shallow concave parabloid profile, wherein the mirror is secured to the frame elements by an adhesive layer between the frame elements and the mirror, and wherein manufacturing tolerances in the frame and/or mirror are compensated for by variations in the thickness of the adhesive. Also contemplated is a mould for forming a parabolic mirror assembly and method of use, that comprises: a mould body having a parabolic upper surface; and a plurality of recessed portions provided in said surface for receiving a frame below the upper surface; wherein a mirror can be conformably positioned on the upper surface of the mould body and adhesive applied between the frame and mirror is cured to form the mirror assembly.

Description

WO 2012/083374 PCT/AU2011/001667 1 Heliostat construction Field of the invention The present invention relates to heliostats, and in particular to parabolic mirror assemblies for heliostats and to their method of construction. The heliostats are 5 preferably for use with a concentrating central receiver system. Means of actuating the heliostats is also disclosed. Background of the invention Fossil fuels will be an important source of energy for many years, as well as a valuable export commodity. However, Australia is also recognised as having the highest solar 10 irradiation (highest average number of sunshine hours) of any continent in the world. Solar tower systems use an array of heliostats (large individually-tracking mirrors) to focus sunlight onto a central receiver mounted on top of a tower. The performance of the array is dependent on its design and control. Results show that there is an optimum size heliostat for a given tower size. For the system to operate 15 efficiently at low cost, good quality control of manufactured heliostats is necessary. The performance of the heliostats is vital to the efficient operation of the system, and small distortions in the mirrors can cause significant loss of focus. In commercial solar power systems, the central receiver tower is high and the heliostats are very large. For these systems, heliostats often comprise multiple flat mirrors, which 20 are individually angled to create an overall concave shape. An optimal heliostat field needs to be efficient not just in terms of energy collection, but in the cost of materials, fabrication and installation. It needs to have a low energy footprint and be able to be easily transported. To maximise commercial success, particularly in regional areas, it is desirable for the heliostats to be assembled by an 25 itinerant and/or unskilled workforce with minimal equipment.

WO 2012/083374 PCT/AU2011/001667 2 The present invention endeavours to address these preferences in its different aspects. Reference to any prior art in the specification is not, and should not be taken as, an acknowledgment or any form of suggestion that this prior art forms part of the common general knowledge in Australia or any other jurisdiction or that this prior art could 5 reasonably be expected to be ascertained, understood and regarded as relevant by a person skilled in the art. Summary of the invention In a first aspect the invention provides a parabolic mirror assembly for a heliostat comprising a mirror and a backing support frame for the mirror having elongate frame 10 elements arranged so that the mirror exhibits a shallow concave parabloid profile, wherein the mirror is secured to the frame elements by an adhesive layer between the frame elements and the mirror, and wherein manufacturing tolerances in the frame and/or mirror are compensated for by variations in the thickness of the adhesive. Preferably, the layer of adhesive is a substantially continuous layer between the mirror 15 and the backing support frame. The variation in the thickness of the adhesive is preferably at least 0.1 micron, more preferably at least 0.1 mm and most preferably at least 0.5 mm. The average adhesive thickness may be in the range of 0.001 to 1.0mm. Preferably, the slope error in the mirror is 2.0 milliradians or less, more preferably 1.5 milliradians or less, and most preferably 1.2 milliradians or less. 20 In its first aspect the invention further provides a heliostat including a parabolic mirror assembly as aforedescribed. The heliostat typically further includes actuator means to effect angular adjustment of the mirror assembly In a second aspect the present invention provides a method of forming a parabolic mirror assembly for a heliostat, the method including the steps of: WO 2012/083374 PCT/AU2011/001667 3 placing a frame within a mould, the mould including recesses for receiving the frame below the mould's upper surface, the upper surface defining a parabolic surface profile; applying adhesive to the upper surface of the frame, the thickness of the 5 adhesive being greater than the distance between the upper surface of the frame and the upper surface of the mould; positioning a mirror onto the mould, such that portions of the mirror contact the mould and portions overlying the frame contact the adhesive; and applying pressure to the mirror such that the mirror bears against the parabolic 10 surface of the mould, such that the adhesive conforms into the gaps to cure, holding the mirror to the frame as a mirror assembly, whereby the mirror adopts the parabolic surface of the mould. Preferably, an upper mould is placed over the mirror, applying the pressure to clamp the mirror between the lower mould and the upper mould. The upper mould has a parabolic 15 surface profile corresponding to the lower mould. The present invention also provides, in a third aspect, a mould for forming a parabolic mirror assembly, the mould including: a mould body having an upper surface defining a parabolic surface profile; a plurality of recessed portions in the upper surface for receiving a frame below 20 the moulds upper surface, the recessed portions including supporting structure for holding a frame within the recesses; wherein a mirror can be positioned on top of the mould body, such that the mirror conforms to the parabolic upper surface and adhesive applied between the frame and the mirror cures, whereby the mirror adopts the parabolic surface of the 25 mould.

WO 2012/083374 PCT/AU2011/001667 4 The lower mould is preferably a female mould, such that the natural sag of the mirror follows the curve of the upper surface profile. The lower mould is advantageously held on a rigid jig assembly, such that it is adjustable to ensure the mould is level prior to use. 5 Advantageously, the mould profile is created by CNC machining the mould body to a desired parabolic shape. The mould body may be solid between the recessed portions to create a plurality of spaced faces defining the upper surface. Alternatively, the mould body may be formed from a plurality of upstanding walls defining the edges of the recessed portions, 10 whereby the upper face of the plurality of spaced walls define the upper surface. There are preferably provided alignment means to ensure components are correctly aligned relative to each other and the mould. The alignment means may be located in the corners of the mould. The mould may be made from wood such as MDF or balsa, polymer, rigid polyester 15 foam such as KlegecellTM, or foamed concrete. The adhesive is preferably polyurethane, for example SikaflexTM. The adhesive is typically between 3-6mm thick. Typically a manufactured mirror and frame assembly relaxes its shape after manufacture, shifting to a longer focal length. To counteract this effect, the shape of the 20 moulds is preferably pre-distorted. This may be performed in the CAD program to cut the desired shape, but could also be achieved by shimming between the female mould and the jig table, or by applying loads to the table with adjusting screws. The amount of pre-distortion is preferably in the order 0.5 to 1mm, and is may be applied at five places. The use of pre-distortion advantageously brings the slope error down from around 25 1milliradian to around 0.5milliradian.

WO 2012/083374 PCT/AU20111/001667 5 The invention further extends to a parabolic mirror assembly formed by the abovedescribed method wherein manufacturing tolerances in the frame and/or mirror are compensated for by variations in the thickness of the adhesive. Brief description of the drawings 5 The invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a representative view of a solar tower system incorporating an array of heliostats; Figure 2 is a rear perspective view of a heliostat incorporating a parabolic mirror 10 assembly according to an embodiment of the present invention; Figure 3 is a plan view of the heliostat depicted in Figure 2, viewed parallel to the plane of the frame; Figure 4 is a rear elevation of the heliostat depicted in Figure 2; Figure 5 is a side elevation of the heliostat depicted in Figure 2, viewed parallel to the 15 plane of the frame; Figure 6 is a front view of the heliostat depicted in Figure 2; Figure 7 is a fragmentary perspective side view of the central hub of the heliostat frame; Figure 8 is a plan view of a rib of the heliostat frame; Figure 9 is a top view of a heliostat arm at a first outer left angle; 20 Figure 10 is a top view of the heliostat arm of Figure 9 at a second outer left angle; Figure 11 is a top view of the heliostat arm of Figure 9 at a second outer right angle; WO 2012/083374 PCT/AU2011/001667 6 Figure 12 is a top view of the heliostat arm of Figure 9 at a first outer right angle; Figures 13 and 14 are side and front views respectively of the frame support bracket of the heliostat of Figure 2;; Figure 15a is a top view of the heliostat post including the mount of the frame support 5 bracket; Figure 15b is a front view of the post of Figure 15a; and Figure 15c is a side view of the post of Figure 15a; Figure 16a is a rear view of a parabolic mirror assembly manufactured according to one embodiment of the present invention; 10 Figure 16b is a cross-sectional end view through line A-A on Figure 16a; Figure 16c is an enlarged view of the section marked C on Figure 16b; Figure 17a is a top view of a mirror and frame in a mould during construction according to an embodiment of the present invention; Figure 17b is a cross-sectional end view through line B-B on Figure 17a; 15 Figure 17c is a cross-sectional end view through line A-A on Figure 17a; Figure 18a is a top view of a frame in a mould during construction according to an alternative embodiment of the present invention with the mirror and male mould not shown; Figure 18b is a cross-sectional end view through line A-A on Figure 18a with the mirror 20 and male mould shown; WO 2012/083374 PCT/AU20111/001667 7 Figure 19a is a top view of a frame in a mould during construction according to another alternative embodiment of the present invention with the mirror and male mould not shown; Figure 19b is a cross-sectional end view through line A-A on Figure 19a with the mirror 5 and male mould shown; Figure 20 is a perspective view of the frame in the mould of Figure 19a; and Figure 21 is a cross-section close up view of the frame and mirror in the mould of Figure 1 9a. Detailed description of the embodiments 10 The heliostat array and central receiver system A concentrating central receiver system 10 is shown in Figure 1 with a close-packed field 18 of two hundred heliostats 15, which each include a mirror 14. These focussing, sun-tracking mirrors 14 are of rectangular shape, typically measuring 2.4m by 1.8m, and have desired focal lengths between 15 and 38 metres. Two hundred heliostats can 15 create a peak of over 500 kWth of solar energy delivered to a central receiver 12 at equinox. The field 18 has been designed for maximum annual energy collection by a receiver aperture of 0.8m diameter at an elevation of 17 metres and an angle of 170 to the horizontal. The average mirror surface error is 3.5mrad and 1 mrad tracking error. The system may be used to produce SolarGasTM, which is formed in a natural gas 20 reforming process that stores solar energy in the chemical bonds of the gas. It is also highly suitable for other applications such as high temperature steam production, hydrogen production, spectral beam splitting and materials testing. The SolarGasm can then be processed into solar hydrogen. This enables solar energy to be stored and transported; this technology serves as a transitional route toward higher levels of solar 25 penetration in the energy mix.

WO 2012/083374 PCT/AU2011/001667 8 The optical performance of the heliostat field 18 and individual heliostats 15 is fundamental to the overall system. The field 18 includes heliostats 15 that are closely packed together to improve the overall optical efficiency of the field 18. Each heliostat 12 is 2.4m by 1.8m; this relatively small size affording the illustrated solar array over 5 900m 2 of surface area to concentrate solar energy. The heliostats 15 are located in the field according to their mirror's focal length. The surface of each mirror 14 is slightly curved in order to direct the sun's rays 100 as a directed beam 102 to a focal point on the central receiver 12. The close packing of the heliostats reduces the footprint of the array, making it 10 adaptable to suburban environments where there is less room available to accommodate larger solar fields. By using advanced optical models, each mirror has been positioned to take into account the change in seasonal position of the sun, without shadowing the mirror behind it. This has enabled the closest packing of heliostats anywhere in the world known to the 15 applicant. The system is able to achieve peak temperatures at the downstream reactor of over 1000 0 C. The field 18 and solar receiver 12 can be operated at lower temperatures by reducing the number of mirrors 14 aimed at the receiver or by controlling the flow rate of the fluid through the receiver. 20 The heliostats A suitable heliostat 15 with respective actuators 60, 62 is illustrated in Figures 2 to 6, and certain individual components are further detailed in Figures 7 to 16. Heliostat 15 includes a large concave mirror 14 fixed by adhesive to a backing frame 20 of rectangular profile. Frame 20 is mounted atop a stand or post 70, by means to be 25 described, and comprises a central hub 23 and ribs 22 that extend radially from central hub 23 to peripheral edge beams 22a. The ribs 22 are pre-manufactured to have a desired shape that provides the frame with particular paraboloid curvature when the WO 2012/083374 PCT/AU2011/001667 9 frame 20 is assembled. The ribs 22 are fastened to corresponding flat radial arms 25 of hub 23 (see Figure 7). The mirror 14 lies on the concave side. The dimensions of the components are determined by a frame pattern which is generated by software. Fasteners such as thread forming screws, rivets, spot welding or bolted joints are used 5 throughout the frame 20. The ribs of the frame are manufactured from sheet metal, e.g. galvanised sheet steel, laser cut (or die cut) and folded. The ribs 22 optionally have slots and tabs cut on a front curved face that may be bent by hand. The ribs 22 are shaped into a broad C or channel cross-section that defines a flat web 32 and edge flanges 28, 30: one of these 10 abuts and is adhered to mirror 14. Web 32 tapers outwardly from a wider inner end fastened to a corresponding hub arm 25. The mirror 14 is glued directly to the frame 20 using a polyurethane based adhesive applied to folded tabs 28a on the inner edge flanges 28 of all the ribs 22, which as already noted collectively define a shallow concave paraboloid shape. The mirror is typically made of 3mm thick glass having a 15 high reflectivity surface, such as a plastic composite, and a low iron content to reduce energy absorption. Suitable such glasses include those manufactured by Sencofein or "Miralite Solar Premium" manufactured by Saint Gobain. The pair of linear screw actuators 60, 62 by which the heliostat orientation is controlled are positioned substantially parallel, so that they both extend generally perpendicular to 20 the mirror 14. This prevents the actuators 60, 62 from colliding during operation whilst giving a greater range of optimal angle to the heliostat 12. The actuators 60, 62 include individual off-the-shelf DC motors 65. The actuators 60, 62 are arranged to provide control in two orthogonal directions so that the focussing point can be maintained for any angle of incident light. One axis is 25 controlled east to west, i.e. side to side, and the other north to south, i.e. upward tilt. However, one axis is controlled relative to the other axis. Specifically, side-to-side rotation occurs about an intermediate mount in the form of a frame support bracket 66, which is itself rotated up or down: this arrangement minimises the amount of space taken up by each heliostat 12.

WO 2012/083374 PCT/AU2011/001667 10 The frame 20 is connected to support bracket 66 at vertically spaced hinges 67 for rotation about an upright axis joining hinges 67. The first linear actuator 60 is mounted between the mirror frame 20 and an arm 64 that projects laterally rearwardly from support bracket 66, for controlling this side-to-side or east-west rotational movement. 5 Bracket 66 is pivotally attached in turn, by bracket pin 68 (Figures 15a-15c), to the top of post 70. Pin 68 defines an inclination axis about which the tilt angle of bracket 66, and thereby of frame 20, is adjustable. The aforementioned upright axis is generally orthogonal to the inclination axis. Arm 64 is rigidly connected to the bracket 66 as close to the inclination axis as possible. Second actuator 62 extends between post 70 and an 10 attachment point 69 at the lower end of bracket 66 for effecting adjustment of the tilt or inclination of the mirror. The angle of the arm 64 to the bracket 66 is selected to provide optimum actuator geometry, with different angles for each heliostat according to positions in the field. The bracket 66 may further include a number of different attachment points 69 for the 15 actuator 62 also selectable to provide the optimum angle according to the individual heliostat's position in the field. Magnetic sensors are used to measure the respective orientation or positional angles of each heliostat, defined by inclination and lateral orientation or declination. Methods of forming a paraboloid mirror 20 The design involves forming a flat sheet of silvered glass mirror 14 into a curved shape. The desired parabolic shape is produced by subjecting the glass to forces in the z direction. This is achieved by using the curved metal ribs 22 behind the mirror 14 and using an adhesive 34 to connect the mirror 14 to the ribs 22. The mirror will experience induced forces in the positive and negative z-direction and be elastically deformed into a 25 focusing shape. The design can be accurately assembled to produce a mirror 14 with small slope errors from the desired shape. The design of the frame also means it is more materially WO 2012/083374 PCT/AU2011/001667 11 efficient than other designs. The design is able to be manufactured simply, easily and reliably. According to a first method of forming the mirror, as shown in Figures 16a to 16c, where like reference numerals are preceded by a 2, the ribs 222 and frame 220 become the 5 template for the mirror shape. The frame 220 is held on a jig so as to be level and adhesive 234 is applied to the top upper edge folds 228; the mirror 214 is then placed on top of the frame 220 and weight (not shown) is applied to the front surface 215 of the mirror 214. The mirror 214 is held against the ribs 222 of the frame 220 while the adhesive 234 cures. The adhesive 234 once cured is typically 0 to 1.0mm thick using 10 this method. In this method, the jig is typically a table having at least five legs. Tuning of the jig during the mirror forming process can be achieved by adjusting levelling feet or inserting shims. The adhesive has a low viscosity and is thixotropic when applied. It cures to form a 15 tough elastomeric compound with properties that are unaltered with 25 years exposure to UV and atmosphere. Suitable adhesives include a range of polyurethane compounds manufactured by SikaflexTm, and commonly used for vehicular and architectural glass bonding. This first, thin-adhesive method is suitable for assembling mirrors on site in remote 20 areas where there is little manufacturing support and requires simple metrology tools. This method is relatively labour intensive. A second method is shown in Figures 17a to 17c, where like reference numerals are preceded by a 3. The shape of the mirror 314 is defined by curvature elements of a mould 336. In this method a male mould element 338 is held on a jig 340 and has an 25 upper surface 342 of the desired parabolic shape of the mirror 314. The mirror 314 is draped over the male mould and the natural sag conforms to the shape of the upper surface 342 under its own weight.

WO 2012/083374 PCT/AU2011/001667 12 Adhesive 334 is then applied to the upper edge folds 328 of the frame 320, which is then placed over the mirror 314. The weight of the frame 320 assists in conforming the mirror to the parabolic shape. When the adhesive is cured, the mirror is permanently affixed to the frame and in the desired parabolic shape. The adhesive in this method is 5 similar to the first method, being 0 to 1.0mm thick. A third, and preferred method, is shown in Figures 18a, 18b, and in Figures 19a, 19b, 20 where like reference numerals are preceded by a 4. The difference between, the embodiment shown in Figures 18a, 18b and Figures 19a, 19b, 20 is that the female mould in Figure 18a is solid, whereas the female mould in Figure 19a is walled with 10 voids 447. The preferred embodiment shown in Figure 19a, 19b, 20 will therefore be described, with that shown in Figures 18a, 18b being equivalent. A mould 436 includes a mould body, being a female mould element 444 that is held on a jig 440. The female element 444 has an upper surface 442 defining a parabolic surface profile. The female element 444 is made from eight box constructions 446 15 spaced apart by recessed portions 448. The box constructions 446 are made up of four walls 450 having upper faces 452 that cumulatively create the upper surface 442. The recessed portions 448 form channels in the upper surface and receive the frame 420. Supporting structure 454 is provided so that the frame sits at the optimum level below the mould's upper surface 442. There is therefore a gap 443 in vertical distance created 20 between the upper surface, or upper edge fold 428, of the frame 420 and the upper surface, or upper faces 452, of the mould. An adhesive 434 is then applied to the upper surface of the frame 420 on upper edge folds 428 at a thickness greater than the gap so to be to, or above, the upper faces 452. The gap is typically 3 to 6mm. A mirror is then positioned on top of the female mould 25 444, such that its natural sag conforms it to the parabolic upper surface 442 of the female mould 444. Downward pressure is then applied to the front of the mirror 414, typically using a male mould element 438, whereby the adhesive 434 is squeezed to the sides of the mirror and frame until the rear of the mirror contacts against the upper surface 442. It is then held in this position until the adhesive cures and sets, whereby WO 2012/083374 PCT/AU2011/001667 13 when it is released from the mould the mirror adopts the parabolic surface of the mould. The male mould element 438 may also be made of a box construction having walls 451. The jig, which is typically a huge table designed to be very rigid, would, for example, be welded up from 100mm x 100mm x 5mm RHS (rectangular hollow section) steel and 5 may include adjustable legs to ensure leveling of the mould for specific locations. The mould elements are precisely machined to the desired curvature by CNC machines. The solid mould method shown in Figures 18a and 18b requires a lot of molding material, and would be best suited to a low density material like balsa, rigid polyester 10 foam (KlegecelITM), or foamed concrete. The voided mould method shown in Figures 19a, 19b and 20 enables the mold to be made from thick sheets of wood (MDF) or polymer. The result is a much lighter mould, which is faster to machine to create the paraboloid surface. In the frame/mirror assembly formed by the above methods, manufacturing tolerances 15 in the frame and/or mirror are compensated for by variations in the thickness of the adhesive. For example, the variation in the thickness of the adhesive is at least 0.1 micron, preferably at least 0.1mm and more preferably at least 0.5mm. The average adhesive thickness is preferably in the range 0.001 to 1.0mm. Slope error in the mirror is typically 2.0 milliradians for less, preferably 1.5 milliradians or less, and more 20 preferably 1.2 milliradians or less. Moreover, the layer of adhesive is a substantially continuous layer between the mirror and the backing support frame. All the various forming methods require alignment means 435, typically in the corners, to ensure that the mirror is correctly aligned with respect to the frame and that the frame is correctly aligned to the mould, so that the subsequent aligning of the mirror results in 25 the correct paraboloid shape.

WO 2012/083374 PCT/AU2011/001667 14 The preferred method of creating the curvature of the mold shapes is to place the entire assembled jig and mould into a CNC machine, and machine the surface in one operation. Typically a manufactured mirror and frame assembly relaxes its shape after 5 manufacture, shifting to a longer focal length (with a slight astigmatism for rectangular mirrors). To counteract this effect, the shape of the moulds can be pre-distorted; this is easily performed in the CAD program to cut the desired shape, but could also be achieved by shimming between the female mould and the jig table, or by applying loads to the table with adjusting screws. The amount of pre-distortion is of the order 0.5 to 10 1mm, and is sufficient to be applied at 5 places only (top, bottom, left, right and centre, as the diagonals are fixed). The use of pre-distortion will typically bring the slope error down from around 1milliradian to around 0.5milliradian. The jig with the CNC machined mould will typically need to be removed from the CNC machine before gluing can take place. Once in its new location, the jig needs to be 15 securely bolted to the floor, acclimatized to the area, and then be leveled to within +/ 0.1mm across the diagonals. Ideally the curvature should be checked again before first use, and regularly thereafter as a quality control measure, using a laser metrology method. The pressure exerted is some methods may be via elastomeric, dead weight, pneumatic 20 or hydrostatic means, to force the mirror to conform to the desired shape. The parabolic surface that the mould generates needs to be held to an accuracy of the order +/ 0.1mm, and the weight of the top and bottom moulds will deflect the jig more than this, so the jig design needs to incorporate this effect, typical effects are observed post-cure, when jigging stresses are relieved, and the edges are below the desired curve and the 25 centre is above the desired curve.

Claims

1. A parabolic mirror assembly for a heliostat comprising a mirror and a backing support frame for the mirror having elongate frame elements arranged so that the mirror exhibits a shallow concave parabloid profile, wherein the mirror is secured to 5 the frame elements by an adhesive layer between the frame elements and the mirror, and wherein manufacturing tolerances in the frame and/or mirror are compensated for by variations in the thickness of the adhesive.

2. A parabolic mirror assembly according to claim 1 wherein said layer of adhesive is a substantially continuous layer between the mirror and the backing support frame. 10

3. The parabolic mirror assembly according to claim 1 or 2, wherein slope error in the mirror is 2.0 milliradians or less, preferably 1.5 milliradians or less, and more preferably 1.2 milliradians or less.

4. The parabolic mirror assembly according to claim 1, 2 or 3, wherein the variation in the thickness of the adhesive is at least 0.1 micron, preferably at least 0.1 mm and 15 more preferably at least 0.5 mm.

5. The parabolic mirror assembly according to any one of claims 1 to 4 wherein the average adhesive thickness is in the range of 0.001 to 1.0mm.

6. A heliostat including a parabolic mirror assembly according to any one of claims 1 to 5. 20

7. A heliostat according to claim 6 further including actuator means to effect angular adjustment of the mirror assembly.

8. A method of forming a parabolic mirror assembly for a heliostat, the method including the steps of: WO 2012/083374 PCT/AU2011/001667 16 placing a frame within a mould, the mould including recessed portions for receiving the frame below the mould's upper surface, the upper surface defining a parabolic surface profile; applying adhesive to the upper surface of the frame, the thickness of the 5 adhesive being greater than the distance between the upper surface of the frame and the upper surface of the mould; positioning a mirror onto the mould, such that portions of the mirror contact the mould and portions overlying the frame contact the adhesive; and applying pressure to the mirror such that the mirror bears against the parabolic 10 surface of the mould, such that the adhesive conforms into the gaps to cure, holding the mirror to the frame as a mirror assembly, whereby the mirror adopts the parabolic surface of the mould.

9. A method according to claim 8 wherein said application of pressure to the mirror is by placing over the mirror an upper mould that has a parabolic surface profile 15 corresponding to the lower mound, and applying the pressure to clamp the mirror between the lower mount and the upper mould.

10. A method according to claim 8 or 9 wherein the lower mould is a female mould, such that the natural sag of the mirror follows the curve of the upper surface profile.

11. A method according to claim 8, 9 or 10 wherein the lower mould is held on a rigid 20 jig assembly, such that it is adjustable to ensure the mould is level prior to use.

12. A method according to any one of claims 8 to 11 wherein the mould profile is created by CNC machining the mould body to a desired parabolic shape.

13. A method according to any one of claims 8 to 12 wherein the mould has a body that is solid between the recessed portions to create a plurality of spaced faces defining 25 the upper surface. WO 2012/083374 PCT/AU2011/001667 17

14. A method according to any one of claims 8 to 13 wherein the mould has a body that comprises a plurality of upstanding walls defining the edges of the recessed portions, whereby the upper faces of the plurality of spaced walls define the upper surface. 5

15. A method according to any one of claims 8 to 14 including providing alignment means to ensure components are correctly aligned relative to each other and the mould.

16. A method according to any one of claims 8 to 15 wherein the mould is pre distorted to counteract relaxation of the mirror after manufacture resulting in a longer focal length. 10

17. A parabolic mirror assembly formed by a method according to any one of claims 8 to 16, wherein manufacturing tolerances in the frame and/or mirror are compensated for by variations in the thickness of the adhesive.

18. A parabolic mirror assembly according to claim 17, wherein said layer of adhesive is a substantially continuous layer between the mirror and the backing support 15 frame.

19. The parabolic mirror assembly according to claim 17 or 18, wherein slope error in the mirror is 2.0 milliradians or less, preferably 1.5 milliradians or less, and more preferably 1.2 milliradians or less.

20. The parabolic mirror assembly according to claim 17, 18 or 19, wherein the 20 variation in the thickness of the adhesive is at least 0.1 micron, preferably at least 0.1 mm and more preferably at least 0.5mm.

21. The parabolic mirror assembly according to any one of claims 17 to 20 wherein the average adhesive thickness is in the range 0.001 to 1.0mm.

22. A mould for forming a parabolic mirror assembly, the mould including: 25 a mould body having an upper surface defining a parabolic surface profile; and WO 2012/083374 PCT/AU2011/001667 18 a plurality of recessed portions in the upper surface for receiving a frame below the mould's upper surface, the recessed portions including supporting structure for holding a frame within the recessed portions; wherein a mirror can be positioned on top of the mould body, such that the mirror 5 conforms to the parabolic upper surface and adhesive applied between the frame and the mirror cures, whereby the mirror adopts the parabolic surface of the mould.

23. A mould according to claim 22 wherein the lower mould is a female mould, such that the natural sag of the mirror follows the curve of the upper surface profile. 10

24. A mould according to claim 22 or 23 wherein the mould has a body that is solid between the recessed portions to create a plurality of spaced faces defining the upper surface.

25. A mould according to claim 22 or 23 wherein the mould has a body that comprises a plurality of upstanding walls defining the edges of the recessed portions, 15 whereby the upper faces of the plurality of spaced walls define the upper surface.