Abstract:
A method and apparatus measure the accuracy of and optically calibrate a scan mirror. Both the method and apparatus may operate over a wide range of environmental conditions. The environmental conditions may include variations in pressure from a vacuum to several atmospheres. Similarly, large variations in temperature may be accommodated. The apparatus includes a laser source, a plurality of facet mirrors and a detector. The laser source projects a beam onto a reflective surface of a rotatable scan mirror, which directs the beam to each of the plurality of facet mirrors. Each facet mirror is positioned at a known angle. Each facet mirror in turn reflects the beam from the reflective surface of the scan mirror substantially back onto itself (autocollimation). The angle detector then receives the reflected beam and measures a value related to a return angle of the beam.

Description:
FIELD OF THE INVENTION 
     The present invention relates to calibration of optical instruments. In particular, the present invention relates to calibration of a reflecting optical device, such as a pointing mirror, using an optical calibration apparatus. The calibration apparatus includes a light source for projecting a beam of light at the reflecting optical device, a plurality of facet mirrors pointed at predetermined angles, and a detector for indicating a properly registered beam. The reflecting optical device steers the beam at the facet mirrors during calibration. 
     BACKGROUND OF THE INVENTION 
     Many monitoring, measuring and input devices utilize scan mirrors to optically scan a field of view. For example, weather satellites, such as the GOES satellite, incorporate scan mirrors for scanning weather patterns over earth. In the GOES satellite, the scan mirror reflects to a detector light received from a portion of the atmosphere at which the mirror is directed. The precise pointing direction of the scan mirror is important, as the detected radiation represents atmospheric data, such as cloud and precipitation data, collected from a precise portion of the earth&#39;s atmosphere. This pointing direction is then used to correlate the atmospheric data collected with the underlying geography of the earth for depiction of weather conditions on weather maps. Other instruments which may have scan mirrors include semiconductor wafer scanners and photocopying machines. In each of these applications, it is necessary to accurately track and control the position of the scan mirror. 
     In the case of satellite and semiconductor wafer scanning mirrors, the scan mirror may operate in a vacuum and under extremes of temperature. In the case of a scan mirror located in a photocopying machine, the scan mirror may be subject to large temperature fluctuations when the photocopier goes from a resting state to a state of continuous copying. 
     The characteristics of the scan mirror may change over temperature and pressure extremes. Therefore, it is desirable to have a device to measure the accuracy of and calibrate a scan mirror over a wide range of temperatures and pressures. It would further be desirable to have a device for measuring scan mirror angles with high accuracy, which itself requires setup and manual manipulation only one time. 
     Prior art devices, such as theodolites, exist for measuring angles. However, theodolites are limited in their angle measuring accuracy. 
     SUMMARY OF THE INVENTION 
     According to the present invention, a method and apparatus measure the accuracy of and optionally calibrate a scan mirror. Both the method and apparatus may operate over a wide range of environmental conditions. The environmental conditions may include variations in pressure from a vacuum to several atmospheres. Similarly, large variations in temperature may be accommodated. 
     According to the method, in a projecting step, a laser beam is projected at a scan mirror. Then, in a commanding step, the scan mirror is commanded to reflect the laser beam successively at a first and a second facet mirror, where the first and second facet mirrors have known angles of reflection. In a reflecting step, the laser beam is reflected substantially back onto itself by each facet mirror. Then, in a determining step, an angle between the first and the second facet mirrors is determined at the scan mirror. Then an error is calculated in a calculating step, based on the determined angle and the known angles. 
     Prior to the projecting step, an offset of the laser beam may be measured and used in the error calculation. Also, the commanding step may include commanding the scan mirror to move until the reflected laser beam is centered. This occurs when a null or near null value is detected by a detector receiving the reflected laser beam. 
     The apparatus includes a laser source, a plurality of facet mirrors and a detector. The laser source projects a beam onto a reflective surface of a rotatable scan mirror, which directs the beam to each of the plurality of facet mirrors. Each facet mirror is positioned at a known angle. Each facet mirror in turn reflects the beam from the reflective surface of the scan mirror substantially back onto itself (autocollimation). The angle detector then receives the reflected beam and measures a value related to a return angle of the beam. 
     The apparatus may further include a processor, coupled to the angle detector and the scan mirror. The processor may command the scan mirror to point the beam at a predetermined facet and may command the scan mirror to move until the angle detector reads a null or near null value. The optical instrument under test typically includes an encoder coupled to the scan mirror for measuring position values corresponding to positions of the scan mirror. The position values may be used to determine angles for comparison with the known angles of the facet mirrors. 
     To achieve environmental stability, the laser source, facet mirrors and the detector are typically mounted to a housing made of a material with a low coefficient of thermal expansion, such as Invar. Moreover, the facet mirrors in a preferred embodiment are mounted to the housing using a three ball kinematic mount. The three ball mounting technique permits precise angular positioning of each facet mirror, even under relatively lax manufacturing tolerances of each facet mirror and the housing itself. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 depicts a metrology device and a scan mirror mounted to a common optical table, according to the present invention. 
     FIG. 2 depicts the metrology device and the scan mirror of FIG. 1 from a vantage point which illustrates a position of the scan mirror relative to the metrology device, according to the present invention. 
     FIG. 3 depicts an internal block diagram of the metrology device, according to the present invention. 
     FIG. 4 depicts a view of a facet mirror which illustrates three ball kinematic mounting of the facet mirror. 
     FIG. 5 depicts an internal block diagram of a device incorporating a scan mirror, according to the present invention. 
     FIG. 6 depicts a method of calibrating a metrology device, according to the present invention. 
     FIG. 7A and 7B depict a method of calibrating a scan mirror according to the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 depicts a metrology device  10  which is used to calibrate and test and an optical instrument  12  according to the present invention. The metrology device  10  includes a housing  22  which is mounted to an optical table  14 . The housing  22  may be mounted to the optical table  14  using bolts, screws, fasteners, adhesive or any other fastening technique. The housing  22  is preferably made of a single or a composite material with a low coefficient of thermal expansion. In a preferred embodiment of the invention, the material is Invar. However, the material chosen for the housing  22  may be any suitable material based on the conditions to be applied during testing of the optical instrument  12 . For example, if the optical instrument  12  is not to be tested over a wide temperature range, then the housing  22  need not be made of a material with a low coefficient of thermal expansion. 
     Referring again to FIG. 1, the metrology device  10  includes a plurality of facet mirrors  16 . The facet mirrors are arranged so that each facet mirror  16  presents a reflective surface to the optical instrument  12  at a known angle. The mounting of the facet mirrors  16 , and angles chosen for the facet mirrors  16 , will be explained in more detail below. 
     In the embodiment depicted in FIGS. 1 and 2, the optical instrument  12  includes a housing  20  which is mounted to the optical table  14 . The housing  20  may be mounted in any convenient fashion, including bolts, screws, fasteners, adhesives or any other convenient technique. A pointing or scanning mirror  18  is rotatably mounted to the housing  20 , typically permitting the mirror  18  to pivot about one or two axes. The mirror may be mounted, for example, using a gimbal, thus permitting two axis rotation of the mirror relative to the housing. The pivot point may be a point on the mirror surface, or may be a point off of the mirror surface. 
     FIG. 2 depicts the metrology device  10  and the optical instrument  12  from a vantage point which illustrates the housing  20  and, a reflecting surface  19  of the pointing mirror  18 . As shown, the housing  12  includes a substantially open face  21 , which may either be open or light transmissive, and a substantially hollow interior between the reflecting surface  19  of the mirror  18  and the open face  21 . The open face  21  and the hollow interior permit light to enter through the open face  21 , reflect off of the reflecting surface  19  of the mirror  18 , and exit again the open face  21 . The hollow interior also permits rotation of the pointing mirror  18 . It will be understood, however, that the configuration of the optical instrument  12  is not important as long as a laser beam can be projected from the metrology device  10  at a reflecting surface  19  of the pointing mirror  18 . For example, the optical instrument  12  may have the pointing mirror  18  mounted on the exterior of the housing  20 . Moreover, the optical instrument  12  may include additional optics, such as lenses and mirrors, between the reflecting surface  19  and the metrology device  10 . 
     FIG. 3 depicts a block diagram of optical and electronic instruments within the metrology device  10 . The metrology device  10  includes a laser source  50 , an optical switch  52 , projection optics  54 , facet mirrors  16 , a beam splitter  55 , a course angle tracker  56  and a fine angle tracker  58 . The metrology device  10  further includes a processor  60  which is coupled over a bus to a clock  62 , a database  64  and a memory  66 . 
     The laser source  50  projects a beam toward the optical switch  52 . The optical switch  52 , under control of the processor  60 , either transmits the beam to the projection optics  54  or toward the beam splitter  55 . in the latter scenario, the beam from the laser source  50  travels directly through the optical switch  52  to the coarse angular tracker  56  and the fine angular tracker  58 . 
     The coarse and fine angular trackers  56  and  58  are typically charge coupled devices (CCD) which include an array of optical detectors. When the beam hits the coarse angular tracker  56 , the course angular tracker  56  produces a value indicative of a location of the beam within the array. This value is provided to the processor  60 . The fine angle tracker  58  is also typically a CCD, and sends a value to the processor  60  indicative of a location of an incident beam. Other detectors such as quadrant photo diodes or lateral diffuse detectors may be used in place of CCDS. 
     Laser beams projected from a laser source have a tendency to drift over time, therefore, diverting the beam from the optical switch  52  directly to the coarse and fine angle trackers  56  and  58 , respectively, affords the opportunity to determine an offset of the beam prior to making any measurements of the optical instrument  12 . The offset value may be used to correct measurements taken during calibration. 
     During calibration, the processor  60  signals the optical switch  52  to transmit the beam. The optical switch  52  thus allows the beam to pass from the laser source  50  to the projection optics  54 . The projection optics  54  may be used to steer or inactively focus the beam on the reflecting surface  19  of the pointing mirror  18 . The beam projects from the projection optics  54  onto the reflective surface  19  of the pointing mirror  18 . The beam then reflects off of the reflective surface  19  in the direction of the array of facet mirrors  16 . The scan mirror  18  may be manually positioned to reflect at one of the facet mirrors  16 . 
     Alternatively, the scan mirror  18  may be commanded, via a calibration profile, to point at one of the facet mirrors  16 , either by a program running on a processor within the optical device  12  or by the processor  60  of the metrology device. When the pointing mirror  18  directs the beam onto a reflective surface of one of the facet mirrors, the beam returns on itself (autocollimated) and is reflected back through the projection optics  54  to the optical switch  52 . The optical switch  52  diverts the beam to the beam splitter  55 . The beam splitter  55  in turn directs portions of the beam to both the coarse angle tracker  56  and the fine angular tracker  58 . 
     In order to properly configure the metrology device  10  for calibration of an optical instrument  12 , it is important to properly configure and know the angle of a reflecting surface of each facet mirror  16 . The angle remains substantially constant over a wide range of temperature when materials having a low coefficient of thermal expansion are used. For example, in a preferred embodiment of the invention, the beam is projected onto the reflecting surface of the mirror  19  which reflects the beam to a facet mirror  16 . The facet mirror  16  in turn reflects the beam substantially back onto itself and the reflecting surface  19 . Subsequently, the reflected beam re-enters the metrology device  10  through the projection optics and is substantially co-located with the incident beam. 
     In order for the incident and reflected beams to be substantially co-located, each facet mirror should have its reflecting surface angled substantially perpendicular to the incident beam after its reflection from the reflecting surface  19  of the scan mirror  18 . Therefore, in a preferred embodiment of the invention, the facet mirrors  16  are mounted to achieve perpendicularity given knowledge of an angle defined by the beam, a point of intersection between the beam and the reflecting surface  19 , and a point, typically the center, on each facet mirror  16 . In a preferred embodiment of the invention, the facet mirrors  16  are mounted to the housing  22  using a three ball mount as shown in FIG.  4 . 
     FIG. 4 depicts an illustration of a three ball mount of a facet mirror  16 . Referring to FIG. 4, a retainer  200  is mounted to a back plate  204 . The back plate  204  may be a face of the housing  22  of the metrology device  10  or it may be a separate plate mounted to the housing  22 . The retainer  200  is mounted to the back plate  204  using the mount screws  206 . The retainer  200  itself includes a clear aperture  202  through which incident light may pass en-route to a reflecting surface of the mirror  16  as well as receptacles for three spring plungers  212 . 
     Mounted to the back plate  204  are three ball posts  208 . Each ball post  208  may be mounted at a proximal end into a receiving counterbored hole of the backplate  204 . Alternatively, the ball posts  208  may be mounted using screws, bolts, rivets, adhesive or any other conventional mounting technique. Each ball post  208  may include a central bore at a distal end of which rests a ball  210 . To achieve a desired angle of the facet mirror, the ball posts  208  may have varying lengths. During mounting, the non-reflecting face of the mirror  16  is positioned against the three balls  210  on the ball posts  208 . The retainer  200  is then placed over the mirror  16  and mounted to the back plate  204  using the mount screws  206 . The plungers  212  are then turned until their balls come into contact with mirror  16 . An additional rotation of the plunger  15  is performed to achieve pre-loading sufficient to disallow motion of mirror  16 . The mirror  16  is thus held into place at three pressure points between the three balls  210  and the three plungers  212 . 
     The three ball mount is advantageous in that it permits manufacturing tolerances of the housing  22  and the facet mirrors  16  to be lax while nonetheless permitting stable angular positioning of the facet mirrors  16 . Alternatives to three ball mounting include mounting the facet mirrors using flexures, adhesives and other conventional techniques such as by using screws, bolts, and rivets. Any of these techniques may be used. However, none of these techniques is as thermally stable or deterministic as the preferred three ball mount. 
     Referring again to FIG. 3, the metrology device  10  is controlled by the processor  60 . The processor  60  is coupled over a bus to the clock  62 , the database  64 , the memory  66 , the laser source  50 , the optical switch  52 , and the coarse and fine angle trackers  56  and  58 , respectively. 
     The memory  66  stores program instructions which determine how the metrology device  10  operates. The memory  66  may include random access memory (RAM), read only memory (ROM), and other storage devices which read or write data from and to electronic media, such as disk drives equipped with hard or floppy disks and CD ROM drives equipped with CD ROMs. The program instructions may be stored on electronic media and then uploaded to memory for execution by the processor  60 . The database  64  is shown separately from the memory  66  but may be part of the memory  66 . Alternatively, the database  64  may be remotely located on a network and may include program instructions which are uploaded to memory. The database  64  further provides a storage facility for storing data collected during calibration. 
     The processor  60  executes the program instructions stored in the memory  66  to accomplish the calibration. Based on the program instructions, the processor  60  controls the operation of the laser source  50 , the optical switch  52 , and the course and fine angle trackers  56  and  58 . The processor  60  also retrieves data from the coarse and fine angle trackers  56  and  58  and the clock  62  during calibration and stores the values as test data in the database  64 . The test data stored in the database  64  may be tabulated and, for example, include time, facet #, angle, coarse angle reading, fine angle reading, and laser offset fields. This is illustrated in the table below: 
     
       
         
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE #1 
               
             
             
               
                   
               
               
                 Test Data Stored in Metrology Device 
               
             
          
           
               
                   
                 Elapsed 
                   
                   
                   
                   
                 Laser 
               
               
                   
                 Time 
                 Facet # 
                 Angle 
                 CAR 
                 FAR 
                 Offset 
               
               
                   
                   
               
               
                   
                 20.00s 
                 1 
                 20 deg 
                 0.1 
                 0.02 
                 0.01 
               
               
                   
                 40.00s 
                 2 
                 30 deg 
                 0.1 
                 0.05 
                 0.01 
               
               
                   
                 40.15s 
                 3 
                 40 deg 
                 0.1 
                 0.09 
                 0.01 
               
               
                   
                 40.30s 
                 4 
                 50 deg 
                 0.1 
                 0.07 
                 0.01 
               
               
                   
                   
               
             
          
         
       
     
     FIG. 5 depicts an internal block diagram of the optical instrument  12 . The optical instrument  12  includes a pointing mirror  18 , mounted to a gimbal  94  which in turn may be mounted to the housing  20 . The optical instrument  12  further includes a motor controller  82 , a multi-axis drive motor  84 , position and motion sensors  86 , a processor  80 , a clock  88 , a memory  90  and a database  92 . The memory  90  stores program instructions for execution by the processor  80 , which controls the operation of the optical instrument  12 . The memory  80  may include random access memory (RAM), read only memory (ROM), and other storage devices which read or write data from and to electronic media, such as disk drives equipped with hard or floppy disks and CD ROM drives equipped with CD ROMs. The program instructions may be stored on electronic media and then uploaded to memory for execution by the processor  60 . 
     The database  92  stores data related to calibration of the optical instrument. For example, the database  92  may store one or more calibration profiles for testing the optical instrument  12 . Each calibration profile specifies the movements that the pointing mirror  18  makes during calibration testing and may illustratively include the following fields: time, facet #, a position value specifying a pointing angle for the mirror  18  to point at a particular facet, as well as acceleration, velocity, and deceleration fields. This is illustratively depicted in the table below. 
     
       
         
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE #2 
               
             
             
               
                   
               
               
                 Calibration Profile 
               
             
          
           
               
                   
                 Elapsed 
                   
                 Position 
                   
                 Max. 
                   
               
               
                   
                 Time 
                 Facet # 
                 value 
                 Accel. 
                 Velocity 
                 Decel. 
               
               
                   
                   
               
               
                   
                 20.00s 
                 1 
                 20 deg 
                 1 m/s 2   
                 3 m/s 
                 1 m/s 2   
               
               
                   
                 40.00s 
                 2 
                 30 deg 
                 1 m/s 2   
                 3 m/s 
                 1 m/s 2   
               
               
                   
                 40.15s 
                 3 
                 40 deg 
                 2 m/s 2   
                 3 m/s 
                 2 m/s 2   
               
               
                   
                 40.30s 
                 4 
                 50 deg 
                 2 m/s 2   
                 3 m/s 
                 2 m/s 2   
               
               
                   
                   
               
             
          
         
       
     
     The database  92  may also store program instructions. The database  92  may be part of the memory  90  or may be located on a network which is coupled to the processor  80 . 
     The processor  80  reads program instructions from the memory  90  and a calibration profile from the database  92  and in response issues commands to the motor controller  82  to move the pointing mirror  18  to specified positions, optionally at specified velocities, accelerations and decelerations. The motor controller  82 , in response, energizes windings within the multi-axis drive motor  84  in a well-known manner to move the pointing mirror  18  in the commanded direction. The position and motion sensors  86  may include an encoder for outputting a position of the scan mirror, as well as optional velocity and acceleration measuring devices. The position and motion sensors  86  provide position and optional velocity and acceleration parameters to the processor  80  and the motor controller  82 . The processor  80  or motor controller  82  may utilize the output of the position and motion sensors as feed back to control the energy applied to the windings of the motor  84  in any of several well-known manners. 
     The measured position and optional velocity and acceleration parameters may be tabulated and stored in the database  92  as measurement data. The measurement data may include elapsed time, facet #, angular position, velocity, acceleration and deceleration for multiple measurements as illustratively depicted in the table below: 
     
       
         
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE #3 
               
             
             
               
                   
               
               
                 Optical Instrument Measurement Data 
               
             
          
           
               
                   
                 Elapsed 
                 Facet 
                 Position 
                   
                 Max. 
                   
               
               
                   
                 Time 
                 # 
                 value 
                 Accel. 
                 Velocity 
                 Decel. 
               
               
                   
                   
               
               
                   
                 20.00s 
                 1 
                 20.05 deg 
                 1.1 m/s 2   
                 2.4 m/s 
                 0.9 m/s 2   
               
               
                   
                 40.00s 
                 2 
                 29.89 deg 
                 1.1 m/s 2   
                 2.5 m/s 
                 0.9 m/s 2   
               
               
                   
                 40.15s 
                 3 
                 39.95 deg 
                 2.1 m/s 2   
                 3.8 m/s 
                 2.0 m/s 2   
               
               
                   
                 40.30s 
                 4 
                 50.02 deg 
                 2.0 m/s 2   
                 3.9 m/s 
                 1.9 m/s 2   
               
               
                   
                   
               
             
          
         
       
     
     The clock  88  keeps track of time or elapsed time during a calibration test. For high-speed, dynamic testing of the scan mirror  18 , the clock  88  is either the same as or synchronized to the clock  62 . 
     FIG. 6 depicts a method of initially calibrating the metrology device  10  for subsequent use in calibrating an optical instrument  12 . In step  100 , the metrology device  10  is positioned and mounted on a optical table. In step  102 , the facet mirrors  16  are mounted on the housing  22 . In a preferred embodiment of the invention, the facet mirrors  16  are mounted on three ball mounts which predetermine the angular locations. In step  104 , the facet mirror angles are measured. Precise angular positioning of the facet mirrors  16  is possible with the assistance of a measuring device, such as a theodolite. 
     In step  106 , the optical instrument  12  is mounted on the optical table  14 . According to the present invention, it is not necessary that the optical instrument be precisely mounted to the optical table  14  when an angular difference between beams directed at different facet mirrors  16  is used for calibration and accuracy measurement. 
     In step  108 , testing and calibration of the pointing mirror  18  is performed. In the preferred embodiment of the invention, the metrology device and the optical table are made of a temperature stable material, such as Invar. In the preferred embodiment, therefore, the facet mirror angles do not change with temperature or pressure. Therefore, testing of the optical device  10  may be done over a wide temperature and pressure range, without adjusting the placement of the facet mirrors  16 . This facilitates testing of optical instruments which are used in varying environmental conditions, such as space or a semiconductor processing apparatus. 
     FIG. 7A illustratively depicts a method of calibrating an optical instrument using the metrology device  20 . In step  150 , the processor  60  sets the optical switch  52  to deflect a beam issuing from the laser source to the angular detectors  56  and  58 . In step  152 , the processor  60  measures and stores an offset associated with the beam. In step  154 , the processor  60  signals the optical switch to project the beam onto the reflecting surface  19  of the pointing mirror  18 . In step  156 , the processor sets the angle detectors  56  and  58  to indicate when a null or near null value is read. The null or near null value corresponds to a particular position on both the coarse and fine angular trackers  56  and  58 . 
     In step  158 , the scan mirror  18  is commanded to point at a first facet mirror until a null or near null value is read. The scan mirror may be commanded initially by the processor  80 , according to a calibration profile stored in the database  92 , to point at a particular facet mirror  16 . Subsequently, the scan mirror  18  may be commanded by the processor  60 , through a link between the processors  60  and  80 , to change angular position until a null or near null value is read. Once a null or near null value is read, then step  160  begins. 
     In step  160 , the processor  80  reads a first encoder value and stores the encoder value in the database  92 . Then in step  162 , the scan mirror  18  is commanded to point at another facet mirror  16  until a null or near null value is read. This is performed in the same manner as in step  158 . When the null or near null value is read, then step  164  begins. In step  164 , the processor  80  reads another encoder value and stores the encoder value in the database  92 . 
     In step  166 , if there are more facet mirror readings to be taken according to the calibration profile, then step  162  begins. If not, then  168  of FIG. 7B begins. In step  168 , the processor  80  determines angles between pairs of facet mirrors  16  based on the encoder readings taken when the null or near null value was read. Then the processor  80  compares the angles or the encoder readings stored in the database  92  with expected values stored in the database  64 . 
     In step  170 , the optical instrument optionally updates a calibration map within the scan mirror. The calibration map correlates positional readings from the position and motion sensors  86  with measured angular deflection. The calibration map may then be used by the processor  80  or the motor controller  82  to properly drive the pointing mirror  18 . In step  172 , the processor  60  optionally computes error in scan mirror  18  angles based on a least squares estimate of angles determined for each facet mirror  16 . 
     In step  174 , dynamic testing begins. The motor  84  drives the scan mirror to, from and past certain facet mirrors  16  under control of the processor  80 . 
     The processor  80  commands the motor based on a calibration profile stored in the database  92 . The process of dynamic testing reveals imperfections in the motor  84 , the motor controller  82 , and the mirror itself. For example, consider the following simple test. Command the scan mirror to move to facet  1 , temporarily stop at facet  1 , accelerate to facet  2 , temporarily stop at facet  2  and then repeat. Rapid acceleration and deceleration causes deflection in the mirror, which is not perfectly rigid. Moreover, the motor  82  and motor controller  84  may cause some oscillation in the mirror  18  during and after stopping at facets  1  and  2 . The control algorithm and angular feedback (encoder) are collectively calibrated in this manner. 
     In step  176 , the processor  60  stores data related to the dynamic calibration test in the database  64 . The data related to the dynamic calibration test may be tabulated and include the expected time that the scan mirror  18  will stop at each facet  16 , the particular facet  16  at which the stop is made, and the location of the beam as measured by the angle trackers  56  and  58 . The dynamic calibration test data may resemble the data stored in table #1. 
     In step  178 , the optical instrument  12  stores a time at which the scan mirror stopped at or passed by a facet  16 , the facet number and a positional, a velocity and an acceleration value read from the position and motion sensors  86 . 
     In step  180 , the processor  60  compares the calibration values stored in the database  92  of the optical instrument  12  with the calibration values stored in the database  64  of the metrology device  10  in steps  176  and  178 . The comparison may show deviations in angle of deflection of the scan mirror under dynamic testing. 
     In step  182 , the processor  80  may optionally update parameters in a calibration map used by the motor controller  82  to control the motor  84 . The parameters may include the maximum allowed acceleration, deceleration or velocity. 
     While specific embodiments have been disclosed, it will be understood by those of ordinary skill in the art that changes may be made to those embodiments without departing from the spirit and scope of the invention.