Abstract:
A system for stabilizing an optical line of sight. An optical system including primary optics and relay optics includes a jitter rejection mirror located within the path of the relay optics. An auto alignment system is provided for maintaining alignment of the jitter rejection mirror in response to a control signal. An auto alignment sensor detects jitter in a reference beam passing through the jitter rejection mirror, and the generated control signal is used to reduce the jitter. The reference beam is supplied by a stabilized source of laser signals which are received by the primary optics, and relayed to the jitter rejection mirror.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to a system for pointing and stabilizing an optical axis of an optical system. Specifically, a system is provided which permits correction of jitter in an optical system using a separate reference laser beam.  
           [0002]    Optical imaging systems and lasing systems are available to provide a magnified image for viewing, and/or for projecting a precision laser for illuminating a distant target. These systems have an optical axis (bore sight) which is positionable in elevation and azimuth. The optical system is susceptible to vibrational forces which tend to impose a jitter on any optical signal being processed by the system. This substantially random motion of the optical system axis produces blurring of an image being magnified by the system. In the case of a lasing system, the motion imparted to the laser disturbs its pointing direction.  
           [0003]    These vibrational influences can be minimized by stabilizing the platform supporting the optical system. The platform supporting the positionable optical system, such as the tripod of a camera or the robotic arm of a surgical laser, have stabilization systems which detect vibrational displacements of the platform, and attempt to apply a counterforce to the platform to oppose the vibrational displacements. However, there are certain performance limitations in this approach. For instance, high performance optical systems, having many optical elements, may have individual elements being disposed with different levels with respect to each other due to the vibrational forces resulting in vibration of the optical axis of the system.  
           [0004]    Optical systems, therefore, have an additional requirement that not only is the platform supporting the system stabilized, but that the optical axis lying along the optical axis or optical line of sight be stable, so that images viewed from the system are stable, as well as maintaining the pointing position of any laser transmission system stable vis-à-vis the optical line of sight. The present invention is directed to maintaining the stability of the optical line of sight in a multi-element optical system.  
           [0005]    In order to keep the optical line of sight stable, an inertial reference must be provided which identifies any apparent motion in the line of sight and which is corrected. In space applications, a star is commonly used as an optical inertial reference because there is no apparent motion relative to the Earth. A star, however, does not work as an inertial reference inside the Earth&#39;s atmosphere because the atmosphere moves, and, therefore, anything viewed through the atmosphere tends to move.  
           [0006]    Accordingly, in order to create the inertial reference and use it to maintain the pointing attitude stability of the optical line of sight, the present invention has been provided.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention provides a system for pointing and stabilizing an optical line of sight in an optical system. The optical system includes a set of primary optics and relay optics which can be used to magnify an image received on one end thereof, or to transmit a laser to a precise location. A jitter rejection mirror is located in the path of the optical system, preferably near the point at which an image is viewed, or in which a laser originates in a laser pointing system. The jitter rejection mirror is positionable in response to an error signal generated by detecting a misalignment due to jitter between the optical system bore axis and a reference axis. The mirror is displaced in a direction to oppose any apparent change in the optical bore sight due to jitter.  
           [0008]    In carrying out the invention in accordance with a preferred embodiment, changes in an inertially stable reference laser beam originating at the object side of the optical system is detected by an auto alignment sensor at the opposite end of the optical system. Jitter imposed on the optical system displaces the reference laser signal which is detected by the auto alignment sensor to generate a correction signal to position the jitter rejection mirror in a direction to cancel the jitter. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    The above-mentioned objects and advantages of the present invention will be more clearly understood when considered in conjunction with the accompanying drawings, in which:  
         [0010]    [0010]FIG. 1 illustrates a stabilized optical system in accordance with a preferred embodiment of the invention;  
         [0011]    [0011]FIG. 2 illustrates the control system used in the stabilized system of FIG. 1;  
         [0012]    [0012]FIG. 3 illustrates the optical jitter correction obtainable using the system of FIG. 1;  
         [0013]    [0013]FIG. 4A illustrates the top view of the stabilized inertial reference unit for generating the line of sight reference laser beam;  
         [0014]    [0014]FIG. 4B illustrates the side view of the inertial reference unit;  
         [0015]    [0015]FIG. 4C illustrates the plan view of the inertial reference unit;  
         [0016]    [0016]FIG. 5A illustrates the top view of the inertial reference unit having a mirror instead of a stabilized reference laser;  
         [0017]    [0017]FIG. 5B illustrates the side view of the inertial reference unit;  
         [0018]    [0018]FIG. 5C illustrates the plan view of the inertial reference unit;  
         [0019]    [0019]FIG. 6 illustrates a separate control system utilized to stabilize the inertial reference unit; and  
         [0020]    [0020]FIG. 7 illustrates the control system for correcting the position of the optical system. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0021]    Referring now to FIG. 1, an optical system having a stabilized line of sight is disclosed in accordance with a preferred embodiment of the invention. The system includes primary optics comprising a reflector  11  and sub-reflector  12 . The primary optics may be used in an imaging system, wherein incoming imaging radiation is received on the reflector  11  and sub-reflector  12 , and forwarded via relay optics  13 ,  17 ,  18 ,  19 , and  20  to an imaging sensor  24 . The system shown can also be implemented as a lasing system, for accurately pointing a laser beam  22  originating from laser source  25 . The laser beam  22  is relayed via the relay optics to the primary optics, and precisely pointed in accordance with the orientation of the primary optics. The primary optics are supported on a gimballed system  16  so that they can be pointed within an arbitrary field of regard.  
         [0022]    In either application, a source of image distortion, as well as a pointing error in a lasing system, results from vibrational disturbances incident to the optical system. These disturbances may operate on different parts of the optical system producing different relative displacements with respect to other components of the optical system, disturbing the optical line of sight (LOS) of the system. In these systems, it is important that the optical LOS remain stable, or the quality of the images received, or precision of pointing of the laser beam will be compromised.  
         [0023]    In accordance with the present invention, the system is stabilized using an inertial reference unit (IRU)  15  which generates a reference laser beam  23  which is fed into the objective side of the primary optics by extended corner cube  14 . The reference laser beam  23  traverses the optical system and is also subject to the same vibrational forces as imaging radiation  10  or pointing beam  22  and experiences jitter with respect to the optical axis of the system. The relative displacement of the reference beam is measured by a two-dimensional optical position detector  27 , which is at the end of the optical path for the system. Displacements detected by sensor  27  are used by auto alignment controller  28  to control the position of a jitter rejection mirror  19 . Auto alignment controller  28  is a servo mechanism controller which operates to control the servo controlled jitter rejection mirror  19  in a direction to effectively cancel changes in the optical LOS of the system resulting from jitter. The sensor  27  may also be stabilized with its own position stabilizing system. Thus, the reference beam  23 , which originates from a positionally stable source  15 , produces an accurate measurement on sensor  27  of displacement of the optical LOS of the system, which is used to stabilize the LOS.  
         [0024]    The inertial reference unit  15  operates as a reference similar to the way a star may be used in space applications because of its apparent positional stability with respect to Earth. The inertial reference unit  15  is stabilized, with its own auto controlled platform, so that any changes in the beam position with respect to the LOS are due entirely to the jitter induced by external forces operating on the optical system platform.  
         [0025]    The attitude of the inertial pointing reference beam  23  is calibrated and initialized in inertial space. Initial reference unit  15  has its own steerable platform and can be commanded to point in any direction in space. The inertial reference unit base is mounted on the primary mirror  11 , and the reference beam  23  is aligned to coincide with the mirror&#39;s  11  axis. The angle between the platform of the inertial reference unit  15  and its base represents the difference between current and desired optical LOS. The difference can be used as an error signal to control a gimbal pointing controller  16  to drive the optical system supporting the primary optics  11  and  12  to obtain the correct LOS.  
         [0026]    The errors in the optical LOS system can be characterized according to their temporal frequency content. The resulting error consists of jitter, bias, and drift. Bias and drift are characteristics representing the best straight line fit to the total position error, and are an indication of the pointing accuracy. The higher frequency error components are jitter, imposed by vibrational forces on the optical system. Jitter can be defined as the standard deviation of the remaining error once bias and drift are subtracted.  
         [0027]    Referring now to FIG. 2, the control system for stabilizing the optical system of FIG. 1 is shown. The optical input scene received by the camera includes jitter from the relay optics along the optical axis of the system. The jitter rejection system  30  reduces the line of sight jitter to produce a stabilized image scene at the imaging sensor  32 . The jitter rejection system  30  includes the jitter rejection mirror and the associated positional components  28 , and sensor  27 .  
         [0028]    The inertial reference unit  15  is stabilized by inertial angular rate detectors, as will be described more particularly with respect to FIGS. 4 and 5. These rate signals are combined with attitude commands in an attitude correction network  38 . The corrected attitude for controlling the inertial reference unit  15  position is filtered in Kalman filter  37 , and time optimal servo (TOS)  36  provides an inner rate loop  34  within the attitude/line of sight control loop. This results in a correctly pointed and stable inertial reference unit  15 . Additionally, the position of the inertial reference unit  15  is used to generate gimbal offload signals, constituting relative angular positions of the inertial reference unit to the gimbal pointing controller  16  of the optical system of FIG. 1, to position the optical system.  
         [0029]    By stabilizing the LOS, an improvement in the pointing of the optical instrument is obtained, as is shown more particularly in FIG. 3. FIG. 3 illustrates the optical jitter, arc sec..2/Hz over the angular frequency bandwidth, with both stabilization, and an unstabilized optical system. The lower curve represents the results of stabilizing the optical system by jitter rejection.  
         [0030]    [0030]FIGS. 4A, 4B, and  4 C illustrate the structure used to stabilize the inertial reference unit. The inertial reference unit includes a plurality of sensors  41 ,  42 ,  43 , and  44  which measure the inertial angular rate of the platform  40 . Platform  40 , in turn, supports the optical collimator of a laser system having a reference laser source  50 . The optical collimator  45  directs the precision beam to the input of the main optical system, where it is used as a reference beam.  
         [0031]    The platform is stabilized by a plurality of actuators, two of which,  47  and  48 , are shown. As the motion of the stabilized platform are measured via the sensors  41 ,  42 ,  43 , and  44 , the control electronics  51  generate control signals for actuators  47 ,  48 . The actuators  47 ,  48  include linear displacement sensors which remeasure the relative displacement between platform  40  and the base of inertial reference unit  15 . The actuators, in turn, apply forces counter to detected vibrational forces to the platform  40  thereby stabilizing the platform. The platform control system thus formed operates in accordance with the configuration of FIG. 2. The entire structure is supported on the primary optical reflector  11  and is steerable by attitude commands.  
         [0032]    The sensors  41 - 44  for detecting the inertial angular rates of the platform  40  require both a low frequency response as well as a high frequency response. It is contemplated that each sensor  41 - 44  may comprise two sensors, one of which is designed for measurement of low displacements rates, and the other of which is designed to measure high displacement rates. The output of each pair of sensors can be blended to provide a broad bandwidth detection of the inertial displacements of the platform  40 .  
         [0033]    The foregoing design provides a two degree of freedom flexure, connecting the stabilized platform to the base of the inertial reference unit  15 . This flexure allows for rotation about the tip and tilt axes of the stable platform. It is rigid to all three directions of translations, and to rotation about the optical axis.  
         [0034]    [0034]FIG. 5 illustrates a second embodiment of generating a reference laser beam using a stabilized mirror. In the embodiment shown, a mirror  46  replaces the collimation optics  45  of FIG. 4. The stabilized platform now constitutes a mirror on which the reference laser can be reflected back to the auto alignment sensor  27 .  
         [0035]    [0035]FIG. 6 is a more detailed illustration of the control system architecture for the inertial reference unit  15 . The system includes two control loops so that the stable platform  40  remains motionless in inertial space when attitude comans are zero. It is contemplated that two sensors will be employed, DC sensor  61  and ARS-12 sensor  62  (available from A-Tech Corporation). The two sensors  61 ,  62  measure the high and low frequency content, respectively, of displacement disturbances incident to the platform  40 . Blending filter  64  is shown which blends the output to provide a single error signal representing the disturbance displacements sensed on platform  40 . The blending filter  64  output is combined with pointing or attitude commands in a summing junction  54 . The inertial reference unit controller  55  then positions the stable platform  60  to assume a position set by the attitude commands, and be stable against the forces  56  as transmitted through the flexure to the platform  40 .  
         [0036]    An E/U core linear voltage differential transformer sensor (LVDT)  63  is used to measure the position of the stabilized platform relative to the inertial reference unit base. This position error is used to drive the gimbal pointing controller  16  for positioning the base of the optical system to eliminate the error. Thus, the primary optics, as well as the inertial reference unit, assumes the same LOS.  
         [0037]    It is also considered possible to implement the inertial reference unit  15  using inertial sensors not mounted on the bottom side of the platform. In this arrangement, the angular rate sensors are mounted to the base. The foregoing strap-down approach generates inertial motion signals which are used to provide the disturbance cancellation on the platform surface.  
         [0038]    An additional control system is used to control the jitter mirror and its respective control system, as was described with respect to FIG. 1, to compensate for LOS errors. The control system is shown more particularly in FIG. 7. Referring now to FIG. 7, the auto alignment sensors  27  generate a two-dimensional displacement image, representing the position of the reference laser beam. The position is used by controller  28  to generate signals for mirror controller  72 . These signals are, in turn, stabilized with feedback signals from the mirror position sensing sensors  74 ,  75 , and  76 . As was in the case of the inertial reference unit stabilization system, two sensors  74  and  75  are used to obtain a wide bandwidth detection of mirror displacements. A blending filter  77  combines DC sensor and high frequency sensors  74 ,  75  outputs to produce the error correction signal for summing junction  71 . The system also includes position feedback, from the E/U core LVDT  76 . Using both rate feedback, and position feed back, combined in summing junction  70 , it is possible to obtain stable control over the mirror position  73 . Mirror  19 , as was disclosed with respect to FIG. 1, cancels the disturbances induced on the reference beam  23 , in accordance with the signal sensed by the auto alignment sensor  27 .  
         [0039]    Thus, there has been disclosed with respect to the one embodiment its illustration and description. Additionally, the disclosure shows and describes only the preferred embodiments of the invention, but is to be understood that the invention of capable of use in various other combinations, modifications, and environments and is capable of changes or modifications within the scope of the inventive concept as expressed herein, commensurate with the above teachings and/or the skill or knowledge of the relevant art. The embodiments described hereinabove are further intended to explain best modes known of practicing the invention and to enable others skilled in the art to utilize the invention in such, or other, embodiments and with the various modifications required by the particular applications or uses of the invention. Accordingly, the description is not intended to limit the invention to the form disclosed herein. Also, it is intended that the appended claims be construed to include alternative embodiments.