Abstract:
A lightweight parabolic mirror includes an open container with a piece of mirror film sealing the open container such that a sealed chamber is defined. A pressure control system coupled to the sealed chamber maintains a predetermined pressure differential between a pressure inside the sealed chamber and a pressure outside of the sealed chamber. The pressure differential causes the mirror film to deflect parabolically in one of a concave or convex fashion thereby defining the mirror&#39;s shape.

Description:
ORIGIN OF THE INVENTION  
       [0001] The invention described herein was made in the performance of official duties by employees of the Department of the Navy and may be manufactured, used, licensed by or for the Government for any governmental purpose without payment of any royalties thereon. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The invention relates generally to parabolic mirrors, and more particularly to a lightweight parabolic mirror design suitable for astronomical use.  
         BACKGROUND OF THE INVENTION  
         [0003]    Glass-backed mirrors are typically used to gather light in astronomical telescopes in order to view objects at great distances. These mirrors are typically circular parabolic mirrors as they need to focus incoming (nearly) parallel rays of light to a small focus point. The ability to resolve faint objects is directly proportional to the size of the aperture used to gather incoming light. Accordingly, glass-backed mirrors used in such long-distance viewing applications are large, e.g., 3-12 feet in diameter. The use of glass is advantageous given its uniform isotropic structure, low coefficient of thermal expansion, and fine grain structure that provides for a highly polished surface. However, the weight, volume and mass of glass becomes a major structural issue as an optically correct parabolic mirror that is only 3 feet in diameter can weight as much as 400 pounds. As the size of the parabolic mirror increases to accommodate longer distance viewing applications, the weight, volume and mass thereof increases exponentially. For example, large parabolic mirrors up to 12 feet in diameter used in various astronomical observatories can weight several tons or more.  
         SUMMARY OF THE INVENTION  
         [0004]    Accordingly, it is an object of the present invention to provide a low mass, lightweight parabolic mirror and method for making same.  
           [0005]    Another object of the present invention is to provide a lightweight parabolic mirror having good thermal and barometric stability for use in astronomical and/or terrestrial observation.  
           [0006]    Yet another object of the present invention is to provide a lightweight parabolic mirror that can hold a near constant focal length over a wide range of temperatures and barometric conditions while minimizing the use of heavy glass to control geometric parameters of the mirror.  
           [0007]    Other objects and advantages of the present invention will become more obvious hereinafter in the specification and drawings.  
           [0008]    In accordance with the present invention, a lightweight parabolic mirror is provided. A container has an opening formed therein. A piece of mirror film is coupled to the container and seals the opening such that the container and mirror film define a sealed chamber. A pressure control system coupled to the sealed chamber maintains a predetermined pressure differential between a pressure inside the sealed chamber and a pressure outside of the sealed chamber. The pressure differential causes the mirror film to deflect parabolically in one of a concave or convex fashion.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    Other objects, features and advantages of the present invention will become apparent upon reference to the following description of the preferred embodiments and to the drawings, wherein corresponding reference characters indicate corresponding parts throughout the several views of the drawings and wherein:  
         [0010]    [0010]FIG. 1 is a is a schematic view of one embodiment of a lightweight parabolic mirror according to the present invention; and  
         [0011]    [0011]FIG. 2 is in part a cross-sectional view and in part a is a schematic view of another embodiment of a lightweight parabolic mirror according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0012]    Referring now to the drawings, and more particularly to FIG. 1, a lightweight parabolic mirror according to the present invention is shown and referenced generally by numeral 10. Parabolic mirror  10  can be shaped/sized and used in a wide variety of applications such as astronomical telescopes, tracking systems, lighting devices, holography, photography, spotting scopes, terrestrial observation systems, solar concentrators, reflector antennas and microwave lens antennas.  
         [0013]    Parabolic mirror  10  includes a pressure container  12  having an opening  14  formed therein. In the illustrated embodiment, opening  14  is a circular opening of diameter D so that parabolic mirror  10  will be a circular parabolic mirror. However, it is to be understood that other parabolic mirror shapes can be created by the present invention by, for example, making opening  14  define other shapes such as a square, rectangle, pentagon, hexagon, etc.  
         [0014]    A piece of thin mirror film  16  covers and seals opening  14  such that the combination of container  12  and mirror film  16  define a sealed chamber  18 . For example, mirror film  16  can be captured about the periphery of opening  14 . As a result, in the case of a circular opening  14 , a circular portion  16 A of diameter D of mirror film  16  extends over and across opening  14 . Mirror film  16  can be any one of the commercially available thin and flexible mirror films such as the VM2000 mirror film produced by 3M Specialty Film and Media Products Division, St. Paul, Minn.  
         [0015]    During construction of parabolic mirror  10 , mirror film  16  is attached/sealed to container  12  such that it is relatively flat, i.e., extends directly across opening  14 . However, during the use of parabolic mirror  10 , circular portion  16 A of mirror film  16  is either drawn into sealed chamber  18  or pushed out of sealed chamber  18  as indicated by dashed lines  17 A and  17 B, respectively, as will be explained further below. Flexing of circular portion  16 A is made possible by the thin and flexible nature of mirror film  16 .  
         [0016]    Parabolic mirror  10  also includes a pressure control system for controlling the pressure in sealed chamber  18 . While a representative structure for such a pressure control system will be described herein by way of example, it is to be understood that other pressure control system structures could be used without departing from the scope of the present invention. In principle, the pressure control system maintains a predetermined pressure differential between the ambient pressure P 1  outside of sealed chamber  18  and the pressure P 2  on the inside of sealed chamber  18 .  
         [0017]    The predetermined pressure differential is selected to parabolically shape circular portion  16 A to provide a particular mirror focus for a given application. To deflect circular portion  16 A parabolically into sealed chamber  18  to create a concave parabolic mirror as indicated by dashed lines  17 A, inside pressure P 2  must be less than outside pressure P 1 . Conversely, to deflect circular portion  16 A parabolically out of sealed chamber  18  to create a convex parabolic mirror as indicated by dashed line  17 B, inside pressure P 2  must be greater than outside pressure P 1 . To insure that all pressure differential deflection occurs only at circular portion  16 A, container  12  must maintain its shape at the predetermined pressure differential.  
         [0018]    One embodiment of the pressure control system for maintaining the predetermined pressure differential includes: a controllable valve  20  installed in container  12  to communicate with sealed chamber  18 , a pump  22  coupled to valve  20 , a controller  24  coupled to valve  20  and pump  22  to control the operation thereof based on pre-programmed control logic/algorithm(s), and sensor(s)  26  and  28  that provide pressure measurements (and/or other measurements such as temperature which are indicative of pressure measurements) of outside pressure P 1  and inside pressure P 2 , respectively.  
         [0019]    In operation, controller  24  is pre-programmed with the predetermined pressure differential (P 1 -P 2 ) needed to shape circular portion  16 A of mirror film  16 . Based on readings from sensor(s)  26  and  28 , controller  24  controls the opening/closing of valve  20  and the operation of pump  22  to reduce or increase pressure in sealed chamber  18 . Accordingly, pump  22  is typically a reversible pump. The particular fluid medium pumped into or out of sealed chamber  18  is not a limitation of the present invention. While any inert fluid will do, a gas such as air is preferred to minimize the overall weight of parabolic mirror  10 .  
         [0020]    To maintain the correct parabolic shape defined by either of dashed lines  17 A or  17 B, it is necessary to maintain the diameter of circular portion  16 A as well as the above-described pressure differential. This is especially true in larger diameter mirrors. One way of fixing the diameter of circular portion  16 A is illustrated in the embodiment of FIG. 2 where like reference numerals are used for those elements that are common with the FIG. 1 embodiment.  
         [0021]    In FIG. 2, a parabolic mirror  100  includes a rigid pressure container  112  (e.g., metal, plastic, composite, etc.) having an opening  114  formed therein. Container  112  includes a peripheral lip  115  provided about opening  114 . Unlike parabolic mirror  10 , opening  114  is not used to define the shape of the parabolic mirror. Rather, a piece of thin mirror film  116  is captured/sandwiched at its periphery between two glass rings  119 A and  119 B which are aligned with one another to define a constant inside diameter D that defines a circular portion  116 A of mirror film  116 . Since glass is a very isotropic material with a small coefficient of thermal expansion, the diameter D of circular portion  116 A will remain constant to assure a constant aperture for parabolic mirror  100  while adding only a small amount of extra weight to the overall mirror assembly. Note that the lower glass ring  119 A can have its inner periphery shaped to facilitate a particular parabolic shape and to provide a smooth edge adjacent mirror film  116 . In addition, as in the previous embodiment, the shape defined by the inner periphery of rings  119 A and  119 B (which defined the shape of the mirror film deflected by the pressure differential) need not be circular.  
         [0022]    The combination of glass rings  119 A and  119 B sandwiching mirror film  116  is clamped to peripheral lip  115  by, for example, the use of a clamping ring  120  and clamping screws  122  cooperating with clamping ring  120  and peripheral lip  115 . As a result, a sealed chamber  118  is defined within the combination of container  112  having glass rings  119 A/ 119 B and mirror film  116  sealing opening  114 . Parabolic mirror  100  includes a pressure control system similar to that described above with respect to parabolic mirror  10  and will, therefore, not be described further herein. Thus, during operation of parabolic mirror  100 , circular portion  116 A of mirror film  116  can be deflected parabolically into or out of sealed chamber  118  as indicated by dashed lines  117 A and  117 B, respectively.  
         [0023]    The advantages of the present invention are numerous. A lightweight parabolic mirror and method of constructing/operating same are provided. The parabolic mirror will adapt to a wide variety of pressure and/or temperature variations. The simple lightweight construction will allow the present invention to be used in portable as well as permanent telescope, tracking system assemblies, etc. The materials used in the present invention are readily available and relatively inexpensive. The light weight of the mirror assembly opens up the possibility of very large diameter apertures since the weight of the overall mirror assembly is more than 500 times lighter than conventional glass-backed mirror assemblies. Further, since the mirror film is thin and has little thermal mass, it stabilizes quickly (e.g., on the order of 1-2 seconds) and homogeneously in varying temperature conditions.  
         [0024]    Although the invention has been described relative to a specific embodiment thereof, there are numerous variations and modifications that will be readily apparent to those skilled in the art in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described.