Abstract:
A system for aligning a segmented mirror includes a source of radiation directed along a first axis to the segmented mirror and a beamsplitter removably inserted along the first axis for redirecting radiation from the first axis to a second axis, substantially perpendicular to the first axis. An imaging array is positioned along the second axis for imaging the redirected radiation, and a knife-edge configured for cutting the redirected radiation is serially positioned to occlude and not occlude the redirected radiation, effectively providing a variable radiation pattern detected by the imaging array for aligning the segmented mirror.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     The invention described herein was made in the performance of work under NASA Contract No. NAS5-02200 and is subject to the provisions of Section 305 of the National Aeronautics and Space Act of 1958 (42 U.S.C. 2457). 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a system and method for testing imaging devices. More specifically, the present invention relates to the alignment of a large segmented mirror to a reference axis originating from a radiation source. 
     BACKGROUND OF THE INVENTION 
     U.S. Pat. No. 4,969,737 issued on Nov. 13, 1990 to Thomas W. Dey (co-inventor) discloses a Foucault knife-edge test for an objective or imaging device. The entire disclosure of this patent is incorporated herein by reference. 
     As disclosed therein and as shown in  FIG. 1 , optical assembly  10  demonstrates the basic principles of the Foucault knife-edge test. The assembly  10  includes a conventional imaging device, i.e. lens  12 , comprising a pair of optical surfaces  14  and  16 , radiation source  18 , collector lens  20 , and conventional photodetector  22  comprising the human eye. The components of assembly  10  are aligned to reference axis  24 . 
     For optical assembly  10 , one may employ the knife-edge test for qualitatively detecting (at eye/photodetector  22 ) the presence of transverse aberrations that may have been introduced into assembly  10  by lens optical surfaces  14  and  16 . Accordingly, knife-edge  26  may be gradually introduced into assembly  10  (shown by way of the staggered arrows in  FIG. 1 ), so that knife-edge  26  sequentially cuts and blocks the image of radiation source  18  at a plane of convergence  28 . This action, in turn, removes source rays from their expected trajectories, so that a variable intensity pattern may be registered by the eye. Finally, a comparison of this intensity pattern with a theoretical intensity pattern for an ideal optical surface may become a qualitative measure of the presence of transverse aberrations introduced by optical surfaces  14  and  16 . 
     Optical assembly  10  may be modified to obtain a quantitative interpretation of the Foucault knife-edge test.  FIG. 2  shows the basic Foucault assembly  10  of  FIG. 1 , but modified to help realize quantitative interpretations of the knife-edge test. It is first noted that the eye has been replaced by a conventional photodetector  30 . For example, photodetector  30  may comprise a matrix (m×n) array of charge coupled devices (CCD) where m is preferably from 64 to 1024, and n is preferably from 64 to 1024. The photodetector device  30  collects the radiation images by imaging device  12  under test, and provides, for each element in the matrix, a value proportional to the radiation intensity at that element.  FIG. 2  shows that the outputs of photodetector  30  may be fed along line  32  to a conventional computing means  34 . 
     Turning next to U.S. Pat. No. 5,020,905, issued on Jun. 4, 1991 to Thomas W. Dey, application of a Foucault knife-edge test to a segmented mirror is described. The entire disclosure of this patent is incorporated herein by reference. As disclosed therein and as shown in  FIG. 3 , segmented optic  38  includes a segmented mirror comprising two physically de-coupled, monolithic mirror sections  40  and  42 . An individual and disjoint entrance pupil contribution by each of the physically de-coupled, monolithic mirror sections  40  and  42  aggregates in sum to form a common entrance pupil  36 , i.e. entrance pupil  36  is developed over the entire surface of segmented optic  38 . 
     The segmented optic  38  of  FIG. 3 , more particularly, may include an aluminized reflective coating on a Pyrex glass substrate. Here, segmented optic  38  has an overall diameter of approximately 125 mm, and a radius of curvature of approximately 2000 mm. 
     The Foucault testing of segmented optic  38  may proceed, with reference to assembly  10  of  FIG. 1 , mutatis mutandis, the required necessary changes being that of (1) replacing lens  12  of  FIG. 1  with that of segmented optic  38  of  FIG. 3 , and (2) re-locating radiation source  18  to accommodate the reflective properties of mirror sections  40  and  42 . 
     The Foucault testing of segmented optic  38  works by reconstructing, or emulating, an idealized monolithic mirror, by using Foucault determined data derived from sections  40  and  42 , to align them into correspondence with the idealized monolithic mirror. Note that the Foucault determined data may be qualitative (for example when photodetector  22  of  FIG. 1  includes the human eye). It is possible that the segmented optic  38  may induce an intensity pattern at the eye, in which the intensity pattern has inherent ambiguities, namely an ambiguity as to which of the two mirror sections  40  or  42  is indeed the source of an optical aberration. For this situation, one may employ the quantitative Foucault techniques described with respect to assembly  10  of  FIG. 2 . 
     Large segmented mirrors, for example segmented concave mirrors used as a primary mirror of an imaging telescope, are significantly misaligned in their initial deployment state. These segmented mirrors must be aligned to properly capture a light beam from an interferometer. Once aligned, the light beam from the interferometer may be used to interrogate (or test) the primary mirror at the mirror&#39;s center of curvature. 
     The segments of the primary mirror must be registered to extreme accuracy in order for the mirror to deliver image quality comparable to that of an equivalent monolithic mirror. Accordingly, the segments of the mirror are mechanically tip-tilted relative to each other, in order to achieve an ideal mirror configuration. 
     The present invention addresses a solution to the problem of how to align a large segmented mirror and achieve an accuracy sufficient for interrogation by an interferometer. 
     SUMMARY OF THE INVENTION 
     To meet this and other needs, and in view of its purposes, the present invention provides a system for aligning a segmented mirror including a source of radiation directed along a first axis to the segmented mirror and a beamsplitter removably inserted along the first axis for redirecting radiation from the first axis to a second axis, substantially perpendicular to the first axis. Also included is an imaging array positioned along the second axis for imaging the redirected radiation, and a knife-edge configured for cutting the redirected radiation. The knife-edge is serially positioned to occlude or not occlude the redirected radiation, effectively providing a variable radiation pattern detected by the imaging array for aligning the segmented mirror. 
     The beamsplitter, the imaging array and the knife-edge are integrated into a housing configured for removably inserting the beamsplitter along the first axis. The knife edge is mechanically coupled to a translation stage configured to move the knife edge in a plane perpendicular to the second axis. Also included is a focusing objective for focusing the radiation directed along the first axis onto a point on the first axis located between the beamsplitter and the focusing objective. Further included is a null assembly positioned along the first axis, and located between the beamsplitter and the segmented mirror, that is configured to receive radiation from the source and reflect the radiation toward the segmented mirror. The position of the source of radiation is adjustable along a length of the first axis, based on the variable radiation pattern detected by the imaging array. 
     Another embodiment of the present invention is a Foucault knife-edge test assembly including a source of radiation directed along a primary axis to a segmented mirror, a beamsplitter removably inserted along the primary axis for redirecting radiation from the primary axis to the Z-axis and forming the beam of radiation, and a knife-edge having an opaque surface in an X, Y plane including V-shaped edges. Each edge is configured to cut the beam of radiation along the Z-axis, with adjoining edges of the V-shaped edges forming a series of successive apexes that are serially positioned to occlude or not occlude the beam of radiation, effectively providing a variable radiation pattern detected by the imaging array for aligning the segmented mirror. 
     In yet another embodiment of the present invention, a segmented mirror is aligned by directing radiation from a source along a first axis to the segmented mirror and temporarily positioning a beamsplitter along the first axis to redirect radiation from the first axis to a second axis perpendicular to the first axis. Then, by serially positioning a knife-edge to cut the redirected radiation along the second axis and imaging radiation along the second axis after the radiation is cut by the knife-edge, a variable radiation pattern detected by the imaging array is used to align the segmented mirror. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The invention is best understood from the following detailed description when read in connection with the accompanying drawing. Included in the drawing are the following figures: 
         FIG. 1  shows a prior art optical assembly for using a Foucault knife-edge test. 
         FIG. 2  shows a prior art optical assembly for using a Foucault knife-edge test including a photodetector. 
         FIG. 3  shows a prior art segmented optic including a segmented mirror. 
         FIG. 4  is a diagram of a center of curvature system showing a fine alignment sensor and a null assembly both in a stowed position in accordance with an embodiment of the invention. 
         FIG. 5  is a diagram of the center of curvature system showing the fine alignment sensor in a stowed position and the null assembly in a deployed position in accordance with an embodiment of the invention. 
         FIG. 6  is a diagram of the center of curvature system showing the fine alignment sensor and the null assembly both in a deployed position in accordance with an embodiment of the invention. 
         FIG. 7  is a detailed diagram showing the fine alignment sensor in a deployed position relative to the null assembly in accordance with an embodiment of the invention. 
         FIG. 8  shows a Foucault knife-edge test assembly in accordance with an embodiment of the invention. 
         FIG. 9  shows a null assembly for testing an optical surface, including an aspheric mirror and a spherical imaging mirror. 
         FIG. 10  shows a system for testing an optical surface that includes the null assembly shown in  FIG. 9 . 
         FIGS. 11A-11P  show intensity patterns resulting from using the Foucault knife-edge test assembly of  FIG. 8 , illustrating occluded and non-occluded light beams incident on an imaging camera of the fine alignment sensor in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 4  shows one embodiment of a center of curvature system  400  for testing an optical surface (not shown), such as the reflecting surface of a primary mirror of a telescope. Center of curvature system  400  includes fine alignment sensor  410 , null assembly  412  and elevator  406 . Center of curvature system  400  further includes focusing objective  408 , aperture  404  and thermal shutter  402 . Thermal shutter  402  is shown in a closed position. 
     In  FIG. 4 , center of curvature system  400  is in a stowed configuration. As shown, fine alignment sensor  410  is in a stowed position that is away from axis  411  and null assembly  412  is in a stowed position when lowered by elevator  406  to a position away from aperture  418 . The stowed positions are indicated, respectively, by the two arrows designated as  413  and  415 . 
     Referring next to  FIG. 5 , fine alignment sensor  410  is still in its stowed position. Null assembly  412 , however, is shown lifted by elevator  406  into a deployed position, as indicated by arrow  413 . The light rays  414  from an interferometer (not shown) are configured to enter center of curvature system  400  via aperture  404  and pass through Petzval focusing objective  408  to form focal point  416 . Light rays diverging from focal point  416  enter null assembly  412 . The operation of null assembly  412  is described below. After passing through null assembly  412 , the light rays exit center of curvature system  400  via aperture  418  if thermal shutter  402  is retracted, as shown. The rays exit aperture  418  and reach the surface under test, such as a large segmented mirror. 
     Referring next to  FIG. 6 , fine alignment sensor  410  is shown in its deployed position. When deployed, fine alignment sensor  410  uses light beam  414  emerging from the interferometer (not shown) to align the segmented mirror. The light rays  414  pass through Petzval focusing objective  408  to form focal point  416 . The light rays diverging from focal point  416  pass through a portion of fine alignment sensor  410  (described below) and enter null assembly  412 . After passing through null assembly  412 , the light rays exit center of curvature system  400  via aperture  418 , when thermal shutter  402  is retracted as shown. The rays eventually reach the surface under test. The light rays  414  are then reflected from the surface under test back into null assembly  412 . As will be described below with regard to  FIG. 7 , the reflected light rays enter fine alignment sensor  410 . 
     Referring now to  FIG. 7 , there is shown greater detail of fine alignment sensor  410  in its deployed position. As shown, fine alignment sensor  410  includes beamsplitter  702 , shown positioned to intercept light rays  414  diverging from focal point  416 . Pupil imaging camera  708  is positioned to intercept light rays emerging from beamsplitter  702  on axis  711 , which is perpendicular to axis  411 . Pupil imaging camera  708  is shown laterally displaced from the beamsplitter. The imaging camera includes an imaging planar array, such as a charge-coupled device (CCD) array  710 , for example, for providing light intensity patterns formed by the mirror under test. The Foucault knife-edge  704  is positioned to perpendicularly cut the light beams in axis  711 . The Foucault knife-edge  704  cuts the light at a focal point designated as  712 . The motion of the knife-edge is controlled by knife-edge translation stage  706 . 
     In operation, light rays  414  enter center of curvature system  400  via aperture  404  ( FIG. 6 ) and pass through Petzval focusing objective  408  to form focal point  416 . With fine alignment module  410  in its deployed position, light rays  414  pass through beamsplitter  702  and enter null assembly  412  ( FIG. 6 ), eventually reaching the surface under test, such as the large segmented mirror (shown in  FIG. 10 ). 
     The light rays  414  are then reflected from the surface of the mirror back into null assembly  412 . The reflected light rays enter beamsplitter  702 , which partially reflects the light rays to form aberrated return beam  420 . Aberrated return beam  420  then passes the cutting plane along focal point  712 . The motion of Foucault knife-edge test  704 , controlled by knife-edge translation stage  706 , cuts return beam  420 . Aberrated return beam  420 , sequentially occluded and not-occluded by the knife-edge, enters pupil imaging camera  708 , to be imaged by CCD array  710 . 
       FIG. 8  shows a Foucault knife-edge in accordance with an embodiment of the invention, generally designated as  802 . Knife-edge  802  is an opaque surface including a plurality of V-shaped edges that are perpendicular to each other to form a series of successive apexes to the Z-axis. The first V-shaped edge  804   a  is larger than the adjacent V-shaped edge  804   b . V-shaped edge  804   b  is larger than the adjacent V-shaped edge  804   c , and so on, up to V-shaped edge  804   n.    
     Operationally, an aligned segmented mirror, in which all segments behave as an ideal mirror, would return a single spot to the focal point  712 . When the segmented mirror is not aligned, however, the light rays returned from the mirror form separate spots  806  for each misaligned segment of the segmented mirror. The spots are occluded by the opaque surface of the knife-edge, when the opaque surface is positioned to cut the light rays of aberrated return beam  420 . When the knife-edge does not cut the light rays, the light rays are not occluded and pass onto the imaging camera.  FIG. 8  shows a sequence of light rays as spots  804   a ,  804   b ,  804   c , up to  804   n , which become sequentially a single cluster of spots (or a single spot) as the segmented mirror is tilt-adjusted to behave like a single mirror. 
     Knife-edge  802  is incrementally driven in the X,Y plane so that the cluster of spots  806  arriving in the Z-plane along axis  711  ( FIG. 7 ) gradually progresses from a state of complete non-occlusion to a state of complete occlusion as the V-shaped edges become smaller. As shown by arrow  808 , the largest V-shaped edge is introduced first into the aberrated return beam  420 , and the smallest V-shaped edge is introduced last. 
     Smaller V-shaped edges of knife-edge  802  are introduced as the segmented mirror is adjusted into fine alignment, ultimately resulting in the spots being coincident at focal point  712 . As each spot of light  806  is occluded by knife-edge  802 , it is possible to determine its vector location and thus align the mirror segments to form a single focal point. Once focus is achieved, fine sensing module  410  may be retracted into its stowed position and light rays  414  from the interferometer may be used to interrogate the segmented mirror for further testing. 
     The pupil imaging camera images the spots passed between the V-shaped edges of the knife-edge.  FIGS. 11A-11P  show the spots as the Foucault knife-edge sequentially occludes and does not occlude the light beams. In  FIG. 11A , the light beams are completely not occluded and in  FIG. 11H , the light beams are completely occluded. As the knife-edge is introduced into the aberrated return beam  420  and as the V-shaped edges become sequentially smaller, more light beams are occluded by the knife-edge.  FIGS. 11-11P  show the opaque surface of the knife-edge sequentially moving out of the light beam so that the light beam is completely occluded, as shown in  FIG. 11I , and is completely noon-occluded, as shown in  FIG. 11P . 
       FIG. 9  shows an exemplary null assembly  902 . Null assembly  902  includes aspheric mirror  912  and spherical imaging mirror  904 . The spherical imaging mirror  904  images the reflecting surface of aspheric mirror  912  either near or onto the optical surface under test (not shown). Spherical imaging mirror  904  is disposed between a light source (not shown) and aspheric mirror  912 . Aspheric mirror  912  is disposed between spherical imaging mirror  904  and the optical test surface. 
     In this exemplary embodiment, light from a light source is focused on to focal point  908 . Light rays diverging from focal point  908  enter through aperture  906  in spherical imaging mirror  904 . Most of the rays from focal point  908 , after passing through aperture  906 , reflect off the surface of aspheric mirror  912 , and travel back to spherical imaging mirror  904 . The rays are then reflected in the other direction through aperture  910  in aspheric mirror  912 , eventually reaching the surface under test. 
     After reflecting from the surface under test, the light rays re-enter null assembly  902  by passing through aperture  910  in aspheric mirror  912 , next reflecting from spherical imaging mirror  904  and then reflecting from aspheric mirror  912  to pass through aperture  906  for further processing. 
       FIG. 10  shows an exemplary embodiment of a system for testing an optical surface that includes null assembly  902 . In this embodiment, light from light source  118 , such as a laser, enters an optical measuring device  120  such as an interferometer. Light emerging from optical measuring device  120  is collimated and focused on to focal point  908  by focusing optics  122 , which may include a focusing mirror or an objective lens. After being focused, the light propagates through null assembly  902  and then impinges onto mirror surface  124  that is under test. After reflecting from mirror surface  124 , the light propagates through null assembly  902  and eventually re-enters optical measuring device  120 . If optical measuring device  120  is an interferometer (as it is in the embodiment shown in  FIGS. 4-6 ), light from the light source  118  and light reflected from the optical surface  124  interfere with each other within the interferometer. The resulting interference pattern may provide a map of deviations for mirror surface  124  as compared to an ideal surface. It will be appreciated that focal point  908  may be focal point  416  shown in  FIGS. 4-7 . 
     Alternative embodiments may have light from the light source  118  entering null assembly  902 , not through optical measuring device  120 , but through some other means. Such means may include an additional focusing optic and other optical apparatus such as beam splitters or mirrors. In this case the light reflected from surface  124  may also enter the optical measuring device  120  as in  FIG. 10 . For example, optical measuring device  120  may be a wavefront shearing interferometer, a Shack-Hartmann wavefront sensor, a phase diversity sensor or any other single pass optical measuring device. In addition, the system may include fine alignment module  410  that may have a null assembly or focusing optics. 
     Ideally, the optical system is achromatic, or as nearly so as possible. Because the system does not have elements with any appreciable chromatic dispersion, null assembly  902  and optical measuring device  120  may use white light, multi-spectral light, multi-wavelength light, or broadband light without degradation due to chromatic aberration or other forms of degradation in the interference pattern. In general, any portion of the electromagnetic spectrum, either in part or in whole, may be used. 
     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.