Patent Application
20130052919
1. Field
The aspects of the present disclosure relate generally to the field of optical element formation, and in particular to a fixture for grinding and polishing meter class optical elements.
2. Description of Related Art
Meter-class optics require support during the grinding/polishing process with large tools. The use of aluminum as a polishing fixture is a standard configuration, with pitch providing a compliant layer to allow support without deformation. Unfortunately, with meter-scale optics, a meter-scale fixture weighs over 120 pounds (approximately 55 kilograms) and may distort the optics being fabricated by loading the mirror and/or tool used in fabrication.
Mounts that are typically used to support large mirrors during fabrication are especially heavy and difficult to handle. The mount must be especially stiff to avoid deformation during the optical fabrication process, where a very large and very heavy lap often can distort the mount and the optic being fabricated. If the optic is placed on top of the lapping tool, the weight of the optic and the fixture can distort the lap. Fixtures to support the mirror during fabrication are often configured from very large plates of aluminum, that are often two (2) inches or more in thickness (approximately 5 centimeters) and have a weight upwards of 150 pounds (approximately 68 kilograms). With the addition of a backing material, such as pitch, and the mirror itself, the panel polishing fixture can typically weigh over 250 pounds (approximately 113 kilograms) for a meter class optic.
For example, the 1-meter Spherical Primary Optical Telescope (“SPOT”) Primary Mirror Segment, developed by NASA, is somewhat light weighted with a scalloped back shape for radius of curvature control. The SPOT will utilize a single ring of point-to-point hexagonal mirror segments. There are three SPOT segments in total. Segment 1 is mounted on a traditional backing structure of two (2) inch thick aluminum plates that weigh close to 160 pounds. With the mirror configured in a “face-up” orientation, the polishing tool, also referred to herein as a “lap” tool, is positioned on top of the mirror. This configuration is difficult to achieve due to the weight of the polishing tool. With the mirror configured in a “face-down” orientation, the polishing tool is positioned underneath the mirror with the mirror resting on top of the polishing tool. Although it was thought that better results might be achieved with the mirror configured in the “face-down” orientation, the polishing tool in this configuration is loaded with the weight of the mirror and the aluminum panel polishing fixture. With the weight of the mirror and aluminum panel polishing fixture easily reaching over 250 pounds, the face-down configuration would likely result in a deformation of the polishing tool or lap and lead to high figure error.
Accordingly, it would be desirable to provide a method or device that addresses at least some of the problems identified above.
As described herein, the exemplary embodiments overcome one or more of the above or other disadvantages known in the art.
One aspect of the exemplary embodiments relates to a polishing fixture assembly for an optical element. In one embodiment, the polishing fixture assembly includes a first composite panel, a second composite panel, and a core member sandwiched between the first composite panel and the second composite panel and coupling the first composite panel to the second composite panel. At least one mounting fixture insert is coupled to each of the first and second composite panels.
Another aspect of the disclosed embodiments relates to a polishing assembly for an optical element. In one embodiment, the polishing assembly includes a polishing machine having a working surface and a grinding/polishing fixture. The optical element is positioned between the polishing machine and the grinding/polishing fixture. At least one pitch button is positioned on a surface of the grinding/polishing fixture between the grinding/polishing fixture and the optical element. A lift interface is coupled to a first side of the grinding/polishing fixture for coupling the grinding/polishing fixture to the polishing machine. A mounting fixture insert is coupled to a second side of the grinding/polishing fixture for coupling the optical element to the grinding/polishing fixture. The grinding/polishing fixture includes a first composite panel, a second composite panel
and a core member sandwiched between the first composite panel and the second composite panel and coupling the first composite panel to the second composite panel.
These and other aspects and advantages of the exemplary embodiments will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. Moreover, the drawings are not necessarily drawn to scale and unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein. In addition, any suitable size, shape or type of elements or materials could be used.
In the drawings:
Referring to
The mounting or fixture inserts 104 are generally configured to couple the mirror to the fixture assembly 100. For example, the SPOT mirror has invar inserts bonded into the glass with low coefficient of thermal expansion epoxy that matches the glass. In one embodiment, such as for the SPOT mirrors, threaded rod 132 is fed through the polishing fixture inserts 104 and engaged in the threaded inserts bonded in the mirror 120. A washer (not shown) and nut 136 is engaged on the other end of the threaded rod 132 and loaded against the polishing fixture inserts 104. In one embodiment, referring to
The lift interface assemblies 106 are generally configured to provide an interface to a lifting apparatus (not shown) for lifting operations to remove the mirror from the polishing machine for metrology. In one embodiment, the lift interface assemblies 106 include a turnover bracket 114 that can be used as a connection point by the lifting machine to lift the fixture 100. In alternate embodiments, the interface assemblies 106 can include any suitable components that allow the fixture 100 and mirror to be removed from the polishing machine. The lift interface assemblies 106 generally comprise inserts flanges that are inserted from the opposite side of the panel 102 to allow the lift load to be spread out over a larger area (see
Referring to
In one embodiment, the mirror 120 is supported on the polishing fixture 100 with the optical surface 124 facing downwards, in direction A towards the polishing machine 130 as shown in
Referring now to
In one embodiment, referring to FIGS. 1 and 6A-6E, the inserts 104 are bonded to the composite panel 102. Holes are cut in each side panel 108, 110, which can comprise carbon face sheets, and the inner core 112, or aluminum honeycomb. The inserts 104 are inserted and bonded in from one of the sides 108, 110 using a standard, low coefficient of thermal expansion epoxy.
The aspects of the disclosed embodiments provide a composite polishing fixture for meter-class optics. In particular, the panel is a lightweight graphite panel with an aluminum honeycomb core. Two composite panels are bonded together with inserts for mirror mounting, assembly handling and interfacing to the polishing machine. The mirror is interfaced to the polishing fixture using pitch buttons to provide a strong, but compliant interface. The inserts are bonded to the composite panels and are capable of withstanding the lifting and polishing loads. The use of composite structures that are lightweight but stiff enough to allow standard polishing and grinding techniques to be used, while providing for a decrease in fixture weight by almost 70 percent.
Thus, while there have been shown, described and pointed out, fundamental novel features of the invention as applied to the exemplary embodiments thereof, it will be understood that various omissions and substitutions and changes in the form and details of devices illustrated, and in their operation, may be made by those skilled in the art without departing from the spirit of the invention. Moreover, it is expressly intended that all combinations of those elements and/or method steps, which perform substantially the same function in substantially the same way to achieve the same results, are within the scope of the invention. Moreover, it should be recognized that structures and/or elements and/or method steps shown and/or described in connection with any disclosed form or embodiment of the invention may be incorporated in any other disclosed or described or suggested form or embodiment as a general matter of design choice. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.
