Abstract:
An in-line night-day boresight with an adjustable wedge-prism assembly is disclosed. The adjustable wedge-prism assembly includes two opposing wedge prisms that are axially rotatable relative to each other. The assembly is arranged in the optical path between the night optics and the day optics. The adjustable wedge-prism assembly allows for compensation of image shift errors introduced by the night optics due to manufacturing errors in the night optics. This in turn allows for a cost-effective and easily aligned in-line night-day boresight.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to boresights, and in particular relates to in-line night-day boresights, and apparatus and methods for aligning same  
       BACKGROUND OF THE INVENTION  
       [0002]     Boresighting optical systems (“boresights”) are used for long-range weapons, such as rifles, to allow the weapon&#39;s user to view a target and align the weapon relative to the target, e.g., to a select bullet impact point. A typical boresight includes a cross-hair reticle, and the boresight is adjusted (“aligned”) so that cross-hairs match the desired bullet impact point for a given target distance. Additional adjustments may be made to realign the boresight to the bullet impact point to take into account such factors as windage, distance, and bullet caliber. Finding the proper boresight alignment for a given set of conditions often involves trial and error, which is time consuming and not always convenient. Consequently, one would generally prefer to perform boresight alignment and/or realignment as infrequently as possible.  
         [0003]     Certain types of boresights have both day-vision and night-vision capability, and are referred to herein as “night-day boresights.” Night vision capability is provided by a night-vision optical system, referred to hereinafter as “night optics.” Likewise, the day-vision capability is provided by a day-vision optical system, referred to hereinafter as “day optics.” Because night vision different imaging capabilities and includes an image intensifier, the optical design of the night optics is different than the day optics. Accordingly, the night optics and the day optics are typically separate optical systems.  
         [0004]     There are three basic approaches to combining both night optics and day optics to form a night-day boresight. The first approach involves having separate night optics and day optics boresights and simply replacing one with the other as desired. However, this requires “rezeroing” each boresight every time it is placed on the weapon. Also, it is not always convenient to swap boresights, such as in combat or hunting situations.  
         [0005]     The second approach involves integrating the day and night optics by combining the night optics and day optics optical paths using an adjustable mirror or beamsplitter. While this approach does not require physically swapping the night and day optics, it is still problematic because it requires boresight realignment when switching from the night optics to the day optics.  
         [0006]     The third approach involved “clipping on” the night optics to the day vision optics. In one version of this approach, the night optics is clipped to a mount that holds the night optics above the day optics. A beamsplitter or mirror is then mounted in front of the day optics to project the night image down from the night optics into the day optics. The position of the mirror is then adjusted to obtain the required boresight alignment. Typically, the mirror or beamsplitter is adjusted until the two lines of sight are parallel, being offset by the difference in mounting height. A predictable point of impact is then available when using the night optics to augment the day optics for night time use.  
         [0007]     In another version of the third approach, the clip-on night optics is mounted in front of (or “in line with”) the day optics. In this arrangement, the night optics is said to be optically “upstream” of the day optics, i.e., the night optics is closer to the target and so receives light prior to the day optics.  
         [0008]      FIG. 1  is a schematic diagram of an in-line night-day boresight  10  shown mounted to a barrel  14 . Boresight  10  includes removable night optics  20  having an input end  22  and an output end  24 . Night optics  20  is arranged upstream of and in line with day optics  30 . Day optics  30  has an input end  32  adjacent the night optics output end  24 , and an output end  34  opposite input end  32 . Night optic  20  and day optic  30  are arranged along an optical axis A 1 . A user  50  is shown viewing through the boresight at output end  34  of day optics  30 .  
         [0009]     In the operation of boresight  10 , light  52  from a distant target (not shown) enters the input end  22  of night optics  20  and is incident an image intensifier tube  56 , which outputs intensified (amplified) light  60 . The intensified light  60  is then relayed to input end  32  of day optics  30  and is relayed to output end  34  to be viewed by user  50 . While night optics  20  is designed for use in combination with day optics  30 , the inevitable manufacturing errors (e.g., mechanical misalignments and tolerance errors) in night optics  20  cause light  60  to take a different path  61  (dashed line). The difference in paths  60  and  61  corresponds to an image shift IS of an amount Δ as seen by user  50 . This image shift typically corresponds to an angular error of about 10 to 15 minutes (i.e., ˜10′-15′) of arc.  
         [0010]     Reducing or eliminate this image shift has been achieved in several different ways. One way is to determine the alignment error due to the image shift due arising from the presence of the night optics, and dial this error into the day optics. Unfortunately, this approach is not preferred because the user has a different set of adjustments when using the night optics and the day optics. Another way is to adjust the optical centerlines during assembly to keep the image shift within a usable margin of error. While this can work in principle, it adds cost to the assembly and testing, with the latter having to be performed frequently until the assembly “settles” due to weapon shock. Yet another way is to provide mechanical adjustment capability to the night optic and day optic mounts. While this is a straightforward solution to reducing or eliminating the resultant image shift, it is not desirable because realignment needs to be performed every time the night optics is mounted and dismounted.  
       SUMMARY OF THE INVENTION  
       [0011]     A first aspect of the invention is a boresight apparatus. The apparatus includes night optics and day optics. The night optics is arranged along an axis, and the day optics is arranged along the same axis optically and downstream of the night optics. The day optics is optically coupled to the night optics. The apparatus also includes an adjustable wedge-prism assembly. The assembly is arranged between the night optics and day optics. The assembly includes first and second opposing wedge prisms. The prisms are rotatably adapted to compensate for an image shift in the day optics caused by the night optics.  
         [0012]     A second aspect of the invention is an image-shift adjustor apparatus for an in-line boresight that has a first optical system and a downstream second optical system. The first and second optical systems may be, for example, night optics and day optics, respectively. The apparatus includes first and second opposing wedge prisms arranged between the first and second optical systems. The wedge prisms are independently rotatable so as to reduce or eliminate an image shift in the second optical system caused by the first optical system.  
         [0013]     A third aspect of the invention is a method of aligning a boresight having in-line night optics and day optics. The method includes determining a first boresight alignment by aligning the day optics to a desired bullet impact point on a target. The method also includes adding the night optics and an adjustable wedge-prism assembly in line with and upstream of the day optics. The assembly has first and second opposing wedge prisms and is arranged between the night optics and the day optics. The method further includes determining an image shift in the desired bullet impact point due to the presence of the night optics. The method also includes adjusting the adjustable wedge-prism assembly to reduce or eliminate the image shift.  
         [0014]     A fourth aspect of the invention is a method of reducing or eliminating an image shift in an in-line boresight that has a first optical system and a downstream second optical system. The method includes arranging first and second opposing and independently rotatable wedge prisms between the first and second optical systems. The method further includes rotating at least one of the wedge prisms so as to reduce the image shift. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]      FIG. 1  is a schematic diagram of an example boresight mounted to a gun barrel, wherein the boresight includes a night optics arranged in-line and upstream of day optics, illustrating the image offset that arises in such an in-line arrangement due to manufacturing errors in the night optics;  
         [0016]      FIG. 2  is a schematic diagram similar to that of  FIG. 1 , illustrating the adjustable wedge-prism assembly used in the boresight to reduce or eliminate the image shift between the day optics and the night optics;  
         [0017]      FIG. 3  is a close-up side view of the adjustable wedge-prism assembly rotatably mounted in an outer housing, showing an example manual wedge-prism adjusting means in the form of dials mechanically coupled to retainers that hold the wedge prisms in the outer housing and that allow for manual rotation of one or both of the wedge prisms;  
         [0018]      FIG. 4  is a cross-section view of  FIG. 3  taken along the line  4 - 4 , showing the wedge prisms being rotatably held by respective retainers within the outer housing;  
         [0019]      FIG. 5  is a schematic diagram showing a target, day optics with a reticle, and a boresight, illustrating the first step in achieving alignment of the night-day boresight of the present invention;  
         [0020]      FIG. 6  is similar to  FIG. 5 , but further including night optics and the adjustable wedge-prism assembly in the optical path between the night and day optics, wherein the adjustable wedge-prism assembly is set to “zero” so that the image shift shows up when light from the target passes through the night optics and day optics; and  
         [0021]      FIG. 7  is similar to  FIG. 6 , but showing how the image shift is removed via adjustment of the wedge prisms in the adjustable wedge-prism assembly. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0000]     Apparatus  
         [0022]      FIG. 2  is a schematic diagram similar to that of  FIG. 1 , further including a adjustable wedge-prism assembly  200  arranged in the optical path between day optics  30  and night optics  20 . In an example embodiment, adjustable wedge-prism assembly  200  is arranged immediately adjacent output end  24  of night optics  20 , as shown.  FIG. 3  is a close-up side view of adjustable wedge-prism assembly  200  (dashed line) as held in an outer housing  202 .  FIG. 4  is a cross-sectional view of adjustable wedge-prism assembly  200  and outer housing  202  taken along the line  4 - 4 .  
         [0023]     With particular reference to  FIGS. 2 and 4 , in an example embodiment, adjustable wedge-prism assembly  200  includes a first wedge prism  210  having a planar surface  212  perpendicular to axis A 1 . Prism  210  also includes a planar wedge surface  214  opposite perpendicular planer surface  212  and having a wedge angle θ 1  relative to axis A 1 . Wedge prism  210  has a thick end  220  and a thin end  222 .  
         [0024]     Assembly  200  also includes a second wedge prism  230  axially spaced apart from prism  210  by a distance d. Prism  230  has a planar surface  232  perpendicular to axis A 1 . Prism  230  also includes a planar wedge surface  234  opposite perpendicular planer surface  232  and having a wedge angle θ 2  relative to axis A 1 . Wedge prism  230  has a thick end  240  and a thin end  242 .  
         [0025]     Prisms  210  and  230  are arranged in opposition, i.e., with their wedge surfaces  214  and  234  facing one another so that thin end  222  of prism  210  is adjacent thick end  240  of prism  230 , and thick end  220  of prism  210  is adjacent thin end  242  of prism  230 . Prisms  210  and  230  are each independently rotatable about axis A 1 . The separation distance d is selected so that prisms  210  and  230  do not make contact when either is rotated relative to the other. In a preferred example embodiment, wedge angles θ 1  and θ 2  are equal (i.e., θ 1 =θ 2 =θ), and this equal-angle example embodiment is considered hereinbelow for the sake of discussion, and the wedge angle are collectively referred to as θ. In an example embodiment, wedge prisms  210  and  230  are identical in shape, size and composition.  
         [0026]     In an example embodiment, wedge angles θ are in the range between about 1′ and about 15′ of arc, which allow for compensating the typical image shifts that arise due to the usual night optics manufacturing errors. Naturally, larger wedge angles can be employed where larger image shifts are anticipated or measured.  
         [0027]     In an example embodiment, wedge prisms  210  and  230  are made from standard optical glass having excellent transmission in the visible and near infrared. An example of such an optical glass is BK-7, available from Schott Glass, Inc. of Duryea, Pa. Also in an example embodiment, one or more of the prism surfaces  212 ,  214 ,  232  and  234  have an antireflection coating formed thereon and designed to improve the overall optical transmission of assembly  200 . The antireflection coating preferably has a broad passband and covers the visible and near-infrared (e.g., 430 nm-730 nm). In particular, it is preferred that the antireflection coating passband include the output wavelength of the image intensifier tube  56  in night optics  20 .  
         [0028]     In an embodiment, wedge prisms  210  and  230  are housed in respective individual prism retainers  270  and  280 . In an example embodiment, retainers  270  and  280  are rotatably mounted within an outer housing  202  (e.g., a lens tube). This can be accomplished in a number of ways, such as via rotatable mounts  286 , e.g., in the form of slip-rings. In an example embodiment, wedge-prisms  210  and  230  are circular in shape, as is usually the case for boresight optical elements.  
         [0029]     In an example embodiment, retainers  270  and  280  are manually rotatable with respect to one another (or rotatable together) using any of the known means of the prior art. For the sake of illustration, retainers  270  and  280  are shown to include respective dials  302  and  304  so that the rotation of one of the dials results in a corresponding rotation of the retainer to which is it mechanically coupled. In an example embodiment, retainers  270  and  280  are mechanically coupled to the respective dials  302  and  304  through respective gear systems (not shown) that allow for fine adjustment of the wedge prisms. Retainers  270  and  280  are also preferably adapted to be secured in a desired fixed position after their adjustment is complete so that motion or shock associated with weapons fire does not alter the adjustment.  
         [0000]     Method of Operation  
         [0030]     As illustrated schematically in  FIG. 2 , adjustable wedge-prism assembly  200  is used to reduce or eliminate the image shift IS between the day optics and the night optics that arises due to manufacturing errors in the night optics. This is achieved by the adjustable wedge-prism assembly introducing a compensating offset to rays  60  so that the image seen by observer  50  has a reduced or eliminated image shift IS. The discussion below sets forth an example method for reducing or eliminating the image shift IS by adjusting adjustable wedge-prism assembly  200 .  
         [0031]      FIG. 5  is a schematic diagram showing a target  502 , day optics  30  with a reticle  504 , and a boresight  51 , illustrating the first step in achieving alignment of a night-day boresight. First, target  502  is acquired via day optics  30  alone (i.e., night optics  20  is removed from the weapon). Reticle  504  is then adjusted such that a bullet impact point  516  hits the target  502  at the desired point, as indicated by cross-hairs  520  at the target. At this point, boresight  510  is aligned for day optics  20 , as indicated by cross-hairs  530  at boresight  510 .  
         [0032]      FIG. 6  is a schematic diagram similar to  FIG. 5 , but that includes the addition of night optics  20 , and adjustable wedge-prism assembly  200 , illustrating the second step in achieving alignment of the night-day boresight. Adjustable wedge-prism assembly  200  is arranged in the optical path between night optics  20  and the day optics  20 . The introduction of night optics  20  introduces an image shift IS into the otherwise aligned boresight due to the aforementioned manufacturing errors present therein. The shifted image is indicated by shifted crosshairs  540 . Note that dials  302  and  304  on adjustable wedge-prism assembly  200  are set to a “zero” position, i.e., a position that introduces no or substantially no image shift. The zero position is indicated by co-linear arrows  552  and  554  that are aligned with respective marks on dials  302  and  304 , which are also aligned with each other at the zero position.  
         [0033]      FIG. 7  is a schematic diagram similar to  FIG. 6  and illustrates the third step in achieving alignment of the night-day boresight  10 . In the third step, one or both of prisms  210  and  230  ( FIG. 4 ) of adjustable wedge-prism assembly  200  is/are adjusted (rotated) so that the image shift IS is reduced or eliminated. The adjustment is illustrated by the separation of arrows  552  and  554 , which reflects the movement of dials  302  and  304 , respectively, from the zero position to the desired image-shift-compensating position. At this position, prisms  210  and  230  introduce an upstream image shift that compensates for (i.e., reduces or eliminates) the image shift IS that occurs in day optics  20  due to the imaging imperfections of night optics  20 .  
         [0034]      FIG. 2  illustrates the embodiment that eliminates the image shift by showing light rays  60  leaving the adjustable wedge-prism assembly  20  at a shifted trajectory corresponding to that required by day optics  30  to form an image at the unshifted location.  
         [0035]     The invention described herein has the advantages of making the alignment of an in-line night-day boresight easier, and making combined night-day boresights less expensive than such prior art boresights and alignment methods. In particular, the compensating capabilities of the present invention allow for relatively loose tolerances on the assembly and manufacturing of the day and night optics, which translates directly into cost reduction of the in-line boresight.  
         [0036]     While the present invention has been described in connection with preferred embodiments, it will be understood that it is not so limited. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined in the appended claims.