Abstract:
A sensor system according to various aspects of the present invention comprises a sensor viewing an area via an optical path and a strut at least partially interposed across the viewing area. The strut is configured to taper along the optical path towards the sensor. In an exemplary embodiment, the strut includes at least two sides forming an angle along their common edge exposed to the sensor along the optical path.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention pertains generally to methods and apparatus relating to sensor systems. 
   2. Description of Related Art 
   For highly sensitive sensors, such as space-viewing sensors and missile defense infrared sensors, the system&#39;s ability to see targets may be limited by emissions from warm surfaces within the sensor&#39;s optical system. For example, in telescope-type systems, a secondary mirror is held relative to a primary mirror using mechanical struts that are within the sensor&#39;s collecting aperture. Traditionally, struts are painted black and are highly emissive and warm, generating a high thermal background contribution. Infrared sensors, however, operate more effectively with minimal thermal self-emission from the sensor and surrounding components reaching the focal plane array (FPA) or other detectors. The geometry around and holding the FPA is typically cooled to minimize the thermal self-emission, but the struts are not, and therefore are visible to the FPA. The strut blocks part of the sensor&#39;s collecting aperture and reflection or emission of light by struts may cause the FPA to view warm parts of the sensor and sense significant thermal background. 
   Newer strut designs use a highly reflective stair-step design that are used to view cold surfaces or outer space. These struts have low emissivity, so the thermal background from struts can be greatly reduced using stair steps. For sensors using advanced composite materials for their mirrors and mounts, however, struts with a stair-step pattern cannot be easily manufactured, as machining sharp corners fractures the brittle composite. Alternative designs, such as an L-shaped strut, occupy significant space. 
   BRIEF SUMMARY OF THE INVENTION 
   A sensor system according to various aspects of the present invention comprises a sensor viewing an area via an optical path and a strut at least partially interposed across the viewing area. The strut is configured to taper along the optical path towards the sensor. In an exemplary embodiment, the strut includes at least two sides forming an angle along their common edge exposed to the sensor along the optical path. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
     A more complete understanding of the present invention may be derived by referring to the detailed description and claims when considered in connection with the figures, wherein like reference numbers refer to similar elements throughout the figures, and: 
       FIG. 1  is a block diagram rendering of a missile according to various aspects of the present invention; 
       FIGS. 2A–B  are side cross-section and top views, respectively, of a sensor system according to various aspects of the present invention; 
       FIG. 3  is a cross-section view of a strut; and 
       FIG. 4  is a cross section view of a primary mirror and the strut. 
   

   DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
   The present specification and accompanying drawing show an exemplary embodiment by way of illustration and best mode. While these exemplary embodiments are described, other embodiments may be realized, and logical and mechanical changes may be made without departing from the spirit and scope of the invention. Thus, the detailed description is presented for purposes of illustration only and not of limitation. For example, the steps recited in any of the methods or process descriptions may be executed in any suitable order and are not limited to the order presented. Further, conventional mechanical aspects and components of the individual operating components of the systems may not be described in detail. The representations of the various components are intended to represent exemplary functional relationships, positional relationships, and/or physical couplings between the various elements. Many alternative or additional functional relationships, physical relationships, or physical connections may be present in a practical system. 
   The present invention is described partly in terms of functional components and various methods. Such functional components may be realized by any number of components configured to perform the specified functions and achieve the various results. For example, the present invention may employ various materials, mirrors, sensors, struts, shapes, sizes, and weights for various components, such as optical components, mechanical components, and the like, which may carry out a variety of functions. In addition, the present invention may be practiced in conjunction with any number of applications and environments, and the systems described are merely exemplary applications of the invention. Further, the present invention may employ any number of conventional techniques for manufacture, deployment, and the like. 
   A sensor system according to various aspects of the present invention may be used for any suitable purpose or application, such as observing remote or nearby targets, gathering information, tracking moving or stationary targets, and guiding systems toward targets. The sensor system receives relevant information relating to a particular condition, target, or other parameter, and is suitably adapted to the particular application to sense relevant data. In various applications, the sensor system  100  may sense any appropriate conditions or signals, for example movement, speed, acceleration, pressure, strain, heat, light, color, chemical composition, electromagnetic waves, magnetic fields, or any other type of physical event or presence capable of being sensed. 
   For example, referring to  FIG. 1 , the sensor system  100  may be integrated into a missile  108 . The missile  108  may comprise any missile system, comprising, for example, a payload  110 , a propulsion system  112 , and a guidance system  114 . The sensor system  100  provides information to the guidance system  114  to identify and/or predict the position of a target relative to the missile  108 . The guidance system  114  receives information from the sensor system  100  and processes the information to control the flight path of the missile  108  and intercept the target. Alternatively, the sensor system  100  may be used in other applications and environments for sensing information, such as astronomy instruments, cameras, and measuring instruments. 
   The sensor system  100  of the present embodiment comprises an infrared sensor system. Referring to  FIG. 2 , the sensor system  100  suitably comprises a main structure  210 , a secondary structure  212 , and one or more struts  214  connecting the secondary structure  212  to the main structure  210 . The main structure  210  and the secondary structure  212  may comprise any appropriate components for the sensor system  100 . For example, the main structure  210  of the present embodiment includes a sensor  216  and a primary mirror  218 , and the secondary structure  212  includes a secondary mirror  220 . The primary mirror  218  reflects incident light to the secondary mirror  220 , and the secondary mirror  220  reflects light to the sensor  216 . 
   The sensor  216 , primary mirror  218 , and secondary mirror  220  may comprise any suitable components for the particular application or environment. For example, in the present embodiment, the sensor  216  comprises an infrared sensor, such as a highly-sensitive infrared focal plane array (FPA) or other suitable system for translating received infrared light into information, such as in the form of electrical or optical signals. Similarly, the primary mirror  218  and the secondary mirror  220  may comprise any suitable reflectors or array of reflectors for reflecting or otherwise transmitting light to the sensor  216 . In the present embodiment, the primary and secondary mirrors  218 ,  220  comprise high-quality composite mirrors configured to reflect and focus infrared light. 
   The sensor  216 , primary mirror  218 , and secondary mirror  220  may also be configured in any suitable configuration, and may comprise additional or fewer sensors, mirrors, and/or other components. In the present embodiment of the invention, the primary mirror  218  has a center hole  224 . The sensor  216  is positioned below the center hole  224 , and the secondary mirror  220  is positioned above the center hole. The primary mirror  218  focuses light on the secondary mirror  220 , which in turn focuses the light on the sensor  216 . Thus, light incident upon the primary mirror  218  is transmitted to the sensor  216  along an optical path  222 . 
   The struts  214  hold the secondary mirror  220  in position. The struts  214  may comprise any appropriate supports for maintaining the position of the secondary structure  212  and may comprise any suitable substantially rigid material, such as a metal, ceramic, plastic, or the like. In the present embodiment, the struts  214  comprise a lightweight composite material for structural rigidity and low weight. The struts  214  are also substantially straight from the main structure  210  to the secondary structure  212 , which simplifies manufacturing, reduces required materials, and reduces spatial requirements for the sensor system  100 . 
   One or more of the struts  214  may extend across the collecting aperture of the sensor  216 , and may thus be visible to the sensor  216 . The struts  214  are suitably narrow relative to the main structure  210  to reduce the obstruction presented by the struts  214  across the sensor&#39;s  216  collecting aperture. Further, the portion of the strut  214  within the sensor&#39;s  216  collecting aperture may be configured such that at least two sides of the strut are exposed to the sensor  216 . In particular, a portion of the strut  214  exposed to the sensor  216  via the optical path is tapered towards the sensor  216  along the optical path. For example, referring to  FIG. 3 , the cross-section of the strut  214  may be triangular, and the angle formed by two sides  310 ,  312  points towards the main structure  210 . The edge  314  facing the sensor  216  along the optical path may be sharp to minimize the amount of surface that may reflect light to the sensor  216  other than from the sides. The areas of the strut  214  that are not visible to the sensor  216  may be configured in any suitable manner, such as flat or multi-sided. 
   Further, the sides of the strut  214  may be enhanced according to any suitable criteria. For example, the areas of the strut  214  visible to the sensor  216  may be treated to inhibit emissions from the strut  214  into the sensor  216 . In the present embodiment, the areas of the strut  214  visible to the sensor  216  include a surface that is substantially reflective of light within the sensor&#39;s  216  frequency range, such as an enhanced gold reflective coating on a polished chemical vapor deposited substrate. 
   Referring to  FIGS. 3 and 4 , light visible to the sensor  216  from the strut  214  is substantially reflected from the surrounding environment, not from the sensor system  100  itself or other parts of the missile  108 . The source of the light reflected from the strut  214  to the sensor  216  via the optical path may be controlled according to the draft angle  316  of the strut  214 . The draft angle  316  of the strut  214  may be selected according to any suitable criteria. For example, many missile systems operate in conjunction with a solar exclusion, which requires that the sun be outside a particular angle relative to the boresight of the missile for proper operation. In the present embodiment, the draft angle  316  may be small to inhibit reflections off the strut  214  from outside the solar exclusion. Consequently, light visible to the sensor  216  that is reflected from the struts  214  is reflected substantially entirely from outside the sensor system  100  and from within the exclusion angle. 
   Although the description above contains many details, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the exemplary embodiments of this invention. The scope of the present invention fully encompasses other embodiments, and is accordingly to be limited by nothing other than the appended claims, in which reference to an element in the singular is not intended to mean “one and only one” unless explicitly so stated, but rather “one or more.” All structural, chemical, material, and functional equivalents to the elements of the above-described exemplary embodiments are expressly incorporated by reference and are intended, unless otherwise specified, to be encompassed by the claims. Moreover, it is not necessary for a device or method to address each and every problem sought to be solved by the present invention for it to be encompassed by the present claims. Furthermore, no element, component, or method step in the present disclosure is intended to be dedicated to the public regardless of whether the element, component, or method step is explicitly recited in the claims. No claim element is to be construed under the provisions of 35 U.S.C. 112, sixth paragraph, unless the element is expressly recited using the phrase “means for.” The terms “comprises”, “comprising”, or any other variation, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus.