Abstract:
The present invention relates to a system for modifying wavefronts of electromagnetic waves. The system includes a flexible mirror having an obverse surface and a reverse surface. The obverse surface has a contour configured to reflect the electromagnetic waves. The system also includes a plurality of actuators positioned along the reverse surface of the mirror for applying at least one transverse magnetic force to the flexible mirror to modify the contour of the obverse surface. Modifying the contour of the obverse surface of the mirror modifies the wavefronts of the electromagnetic waves.

Description:
FIELD OF INVENTION 
     The present invention relates, in general, to optical wavefront correction via flexible mirror. The flexible mirror may be included in a telescopic system located in a terrestrial environment or outer space. The flexible mirror includes a lightweight (low mass/low density) aerogel core sandwiched between an obverse layer and a reverse layer (e.g. sapphire substrate material) on each respective side. A surface of the obverse layer is polished and coated to specific optical qualities (optical layer) for reflecting electromagnetic waves (e.g. light, laser), while the reverse layer provides stress relief to the mirror. In general, the flexible mirror is mounted over a honeycomb support structure which houses eddy current actuators. The eddy current actuators are modulated to magnetically push on the reverse surface of the flexible mirror thereby providing wavefront modification on the scale of a fraction of a wavelength. 
     BACKGROUND OF THE INVENTION 
     In conventional systems, large aperture mirrors tend to have significant mass and therefore are susceptible to unwanted deformation due to gravitational and inertial forces. A number of actuators which may be physically attached to the reverse surface of the mirror (for flexing the mirror) also add to the mass and provide both thermal and mechanical stress points. In a terrestrial environment, such as on Earth, gravitational forces as well as any attachment points act on the mirror from various angles. This gravitational force deforms the mirror thereby producing wavefront errors in the received light waves. In a space environment, such as on a satellite orbiting the Earth, inertial forces (due to acceleration of the satellite) and forces due to lack of gravity (gravitational unload after test and alignment under gravitational load) also act on the mirror from various angles thereby deforming the mirror and producing wavefront errors. In general, the gravitational and inertial forces are proportional to the mass of the mirror. 
     SUMMARY OF THE INVENTION 
     To meet this and other needs, and in view of its purposes, the present invention provides a system for modifying wavefronts of electromagnetic waves. The system includes a flexible mirror having an obverse surface and a reverse surface, where the obverse surface has a contour configured to reflect electromagnetic waves. The system also includes a plurality of actuators positioned along the reverse surface for applying at least one transverse magnetic force to the flexible mirror to modify the contour of the obverse surface. In general, modifying the contour of the obverse surface modifies wavefronts of the electromagnetic waves. 
     The flexible mirror includes obverse and reverse layers, and a low density core sandwiched between the obverse and reverse layers. The obverse layer includes the obverse surface, and the reverse layer includes the reverse surface. In one embodiment, the obverse and reverse layers are each comprised of sapphire, and the low density core is comprised of aerogel. 
     The system also includes a mirror base positioned along the reverse surface, where the mirror base includes a plurality of structures for housing the plurality of actuators. In one embodiment, the mirror base is comprised of silicon carbide. The plurality of actuators in the system include at least one eddy current actuator having a field coil and a disk armature. The disk armature of the eddy current actuator is attached to the reverse surface of the flexible mirror, and a gap is formed between the disk armature and the field coil. In general, the transverse force is a magnetic force applied through the gap between the disk armature and the field coil. 
     The plurality of actuators may also include at least one linear voice coil motor having an armature attached to the reverse surface of the mirror, where the transverse force is applied by the armature to the reverse surface of the mirror. The system may also include a filter having an array of openings for receiving the electromagnetic waves reflected from the flexible mirror and forming a pattern. In general, a sensor detects the pattern and a processor receives the pattern from the sensor to determine wavefront errors in the electromagnetic waves. The processor provides a signal to at least one actuator to provide the at least one transverse force to the flexible mirror to modify the electromagnetic wavefront. 
     The system may be a telescopic system that includes a primary flexible mirror having an obverse surface and a reverse surface, where the obverse surface has a contour configured to reflect electromagnetic waves received by the telescopic system. The system may also include a secondary mirror configured to receive the electromagnetic waves reflected from the primary mirror, and a plurality of actuators positioned along the reverse surface for applying a transverse magnetic force to the flexible mirror to modify the contour of the obverse surface. In general, modifying the contour of the obverse surface modifies the wavefront of the electromagnetic waves received by the telescopic system. 
     In one embodiment, the plurality of actuators includes at least one eddy current actuator having an field coil and a disk armature, where the disk armature is attached to the reverse surface of the flexible mirror and the field coil is positioned at a distance from the disk armature for generating an eddy current loop in the disk armature. 
     In one embodiment, the system may include a central opening in the obverse surface, and a detector for receiving the electromagnetic waves reflected from the secondary mirror through the central opening of the primary mirror. In general, the primary mirror may be configured in a catadioptric telescopic system. 
     In one embodiment, the system may include a continuous obverse surface, and a detector for receiving the electromagnetic waves reflected from the secondary mirror. The electromagnetic waves are perpendicularly folded by the secondary mirror through an opening in the side of the telescopic system when the primary mirror is configured in a catoptric telescope. 
     A Shack-Hartmann filter for filtering the electromagnetic waves may also be included, where the filter converts the electromagnetic waves into a light dot pattern. A processor computes wavefront errors based on the light dot pattern produced by the filter. In one embodiment, the processor computes a pixel location and a pixel intensity of each light dot in the pattern image and compares the location and intensity to a predetermined location and a predetermined intensity for determining the wavefront errors. 
     The system may include a concave mirror base positioned along the reverse surface of the primary mirror for housing the plurality of actuators. The concave mirror base supports the primary mirror. The magnetic force applied to the reverse surface of the mirror has a magnitude, direction, and duration in correlation with a desired wavefront modification. In general, the magnitude, direction and duration of the magnetic force contours the mirror to perform low frequency wavefront correction and high frequency adaptive wavefront control. 
     The mirror in the system may include an aerogel core, an obverse sapphire layer and a reverse sapphire layer. In general, the aerogel core is bonded between the obverse sapphire layer and the reverse sapphire layer, and the obverse sapphire layer is coated with an optical layer for reflecting electromagnetic waves. In one embodiment, the aerogel core is comprised of a plurality of hexagonal structures having hollow centers arranged in a honeycomb. 
     It is understood that the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1   a  is a top view of a mirror base including a central opening for supporting a flexible mirror (transparent for display purposes) with a central opening, where the mirror base and flexible mirror have different diameters, according to an embodiment of the present invention. 
         FIG. 1   b  is a top view of a continuous mirror base for supporting a continuous flexible mirror (transparent for display purposes), where the mirror base and flexible mirror have the same diameter, according to an embodiment of the present invention. 
         FIG. 1   c  is a cross-sectional view showing the layers of a flexible mirror supported by the mirror base in  FIGS. 1   a  and  1   b , according to an embodiment of the present invention. 
         FIG. 2   a  is a side view of a Schmidt-Cassegrain telescope including the flexible mirror mounted on the mirror base in  FIG. 1   a , according to an embodiment of the present invention. 
         FIG. 2   b  is a side view of a Newtonian telescope including the flexible mirror mounted on the mirror base in  FIG. 1   b , according to an embodiment of the present invention. 
         FIG. 3  shows two light patterns produced by a Shack-Hartmann array, according to an embodiment of the present invention. 
         FIG. 4  is a cross-sectional view of a linear voice coil motor mounted to the mirror base in  FIGS. 1   a  and  1   b , and mounted to the reverse surface of the flexible mirror in  FIG. 1   c , according to an embodiment of the present invention. 
         FIG. 5  is a cross-sectional view of an eddy current actuator housed in one of the hexagonal structures in  FIGS. 1   a  and  2   b , according to an embodiment of the present invention. 
         FIG. 6   a  shows a laser beam shaping system for modifying a profile of a laser beam with the flexible mirror system in  FIG. 1   a , according to an embodiment of the present invention. 
         FIG. 6   b  shows another laser beam shaping system for modifying a profile of a laser beam with the flexible mirror system in  FIG. 1   a , according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As will be described, the present invention provides an optical system (e.g. a telescope) having a low mass flexible mirror in which the surface contour is modified by eddy current actuators. Modulation of the actuators modify wavefronts thereby correcting wavefront errors in received electromagnetic (e.g. light) waves (on the scale of fractions of a wavelength). Using the present invention, optical wavefront errors caused by factors such as fabrication errors, alignment errors, atmospheric conditions as well as unwanted deformations due to gravitational and inertial forces may be corrected in real-time. Such wavefront corrections provide an adaptable system which increases energy convergence of the received light waves, thereby improving image quality. The present invention may also be used to alter the spatial phase profile of a coherent light source (e.g. laser beam) reflected from the mirror&#39;s surface. Altering the shape of a laser beam may be useful in certain laser milling applications to improve kerf quality. 
     In one embodiment,  FIG. 1   a  shows a top view of optical system  100  looking through flexible mirror  102  (transparent for display purposes) to a mirror base  110 . Mirror  102  is a low mass flexible mirror resting on a plurality of hexagonal supports  111  arranged in a honeycomb that make up mirror base  110  (which may be manufactured from variety of rigid light weight low expansion materials such as silicon carbide and ultra low expansion glass). In this embodiment, hexagonal supports  111  are arranged as a honeycomb, each housing a respective eddy current actuator  106  (See  FIG. 5  for details). Mirror base  110  also supports linear voice coil motors  104  in which the armatures of the motors are physically attached to a reverse surface of mirror  102 . In this embodiment, both mirror base  110  and flexible mirror  102  may have roughly the same inner perimeter  118  (producing central opening  112 ). Also, the outer perimeter  114  of mirror base  110 , and the outer perimeter  115  of mirror  102  may be different as shown in  FIG. 1   a , or may be the same (not shown). 
     Eddy current actuators  106  which are disposed in circular center cavity  117  of each hexagonal support  111 , magnetically interact with metalized disk armatures  116  mounted on a reverse surface (non-reflecting side) of mirror  102 . Mounted around the outer perimeter and inner perimeter of mirror base  110  are low mass linear voice coil motors  104  which are physically attached to the reverse side of mirror  102  (See  FIG. 4  for details). Linear voice coil motors  104  may be arranged in clusters around the inner perimeter  118  or mounted separately around the outer perimeter  114  to attach mirror  102  to mirror base  110 . In another embodiment, a three point kinematic mount may be arranged on mirror base  110  and physically attached to the reverse side of mirror  102 . In general, the three point kinematic mount (not shown) may be used in place of, or in conjunction with the linear voice coil motors to hold mirror  102  and base  110  together. 
     Soft stops  108  may also be mounted at the vertex of each hexagonal support  111  between the top of the hexagonal support and the reverse surface of the mirror (See  FIGS. 4 and 5  for details). In general, soft stops  108  prevent mirror  102  from being damaged as a result of contact with mirror base  110 . It should be noted that the size and arrangement of hexagonal supports  111  may be varied based on certain applications. Mirror base  110  may also include shapes other than hexagonal (e.g. circles, squares, triangles, etc. . . . ) to house the eddy current actuators. Furthermore, the number and the arrangement of both the linear voice coil motors and eddy current actuators may vary based on certain applications. For example, mirror base  110  could be arranged with many smaller hexagonal structures to create a uniform configuration to correct low frequency wavefront errors. 
     Optical system  100  in  FIG. 1   a , having central opening  112 , may be a primary mirror in a catadioptric telescope (e.g. Schmidt-Cassegrain). In catoptric telescopes (e.g. Newtonian), however, a central opening may not be needed.  FIG. 1   b  shows an optical system  150  which is similar to optical system  100  with the exclusion of central opening  112 . Specifically, mirror base  110  and mirror  102  (transparent for display purposes) in  FIG. 1   b  are continuous (do not have a central opening). Central opening  112  is replaced in  FIG. 1   b  with additional hexagonal structures  111  (housing eddy current actuators) and a central cluster of linear voice coil motors  104 . Mirror base  110  and mirror  102  may also have roughly the same outer perimeters  114  and  115  (although this is not necessary). As in  FIG. 1   a , a three point kinematic mount (not shown) may also be included in system  150 . 
     In certain applications (e.g. reflecting telescopic applications), mirror base  110  may have a concave shape (parabolic) with respect to incoming light waves. The degree of curvature of the parabolic mirror base may be dictated by design needs for a specific telescope and application. By curving mirror base  110 , mirror  102  (which rests on base  110 ) is also held in a nominal parabolic position for reflecting concentrated light waves to a destination. 
     Shown in  FIG. 1   c  (cross sectional view) is an embodiment of mirror  102 . In general, mirror  102  includes a light weight flexible (flexible on the order of the wavelength of incident radiation) aerogel core  126  sandwiched between a sapphire obverse layer  124  (layer facing incoming light waves), and a sapphire reverse layer  128  (layer facing mirror base  110 ). The aerogel core as shown in  FIG. 1   c  may be a solid structure, a honeycomb structure  126  (similar to the mirror base) or any other structure sufficient to provide support and flexibility to a given mirror design. Low density aerogel core  126  provides stability as well as reduces the mass of mirror  102  (due to the rigid and sparsely filled structures). The aerogel core may be a monolithic aerogel. Moreover, the aerogel core provides a thermal barrier with a low coefficient of thermal expansion. Sapphire layers  124  and  128  provide structural support for the aerogel core as well as a substrate for optical coating  120 . 
     During manufacturing, obverse layer  124  is coated with a reflective layer  120  in the electromagnetic radiation band of interest, while reverse layer  128  provides stress relief to the curved structure during the bonding process. In general, reverse surface  121  provides a surface for adhering metalized eddy current disk armatures  116  for interacting with field coils of the eddy current actuators (See  FIG. 5  for details). Also, optical layer  120  provides an obverse surface  123  for reflecting incoming electromagnetic waves  130 . 
       FIG. 2   a  shows a cross-sectional view of a Schmidt-Cassegrain (catadioptric) telescope  200  utilizing optical system  100  (cross sectional view) as a primary mirror. As light waves  130  enter telescope tube  206  through corrective lens  212 , they are reflected by obverse surface  123  of mirror  102  towards convex secondary mirror  202 . The light waves are then reflected through central opening  112  of mirror  102  and split into two beams ( 131  and  132  respectively) by beam splitter  214 . 
     Beam  131  is folded by mirrors  220  and  218  respectively, and refracted by lens  224 . Refracted beam  131  is then received by image sensor  230  where it may be viewed by a user. Beam  132  is folded by mirror  216 , collimated by collimating lens  222 , filtered by light filter  226  and received by sensor  228  (e.g. imager) where the signal is detected for processing. 
       FIG. 2   b  shows a cross-sectional view of a Newtonian (catoptric) telescope  250  utilizing optical system  150  (cross sectional view) as a continuous primary mirror (without a central opening  112 ). As light waves  130  enter telescope tube  206 , they are reflected by obverse surface  123  of mirror  102  towards fold mirror  232  which is suspended from tube  206  by support  234 . The light waves are then reflected through a side opening  236  in tube  206  and split into two beams ( 131  and  132  respectively) by beam splitter  214  (similar to the Schmidt-Cassegrain telescope in  FIG. 2   a ). 
     Similar to system  100 , beam  131  is folded by mirrors  220  and  218  respectively, and refracted by lens  224 . Refracted beam  131  is then received by image sensor  230  where it may be viewed by a user. Beam  132  is folded by mirror  216 , collimated by collimating lens  222 , filtered by light filter  226  and received by sensor  228  where the signal is detected for processing. 
     Filter  226  in systems  100  and  150  may be any type of light filter or grating which produces a light pattern which may be analyzed. For example, filter  226  may produce a set geometric pattern of light points (e.g. Shack-Hartmann Lens-let array). Shown in  FIG. 3  are two examples of a Shack-Hartmann light pattern produced by filter  226 . Collimated light from lens  222  enters filter  226  and is filtered into a Shack-Hartmann pattern of light intensity dots. The light dots output from the filter are received by sensor  228  where they are analyzed by a processor (not shown). In general, each dot may illuminate N number of pixels in sensor  228 , where the ideal size (number of pixels per dot), location (which pixels) and brightness (pixel intensity) of each dot is known to the processor. During analysis, the size, location and intensity of each light dot in a given pattern is compared to the ideal size, ideal location and ideal intensity values known to the processor. 
     Pattern  302  in  FIG. 3  shows an ideal pattern of light intensity dots produced by the filter when the light waves received by telescope  200  or  250  have insignificant wavefront errors. Pattern  304  in  FIG. 3 , however, shows a non-ideal pattern where the sizes, shapes, locations and intensities of the light dots have been altered due to wavefront errors. This non-ideal pattern may be a result of wavefront errors produced when mirror  102  is deformed due to gravitational or inertial forces. 
     Analysis of the non-ideal dots may include computing a centroid of each ideal dot and non-ideal dot. A vector may be computed from the center of the non-ideal dot to the center of the ideal dot. This vector describes the overall movement of each dot (how far and at what direction the dot has traveled from the ideal location). Based on this information, as well as dot size and intensity information, the processor (not shown) modulates the linear voice coil motors and eddy current actuators to flex the mirror at various points along its surface. By modifying the surface contour of mirror  102 , wavefront errors may be reduced or eliminated. 
     An interferometer (not shown) may also be used to analyze the wavefronts. The interferometer may convert the electromagnetic waves into a light-dark fringe pattern. The processor may then compute the wavefront errors based on the light-dark fringe pattern, and modulate the actuators accordingly. 
     In general, control of the motors and actuators may be performed in a variety of ways, intermittently or continuously to maintain a desired contour of the mirror (from initial calibration corrections to counteracting deformations caused by gravitational load/unload and inertial forces). A steady state condition may be desired to calibrate or correct for known errors. In general, low frequency modulation may be used for inertial wavefront correction while high frequency modulation may be used for agile adaptive wavefront control. The controller modulates the motors and actuators until the ideal dot pattern is approached or achieved (i.e. the mirror has obtained the desired surface contour to reduce or eliminate the wavefront errors and or produce the desired impulse response). 
       FIG. 4  shows a cross sectional view of mirror base  110  including linear voice coil motor  104  housed in the center of hexagonal support walls  412 . Stator  408  of motor  104  is physically attached to mirror base  110  in the center of hexagonal support  111 . Armature  406  is physically attached to reverse surface  121  of mirror  102 . 
     During operation, an electromagnetic field is induced in the coils  416  of stator  408  which magnetically interacts with coils  414  of armature  406 . This magnetic force either pushes or pulls armature  406  up or down respectively in the transverse (+/−) Y direction. When armature  406  is pulled down in the (−) Y direction, mirror  102  flexes towards mirror base  110 . If mirror  102  is pulled down too far, soft stops  108  prevent the reverse surface of the mirror from contacting the top of walls  412 . In contrast, when armature  406  is pushed up in the (+) Y direction, mirror  102  flexes away from mirror base  110 . 
     Referring to  FIG. 5  there is shown an embodiment of an eddy current actuator  106  including field coil  502  mounted on the top of a screw drive motor which includes a movable part  506  and a stationary part  504 . Also included in system  500  is a light weight metalized disk armature  116  adhered to reverse surface  121  of mirror  102 . Field coil  502  and disk  116  are spaced away from each other at a nominal distance  508  which allows eddy currents to flow. In general, the eddy currents produce a repulsive magnetic force pushing in the (+) Y direction on reverse surface  121  of mirror  102 . The screw drive motor  504  and  506  (using threads  514  and  516 ) may be used to initially position field coil  502  to within working distance  508  of disk armature  116  prior to modulating the actuator. This nominal air gap  508  ensures that eddy currents flow in armature  116 . Also, shown in planar view (above the mirror) disk  116  is a doughnut shape including an outer and inner perimeter. 
     During operation, an electromagnetic field is induced in field coil  502  which pushes armature  116  up in the (+) Y direction with varying amounts of force depending on the correction needed. Eddy currents are induced in armature  116  which induce a magnetic field. The magnetic field in disk  116  interacts with a magnetic field in coil  502  through magnetic repulsion (note that the nominal gap  508  is maintained between field coil  502  and armature disk  116  by the voice coil motors). 
     In general, the pull of the voice coil motors in the (−) Y direction and the push of the eddy current actuators in the (+) Y direction are sufficient to hold the mirrors center of mass static (floating in space) with respect to the mirror base in the presence of gravitational and inertial forces. The amplitude of the individual eddy current actuator modulations is designed to correct the local wave front while the average push is sufficient to hold mirror  102  fixed in space with respect to mirror base  110 . 
     A benefit of eddy current actuators is that they do not add significantly to the mass of mirror  102  (field coils  502  do not physically contact the eddy current disk armatures  116  on the reverse surface of the mirror). Moreover, the eddy current actuators do not add stress points to the mirror that typically result from actuators that are physically attached to the mirror. 
     As previously described, wavefront errors may be caused by many conditions such as gravitational and inertial forces. In one example, telescope  200  may be included in a space vehicle. When the space vehicle accelerates in any given direction, inertia may distort the contour of the mirror. To compensate for these inertial forces, the voice coil motors and eddy current actuators are energized (modulated) to push/pull (hold) the mirror. The amplitude, direction and duration of the forces exerted by the motors and actuators are controlled by the processor to correct the wavefront errors. In one example, the amplitude, direction and duration of the forces exerted by the motors and actuators may be equivalent (correlated) to the mirror&#39;s gravitational or inertial forces. 
     When system  100  is included in a space vehicle as described in the example above, safely controlling the movement and vibration of mirror  102  during space launch may also be beneficial. During space launch, mirror  102  may be held against soft stops  108  in response to a pulling force exerted by motors  104 . In another example, soft stops may be adhesively connected to both the vertices of the hexagonal supports  111  and reverse surface  121  of mirror  102 . The adhesive may then be melted away after launch is complete, thereby freeing mirror  102 . 
     In another example, system  100  may be included in a terrestrial observatory. Due to atmospheric conditions and gravitational forces acting on the mirror, wavefronts received from celestial objects may be distorted. Similar to the above described example, the motors and actuators are modulated to maintain a desired impulse response by changing the contour of mirror  102 , thereby correcting wavefront errors and providing the desired image quality at the plane of focus. 
     The reciprocities in systems  100  and  150  described above also allow the systems to transmit electromagnetic energy. Systems  100  and  150  may be utilized in applications that require beam shape control of an emitted laser beam. Modifying the beam profile of a laser is a technique used in the laser milling and machining industry to improve kerf quality when laser cutting various materials. 
     Surface  120  of the flexible mirror  102  may be modified to control the projection of the second moment (beam diameter) of a coherent electromagnetic source (laser) in the far field. The mirror surface  120  is modified by the eddy current actuators to control the spatial phase profile of the beam, thus producing various beam shapes which may include but are not limited to Gaussian, rectangular and elliptical super-Gaussians. 
       FIG. 6   a  shows a laser beam shaping system  600  including optical system  100 . A laser beam  622  is emitted from laser  618 , expanded by lens  612  and reflected by secondary mirror  616  and primary mirror  102  towards beam splitter  620 . Beam splitter  620  diverts most of the beams power (beam  646 ) towards lenses  630  and  632  where the beam is focused at a location  636  on workpiece  634  (i.e. the material to be cut). Some of the laser beams power (beam  648 ) passes through splitter  620 , where it refracted and collimated by lenses  622  and  624  respectively. The beam is then filtered by Shack-Hartman array  626  and detected by sensor  628 . 
     In general, the Shack-Hartman pattern detected by sensor  628  correlates to a specific beam shape. For example, a Gaussian beam shape produces a unique pattern different from a rectangular beam shape. The system therefore, modulates the eddy current actuators to modify mirror surface  120 , thereby obtaining a desired pattern which correlates to a desired beam shape. 
     System  650  in  FIG. 6   b  shows another laser beam shaping system including a reflecting path. Laser beam  622  is emitted by laser  618 , folded by mirror  640  and split by beam splitter  642 . The laser beam is reflected by beam splitter  642  towards secondary mirror  616 , primary mirror  102  and folding mirror  644 . The beam is then passed through lenses  630  and  632 , where it is focused at location  636  on workpiece  634 . 
     In general, workpiece  634  has a reflectance which reflects some of the lasers power back through the system where it is filtered by  626  and detected by sensor  628  (i.e. the laser is reflected by the workpiece through lenses  632  and  630 , reflected by mirrors  644 ,  102  and  616  and passed to the sensor  628  through beam splitter  642 ). This configuration allows the system to receive and analyze a reflected beam from the workpiece, thereby determining the laser beam shape at the focus point  636 . Due to analyzing the beams reflection, a more intelligent control may be implemented to more accurately shape laser beam  622 . This configuration also allows for efficient power transfer from laser  618  to workpiece  636  (i.e. only a fraction of the beams power transmits through the beam splitter  642  to reach the Shack-Hartman array  626 ). In this embodiment, mirror  640  may also be replaced with a steerable mirror to alter the position of the laser beam on the workpiece (i.e. steering the laser beam). 
     Although the invention is illustrated and described herein with reference to specific embodiments, the invention is not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the invention.