Abstract:
A sight includes a housing with first and second openings, and includes an optics section that is disposed within the housing, that has image erecting optics and eyepiece optics, and that optically influences radiation received through the first opening from a scene, so as to deliver through the second opening a viewable image that is a function of the radiation. An adjusting section facilitates adjustment of the position of the image erecting optics and the eyepiece optics within the housing.

Description:
TECHNICAL FIELD OF THE INVENTION 
   This invention relates in general to techniques for zeroing a sight and, more particularly, to techniques for internally zeroing a sight. 
   BACKGROUND OF THE INVENTION 
   Over the years, various techniques and devices have been developed to help a person accurately aim a weapon such as a rifle. One common approach is to mount a sight or scope on the weapon. A person then uses the sight or scope to view an intended target in association with a reticle, often with a degree of magnification. Although existing weapon sights have been generally adequate for their intended purposes, they have not been satisfactory in all respects. 
   For example, after mounting a sight on a rifle, there is always some pointing error for a specific target distance that must be “zeroed” out in order to shoot accurately. In other words, for the target distance in question, an adjustment needs to be made so that the reticle cross-hairs of the sight will be centered on the point where the bullet will strike at that distance. One known approach is to use an external adjustment mechanism that adjusts the position of the entire sight in elevation and azimuth with respect to its mount on the rifle. A different known approach is to move an optical element internally within the sight, in order to readjust the effective pointing direction of the sight. 
   The traditional approach for internal zeroing is to move the optical erecting element within the sight. This essentially moves the secondary image plane so as to correct for the zeroing error. Some sights that use this approach are relatively long, because the objective image is reimaged by the erecting element to a conjugate plane, which is then viewed through the eyepiece. Shorter and more compact sights use a prism to erect the image. The prism can be internally moved with respect to other optical components, in order to displace the image at the reticle plane. 
   Although known techniques for internal zeroing have been generally adequate for their intended purposes, they have not been entirely satisfactory in all respects. One consideration is that the known techniques for internal zeroing will realign the aim point of the rifle, but also involve a degradation of the image quality. For example, moving an erecting prism within the light path relative to other optical components can produce axial coma and astigmatism that are associated with a loss of resolution for the axial and off-axis fields. This limits the effective zeroing range for this approach to a fraction of a degree. 
   SUMMARY OF THE INVENTION 
   One of the broader forms of the invention relates to zeroing a sight having a housing with first and second openings, and having an optics section within the housing that optically influences radiation from a scene received through the first opening so as to deliver through the second opening a viewable image that is a function of the radiation, where the optics section includes image erecting optics and eyepiece optics. This form of the invention involves adjusting the position of the image erecting optics and the eyepiece optics relative to the housing. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A better understanding of the present invention will be realized from the detailed description that follows, taken in conjunction with the accompanying drawings, in which: 
       FIG. 1  is diagrammatic perspective view of an apparatus that is a weapon sight configured to be removably mounted on a not-illustrated weapon such as a rifle, and that embodies aspects of the present invention. 
       FIG. 2  is a further diagrammatic perspective view of the sight of  FIG. 1 , taken from a different direction. 
       FIG. 3  is a diagrammatic side view of the sight of  FIG. 1 , partly in section. 
       FIG. 4  is a diagrammatic central sectional side view of the sight of  FIG. 1 . 
       FIG. 5  is a diagrammatic exploded perspective view of several selected components from the interior of the sight of  FIG. 1 . 
       FIG. 6  is a diagrammatic sectional view of the sight, taken along the section line  6 - 6  in  FIG. 4 . 
       FIGS. 7 and 8  are respective diagrammatic vertical sectional views of an adjusting mechanism that is part of the sight of  FIG. 1 , taken along respective vertical section planes that are offset by 90° with respect to each other. 
       FIG. 9  is a diagrammatic view showing the optical elements of the sight of  FIG. 1 , and showing chief and marginal rays for radiation from a not-illustrated scene that is traveling through the sight. 
       FIG. 10  is a diagrammatic view similar to  FIG. 9 , but showing a pointing error of θ° degrees. 
       FIG. 11  is a diagrammatic view similar to  FIG. 10 , but showing the relative position of the optical components after a correcting pivotal movement of certain optical components. 
       FIG. 12  is a graph showing the extent of change in accommodation of an eye that is required (for axial tangential and sagittal lines) as the zero setting is changed. 
       FIG. 13  shows the effect on Modulation Transfer Function (MTF) as the zero error correction is increased. 
   

   DETAILED DESCRIPTION 
     FIG. 1  is diagrammatic perspective view of an apparatus that is a weapon sight configured to be removably mounted on a not-illustrated weapon such as a rifle, and that embodies aspects of the present invention.  FIG. 2  is a further diagrammatic perspective view of the sight  10 , taken from an opposite side of the sight. 
   With reference to  FIGS. 1 and 2 , the sight  10  includes a tubular housing  12 . A ring  16  is mounted on one end of the housing  12 . The ring  16  defines an inlet opening  17  that permits radiation from a non-illustrated scene to enter the housing  12 . A further ring  18  is mounted on the opposite end of the housing  12 . The ring  18  defines an eyepiece opening  19  that allows radiation to exit the housing  12 , and to be viewed by the eye of a person. 
   A mounting mechanism  21  is secured to the underside of the housing  12 . The mounting mechanism  21  can be used in a known manner to removably mount the sight  10  on a standard mounting rail of a non-illustrated weapon, such as a rifle. The mounting mechanism  21  has a knob  22  that can be manually rotated in order to attach the sight  10  to a weapon, or to detach the sight from a weapon. A cylindrical opening  23  extends through part of the mounting mechanism  21  in a direction lengthwise of the sight  10 , in order to reduce the overall weight of the sight. Three rubber bumpers  26 - 28  are removably mounted by screws  29  to the top and sides of the housing  12 . The bumpers  26 - 28  provide a degree of shock protection for the sight  10 . 
   A cylindrical tube  31  is fixedly coupled to one side of the housing  12 , between the mounting mechanism  21  and the rubber bumper  26 . The tube  31  extends lengthwise of the sight  10 . The tube  31  has an internally threaded opening at one end, and a removable battery cover  32  has threads that engage the threaded opening. A battery compartment is provided within the tube  31  at the end thereof nearest the cover  32 , and the cover  32  can be removed in order to replace a battery. A flexible retaining strap  33  has one end coupled to the battery cover  32 , and its other end coupled to the mounting mechanism  21 , so that the battery cover  32  will not be inadvertently lost or misplaced when it is removed for battery replacement. 
   A rotatable control knob  36  is provided at the opposite end of the cylindrical tube  31 . In the disclosed embodiment, the control knob  36  can be rotated to any of 15 different positions. The position of the knob  36  controls the brightness of an illuminated reticle, as discussed in more detail later. Two adjusting knobs  38  and  39  are rotatably supported on the housing  12 , and are offset from each other by an angle of 90° with respect to an optical axis of the sight  10 . The adjusting knobs  38  and  39  are disposed near the end of the sight  10  that has the eyepiece opening  19 , and are used to “zero” the sight  10  in a manner discussed in more detail later. A spring retaining cap  42  has threads that engage a threaded opening provided through a side of the housing  12  opposite from the adjusting knobs  38  and  39 . The spring retaining cap  42  is discussed in more detail later. 
     FIG. 3  is a diagrammatic side view of the sight  10 , partly in section. A wall  51  is fixedly mounted within a cylindrical opening  52  that extends through the cylindrical tube  31 . A circular disk  53  is axially movably supported within the opening  52 , between the wall  51  and the removable battery cover  32 . Two resiliently flexible O-rings  54  are provided between the wall  51  and the disk  53 . An electrically conductive metal contact  55  is mounted in the center of the disk  53 , and serves as an electrical contact. An electrically conductive metal coil spring  57  is mounted on the inner side of the battery cover  32 , and also serves as an electrical contact. A battery  58  is provided between the disk  53  and the spring  57 , with one end engaging the electrical contact  55 , and the other end engaging the electrical contact defined by the spring  57 . In the disclosed embodiment, the battery  58  is a commercially-available 3-volt lithium battery, but it could alternatively be any other suitable type of battery. 
   On the side of the wall  51  opposite from the battery  58 , a multi-position electrical switch  61  is fixedly mounted within the opening  52  through the tube  31 . The switch  61  has a rotatable shaft  62 , and the control knob  36  is fixedly mounted on the shaft  62 . The opposite side of the switch  61  has several electrical terminals, and a circuit board  63  is soldered to the terminals of the switch  61 . The circuit board  63  is electrically coupled to the electrical battery contacts  55  and  57  by respective wires, which are not shown in  FIG. 3 . The switch  61  and the circuitry on the circuit board  63  serve to permit variation of the brightness of an illuminated reticle, which is discussed later. In this regard, the switch  61  has 15 operation positions. In one of these positions, the switch  61  turns off the circuitry on the circuit board  63 . The other 14 positions of the switch  61  correspond to respective different levels of brightness for the illuminated reticle. 
     FIG. 4  is a diagrammatic central sectional side view of the sight  10 .  FIG. 5  is a diagrammatic exploded perspective view of several selected components from the interior of the sight  10 . With reference to  FIGS. 4 and 5 , an annular retainer  71  is fixedly mounted within the opening through the tubular housing  12 , at an end of the housing near the ring  16 . A resilient O-ring  72  is supported on a side of the retainer  71  opposite from the ring  16 . A further annular retainer  73  is fixedly mounted in the opening through the housing  12 , a short distance inwardly from the retainer  71  and O-ring  72 . An annular bearing member  76  is supported on the annular retainer  73 , and has an inwardly-facing bearing surface  77  that is a portion of a spherical surface. 
   An approximately cylindrical tube  81  is provided within the housing  12 , and has a central opening  82  extending therethrough. The tube  81  has an annular flange  83  that projects radially outwardly from one end thereof, and an axially facing end surface  84  is provided on the flange  83 . The flange  83  also has an outwardly-facing annular bearing surface  86 , which is a portion of a spherical surface. The annular flange  83  is disposed between the O-ring  72  and the bearing member  76 , with the end surface  84  engaging the O-ring  72 , and the bearing surface  86  slidably engaging the bearing surface  77 . Due to the sliding engagement of the bearing surfaces  77  and  86 , the tube  81  is capable of a very small amount of pivotal movement relative to the housing  12 . In particular, this relative pivotal movement occurs about a pivot point  87  that is located at the centerpoint of the partial spherical surfaces  77  and  86 . As the tube  81  pivots, the flange  83  will slightly compress one side of the resilient O-ring  72 . 
   The tube  81  has an axially-extending slot  88  through the upper wall thereof, and a similar axially-extending slot  89  through the bottom wall thereof. Two vertically extending grooves  91  are provided on opposite sides of the tube  81 , at a location spaced along the tube  81  from the flange  83  by a distance that is approximately three-quarters of the length of the tube. Within the tube  81  are two spaced, vertical, planar surface  92  that are axially aligned with the grooves  91 , and that are parallel and face each other. In each of the vertical grooves  91 , the inner surface has a horizontally-extending slot  93  that opens through the associated planar surface  92 . Axially-facing shoulders  96  and  97  are provided at the opposite axial ends of each of the surfaces  92 . The top surface of the tube  81  has a transverse groove  101  adjacent an end of the slot  88  remote from the flange  83 . A cylindrical pin  102  is disposed partially within the groove  101 . The sight  10  also includes another similar groove and pin that are offset by 90° from the groove  101  and pin  102 , and that are not visible in  FIGS. 4 and 5  because they are on the side of the tube  81  that is not visible in  FIG. 5 . 
   A circular aperture stop  106  is fixedly mounted within the opening  82  that extends through the tube  81 , in engagement with the two axially facing shoulders  96 . The aperture stop  106  has a circular aperture opening  107  provided through the center thereof. In the disclosed embodiment, the aperture stop is held in place by a commercially-available adhesive, such as an epoxy adhesive. 
   An annular lens support  118  is fixedly supported within the ring  16 . Two objective lenses  121  and  122  are supported by the lens support  118 , and together form a lens doublet that collects and focuses incoming radiation from a not-illustrated scene. In the disclosed embodiment, the objective lens  121  is positioned so that a rear nodal point thereof is disposed at the pivot point  87  for the tube  81 . 
   A prism assembly  124  includes a prism arrangement of a known type, including a lower or entrance prism  126 , and an upper or rear prism  127 . The lower prism  126  is a type of prism sometimes referred to as a roof prism. In the disclosed embodiment, the prism assembly  124  is a Pechan prism system of a type known in the art, but it could alternatively be a Porro prism system, an Abbe-Koenig prism system, or a suitable arrangement of lenses. The prism assembly  124  includes rectangular support plates  129  that are provided on opposite sides of the two prisms  126  and  127 . In the disclosed embodiment, the support plates  129  are adhesively secured to the two prisms  126  and  127 , for example with a commercially-available epoxy adhesive. The support plates  129  maintain the prisms  126  and  127  in a fixed positional relationship, with a very small air gap between two adjacent and parallel surfaces on the prisms. 
   The prism assembly  124  inverts and reverts an optical image formed by the objective lenses  121 - 122 , so as to effect image erection. An upper surface  131  of the upper prism  127  serves as an image plane of the sight  10 , and has a reflective coating thereon to reflect light from a scene that is traveling through the sight  10 . The reflective coating on the surface  131  has a not-illustrated reticle pattern etched therethrough. A light emitting diode (LED)  133  is supported by an LED holder  132 , and the LED holder  132  is adhesively secured to the outer side of the reflective coating on surface  131 . The terminals of the LED  133  are electrically coupled by not-illustrated wires to the circuit board  63  ( FIG. 3 ). When the LED  133  is illuminated, light from the LED passes through the reticle pattern etched in the reflective coating, so as to superimpose an image of the reticle onto an image of the scene that is passing through the sight  10 . 
   The outer sides of the support plates  129  each engage a respective one of the vertical planar surfaces  92  that are located within the tube  81 . During factory assembly of the sight  10 , when the prism assembly  124  has been properly positioned within the tube  81 , an adhesive is injected through each of the horizontal slots  93 , so that the adhesive is present between the surfaces  92  and the plates  129 . The adhesive secures the plates  129  to the surfaces  92 , thereby fixedly securing the prism assembly  124  within the tube  81 . 
   Three eyepiece lenses  136 ,  137  and  138  are fixedly supported within the tube  81  at the end thereof remote from the flange  83 . The lenses  136  and  137  form a lens doublet. The eyepiece lenses  136 - 138  optically process radiation from the prism assembly  124 , including color correction, so that the image can be properly viewed by the eye of a person using the sight  10 . 
   An optically-transparent window  141  is fixedly secured within the ring  18  at the eyepiece end of the housing  12 . The window  141  has no optical power, and is provided to physically seal the eyepiece end of the housing  12 . The housing  12  can thus be optionally filled with a gas such as dry nitrogen. As explained earlier, the tube  81  is capable of a very limited amount of pivotal movement about the pivot point  87 . It should be noted that the aperture stop  106 , the prism assembly  124 , and the eyepiece lenses  136 - 138  are all supported on the tube  81 , and thus all pivot with the tube  81 . In contrast, the objective lenses  121 - 122  and the window  141  are fixedly supported on the housing  12 , and do not move with the tube  81 . 
     FIG. 6  is a diagrammatic sectional view taken along the section line  6 - 6  in  FIG. 4 . As discussed earlier, the spring retaining cap  42  threadedly engages an opening in a wall of the housing  12 . A coil spring  146  has one end stationarily supported on the spring retaining cap  42 . The other end of the spring  146  is disposed in a shallow circular recess provided in the exterior surface of the pivotal tube  81 , near an end of the tube  81  remote from the pivot point  87  ( FIG. 4 ). The spring  146  resiliently urges this end of the tube upwardly and rightwardly in  FIG. 6 . 
     FIG. 6  shows the pin  102  that, as discussed above, engages a groove provided in the top surface of tube  81 . As also mentioned earlier, a similar groove is provided in a side surface of the tube  81 , and a further pin  148  engages this further groove. The adjusting knob  38  is part of an adjusting mechanism that is secured in a horizontal opening through the housing  12 , and that operatively engages the pin  148 . When the knob  38  is manually rotated in one direction, its adjusting mechanism moves the pin  148  and the associated end of the tube  81  leftwardly in  FIG. 6 , against the urging of the spring  146 . When the knob  38  is manually rotated in the opposite direction, its adjusting mechanism retracts and allows the spring  146  to move the tube  81  and pin  148  rightwardly in  FIG. 6 . 
   The adjusting knob  39  is part of a similar adjusting mechanism that is mounted in a vertical opening through the housing  12 , and that cooperates with the pin  102 . When the knob  39  is manually rotated in one direction, its adjusting mechanism moves the pin  102  and tube  81  downwardly in  FIG. 6 , against the urging of the spring  146 . When the knob  39  is manually rotated in the opposite direction, its adjusting mechanism retracts and allows the spring  146  to move the tube  81  and pin  102  upwardly in  FIG. 6 . Thus, by manually rotating either or both of the adjusting knobs  38  and  39 , the pivotal position of the tube  81  can be adjusted. 
   The two adjusting mechanisms that respectively include the knobs  38  and  39  are effectively identical. Therefore, only one of these adjusting mechanisms will be illustrated and described in detail. In this regard,  FIGS. 7 and 8  are respective diagrammatic vertical sectional views of the adjusting mechanism that includes the knob  39 .  FIGS. 7 and 8  are taken along respective vertical section planes that are offset by 90° with respect to each other about the vertical axis of rotation of the knob  39 . 
   With reference to  FIGS. 7 and 8 , the adjusting knob  39  is approximately cup-shaped. A vertical and approximately cylindrical sleeve  161  has an upper end that extends into the knob  39 . The sleeve  161  has external threads  160  at its lower end, and the threads  160  engage a threaded vertical opening provided through the housing  12 , in order to fixedly secure the adjusting mechanism to the housing. A retaining ring  162  encircles the sleeve  161 , and engages a downwardly-facing shoulder  163  provided on the sleeve  161 . As indicated at  164 , external threads on the retaining ring  162  engage internal threads provided on the knob  39 . The knob  39  and retaining ring  162  are fixedly secured to each other by an adhesive disposed within the threads  164 . The knob  39  and retaining ring  162  can rotate relative to the sleeve  161 . The upper end of the sleeve  161  has an external surface with an annular groove  166 , and a resilient O-ring  167  is provided in the groove  166 , and slidably engages an inner surface of the knob  39 . 
   A detent ring  168  is fixedly secured within the sleeve  161  by an adhesive. The inner surface of the detent ring  168  has a plurality of vertically extending ribs that are angularly spaced, so as to define a series of detents. A rotor  171  is supported within the sleeve  161  for rotation with respect to the sleeve about a vertical axis. As indicated at  172 , external threads on the rotor  171  engage internal threads provided on the lower end of the sleeve  161 , so that relative rotation of the rotor  171  and sleeve  161  causes vertical axial movement of the rotor  171  within the sleeve  161 . A detent pin  174  is radially movably supported within a horizontal radial bore in the rotor  171 . The detent pin  174  is yieldably urged radially outwardly by a small coil spring disposed within the bore. The detent pin  174  has a point at its outer end, and the point cooperates with the vertical ribs on the detent ring  168 , in order to yieldably resist rotation of the rotor  171  with respect to detent ring  168  and sleeve  161 . 
   Two cylindrical rods  176  and  177  each have an upper end fixedly secured to the knob  39 , and have lower ends that are slidably received within respective vertical openings provided in the rotor  171 . A plunger  181  is supported at the lower end of the rotor  171 , and can rotate with respect to the rotor. There is no direct attachment between the plunger  181  and the rotor  171 . Instead, when the adjusting mechanism of  FIGS. 7-8  has been installed in the sight  10 , the plunger  181  is operationally held between the rotor  171  and the pin  102  ( FIGS. 4 and 6 ), and is not able to escape. 
   When the knob  39  is manually rotated with respect to the sleeve  161  and the detent ring  168 , the rods  176  and  177  cause the rotor  171  to also rotate with the knob. As the rotor  171  rotates relative to the detent ring  168 , the detent pin  174  is pushed inwardly against the force of its coil spring as the tip of the pin  174  slides over each vertical rib on the detent ring  168 . When rotation of the knob  39  ceases, the tip of the pin  174  engages the vertical groove between two adjacent vertical ribs on the detent ring  168 , thereby yieldably resisting inadvertent rotation of the knob  39  and rotor  171  with respect to the sleeve  161  and the detent ring  168 . 
   Due to the threaded engagement between the rotor  171  and the sleeve  161  at  172 , relative rotation of the rotor  171  and sleeve  161  causes the rotor  171  and the plunger  181  to move axially with respect to the sleeve  161 , or in other words vertically in  FIGS. 7 and 8 . As discussed above in association with  FIG. 6 , downward movement of the plunger  181  forces one end of the tube  81  downwardly against the force of the coil spring  146 , and upward movement of the plunger  181  permits the coil spring  146  to move that end of the tube  81  upwardly. 
     FIG. 9  is a diagrammatic view showing the optical elements of the sight  10 , and also showing the chief and marginal rays of radiation from a not-illustrated scene that is traveling through the sight  10 . In use, the eye  191  of a user is positioned at the exit pupil of the sight  10 , where the exit pupil is the image of the aperture stop  106  as produced by the eyepiece lenses  136 - 138 . The incoming radiation from the scene enters through the objective lenses  121 - 122 , which form an image. This image is inverted and reverted by the prisms  126 - 127  so that, when viewed through the eyepiece lenses  136 - 138 , the image of the scene and reticle at the image plane defined by surface  131  has the correct orientation. When the sight  10  is properly aligned to a rifle or other weapon on which it is mounted, the center of the reticle cross-hair pattern (which defines the line of sight) will be aligned with the striking point of a bullet fired to a particular object distance that is commonly known as the “sighting in” distance. 
   As discussed earlier, the aperture stop  106 , the prisms  126 - 127  and the eyepiece lenses  136 - 138  all pivot with the tube  81  about a pivot point  87 . As also discussed earlier, this pivot point  87  is disposed at the rear nodal point of the objective lens  121 . It would alternatively be possible for the pivot point  87  to be at a different location. For example, the pivot point  87  could be at a location shifted rightwardly in  FIG. 9  from the rear nodal point of the lens  121 . However, as the pivot point  87  is moved rightwardly in  FIG. 9  toward the eyepiece, any pivotal movement of the tube  81  away from a centered position causes the chief ray of the axial field (the true zero field) to become decentered on the objective lenses  121 - 122 , so that the objective lenses  121 - 122  work as a decentered optic. This decentration introduces aberrations that reduce the resolution of the sight  10 . 
   In  FIG. 9 , the sight is shown in its nominal zero pointing error position. The eyepiece lenses  136 - 138  and the prisms  126 - 127  are centered on the axial chief ray, which passes through the front and rear nodal points of the objective lens  121 , and through the center of the aperture stop  106 . Assuming for the sake of discussion that the target is a point object located at the sighting distance, then at the zero degree field (the shot-targeted field), the point object will be imaged to the center  193  of the reticle cross-hair. 
     FIG. 10  is a diagrammatic view similar to  FIG. 9 , but showing a pointing error of θ° degrees. In particular, when a pointing error of θ° degrees is present, the zero degree field is imaged to a image height h error    196  at the image plane and reticle of: 
             h   error     =       f   ob     ⁢     θ   [     3.14   180     ]             
where f ob  is the focal length of the objective lens arrangement  121 - 122 . Typically, θ would be less then 2°. This would represent a mechanical misalignment of 3.5 mm over a length of 100 mm, or in other words a considerable misalignment.
 
   In order to adjust this image height error h error    196  to zero, one or both of the adjusting knobs  38  and  39  are manually rotated in order to pivot the tube  81  that supports the aperture stop  106 , prisms  126 - 127 , and eyepiece lenses  136 - 138 . In particular, the tube  81  is pivoted about the pivot point  87  by an angle of −θ°.  FIG. 11  is a diagrammatic view similar to  FIG. 10 , but showing the relative position of the optical components after the correcting pivotal movement of the tube  81  and the optical components thereon. Following this pivotal corrective movement of −θ°, the chief ray for the true 0° field will pass through the rear nodal point of the lens  121 , and through the center of the aperture stop  106 , so that the prisms  126 - 127  and the eyepiece lenses  136 - 138  again work as a centered optical system. Since the chief ray passes through the rear nodal point at  87 , the objective lenses  121 - 122  are also centered, but are effectively tilted at an angle of θ°. Consequently, since the chief ray of the axial field is centered at the objective lenses  121 - 122  and also at the eyepiece cluster  136 - 138 , good imagery is obtained. 
   To evaluate the performance of the disclosed zeroing technique, it is useful to look at the performance fall-off in terms of resolution, and also astigmatism as a function of zero setting for the axial field. Resolution is typically characterized in terms of the value of the Modulation Transfer Function (MTF), evaluated for a particular image spatial frequency. This can be calculated for a sinusoidal object, which in an a focal system is an object with an ideal image intensity variation with angle θ at (angular) frequency f of A(1+cos(2πfθ)). The calculation includes measuring the intensity variation (Imax, Imin) at the image plane, and calculating MTF(f)=((Imax−Imin)/(Imax+Imin)). The limit of resolution of the human eye is generally accepted to be 1 arc minute at the retina for a 4 mm eye pupil, and this corresponds to a spatial frequency of 0.5 cycles/arc minute. The modulation or MTF delivered at this frequency by an optical system to the eye should be at least 0.25 in order to be detectable by the eye. 
   Depending on the zeroing mechanism, when a sight is zeroed, various effects may be seen on the resolution for the axial field. The most significant contributors to resolution loss are axial coma and astigmatism. The effect of coma on a point object is to produce a comet-like flare in the image, thereby reducing the resolution. Astigmatism occurs when horizontal lines (tangential for y fields) and vertical lines (sagittal for y fields) are not seen at the same image distance. Commonly, a visual instrument will be set up during assembly so that the eye must accommodate to see the image approximately 1.3 meters away. For the axial field, this is a −0.75 Diopter setting and represents a preferred focus position for the “relaxed eye”. Astigmatism may be present in the visual instrument (for example the vertical line might effectively be focused at 1.5 meters or 0.67 Diapers, while the horizontal line might effectively be focused at 1.1 meters or 0.91 Diopters, so that the astigmatism is 0.24 Diopters). In the case of astigmatism, the eye can refocus to image one but not both of the two line orientations sharply, and will tend to adjust to some intermediate image position with loss of resolution for both images. In general, astigmatism below about 0.25 Diopters is considered to be acceptable for visual instruments. 
     FIGS. 12 and 13  are graphs showing the impact of zeroing on the astigmatic performance of the sight  10  using the disclosed zeroing technique, and considering only the axial field. This data was obtained as follows:
         1. The sight eyepiece was initially set to a diopter setting of −0.75 for an axial field with no zero error, prior to adjustment for the specified zero errors. The objective distance was set so that an object at 100 meters would be exactly focused to the reticle plane.   2. A zero error angle was imposed on the sight, and then the prism-eyepiece cluster was pivoted by the negative of this angle to correct the zero.   3. The eye was then allowed to refocus as required for the new zero setting. This means that the object at 100 meters is no longer focused exactly to the reticle plane.       
     FIG. 12  shows the extent of change in accommodation of the eye that is required (for axial tangential and sagittal lines) as the zero setting is changed.  FIG. 13  shows the effect on MTF at 0.5 c/arc minute as the zero error correction is increased. It can be seen that, out to a zero correction of 1.5°:
         1. There are approximately 0.04 Diopters of shift in location of the “best” image plane with the change in zeroing. This is quite a small change in eye accommodation. This represents a change in apparent object distance of about 5 meters for an object at 100 meters, and would contribute virtually no parallax, so that the effective “sighting in” distance is unchanged with zero setting.   2. The tangential and sagittal image planes are within 0.04 Diopters of each other. This is an insignificant amount of axial astigmatism.       
   3. There is a reduction in mean axial MTF of about 0.1. Since the axial performance of the sight is good, the loss of 0.1 in MTF leaves the performance well above the minimum MTF target of 0.25. 
   The results discussed above are specific to the disclosed sight  10 , but they illustrate the general result that, for this zeroing method, the axial field performance is relatively unaffected, even for fairly large zero adjustments. The effects of zeroing on performance across the field are also not severe. 
   In the disclosed embodiment, the prism assembly and the eyepiece cluster are all pivoted about a common pivot point. The aperture stop is also pivoted about this pivot point, because it is spaced from the front objective. Alternatively, however, the aperture stop could be stationary with respect to the housing if it was located adjacent the front objective. In the disclosed embodiment, the pivot point is advantageously located at the rear nodal point of an objective lens of the sight. The position of this pivot point is mechanically controlled very accurately, due to the use of cooperating machined spherical surfaces. 
   By moving both the prism assembly and the eyepiece cluster, as well as the aperture stop where appropriate, these optical components are always working as a centered optical system. This avoids the aberrations associated with decentration, and allows a large range of zeroing before aberrations cause any significant reduction in image quality. Because aberrations are small, the sight resolution for the axial and near to axis fields is only minimally affected by zeroing adjustments, giving good image quality throughout the zeroing range. Since the prism assembly and eyepiece cluster are adjusted together, the reticle cross-hair is always centered in the field of view, and the eye always has the same focus at the reticle plane. This approach works very well for prism-based sights that are relatively short, because the movement needed for the eyepiece optics during pivoting is at most a few millimeters. 
   Although one selected embodiment has been illustrated and described in detail, it will be understood that various substitutions and alterations are possible without departing from the spirit and scope of the present invention, as defined by the following claims.