Patent Publication Number: US-9835491-B1

Title: Spectral, polar and spectral-polar imagers for use in space situational awareness

Description:
PROVISIONAL PRIORITY CLAIM 
     Priority based on Provisional Application Ser. No. 61/650,811 filed May 23, 2012, and entitled “SPECTRAL, POLAR AND SPECTRAL-POLAR IMAGERS FOR USE IN SPACE SITUATIONAL AWARENESS” is claimed. The entirety of the disclosure of the previous provisional application, including the drawings, is incorporated herein by reference as if set forth fully in the present application. 
    
    
     BACKGROUND 
     In Space Situational Awareness (SSA), distant and dim unresolved objects are evaluated to assess configuration, activity and level of threat. Observed signatures depend on the specific illumination and viewing conditions, the materials of the space-borne object and the object orientation. Since the object is unresolved, characterization is often limited to analysis of the resulting (or, a resultant) light curve, which represents the temporal signature variations. The observed signature, or light curve, associated with an object changes as that object spins or otherwise changes in time. This temporal evolution of the light curve provides a fingerprint which can be used to assess the state of the satellite (object). While increasing the integration time may be used to increase the signal of dim objects, the integration time is limited by the temporal resolution required to resolve salient features of the light curve, thus placing a premium on sensor efficiency. 
     Although light curves are established as a powerful tool in SSA, other aspects of optical wavefronts, beyond the reach of traditional SSA, remain unexploited. 
     Accordingly, a need exists for a high-efficiency approach to spectral imaging that augments light curve analysis with spectral information, an approach conceived specifically with the efficiency and temporal sampling requirements of SSA in mind, with the goal of extracting as much information as possible from the optical wavefronts presented by distant, dim unresolved objects. 
     SUMMARY 
     Various implementations of the present invention provide an efficient method for is spectral imaging with an astronomical telescope. In each of various configurations, a coupling optic is employed to form a displaced image of the pupil across an array of lenslets. Each of the lenslets forms an image in a unique spectral band (hereinafter, referred to alternatively as “wavelength set”), and the array of lenslets forms a multi-spectral image. Alternatively implemented, this configuration provides snapshot-mode spectral imagery with high photon collection efficiency that is scalable to ten or more spectral bands. 
     An approach alternative to that described above relative to spectral imaging is configured as a polar imager by using a birefringent material to displace the beam(s) based on polarization instead of wavelength. In one illustrative such configuration, only two lenslets are used to form two images. The argument for using this approach for polar imaging alone is not very compelling since alternative optical configurations that form two images are readily available. However, a spectral-polar imager is envisioned according to which the spectral and polar beam displacement techniques are combined in a single instrument. In accordance with at least one such configuration, the axis of spectral displacement is orthogonal—or at least non-parallel—to that of polar displacement. As with the spectral imaging implementation, this latter approach to spectral-polar imaging is advantageous because it provides snapshot-mode spectral imagery with high photon collection efficiency that is scalable to several (e.g., ten or more) spectral bands in each of two polarizations. 
     One implementation prescribes altering the functionality of a conventional telescope by adding an optical assembly after the focal plane. The additional optics include a collimator, a beam displacer and a lenslet array. An imaging-sensor array (e.g., conventional focal plane array) is positioned at the image plane of the lenslet array, and can comprise, for example, a CCD or CMOS array appropriate for the spectral band of interest. An imaging-sensor array capable of snapshot mode integration facilitates simultaneous sampling. 
     The collimator images the entrance pupil of the telescope onto the plane of the lenslet array. The focal length of the collimator is configured so that the image of the pupil is approximately the same size as one of the individual lenslet apertures. The f-number of the collimator matches that of the telescope, while the field of view is less than or equal to that of the telescope. 
     The beam displacer displaces the parallel pencils of light based on wavelength, polarization or both. Ideally, the beam displacer introduces only displacement and no net angular deviation into the rays since angular deviation will introduce chromatic blur into the resulting imagery, thereby degrading performance. 
     The lenslet array is placed at plane of the exit pupil of the collimator. While the lenslets are similar with the same magnification and f-number, they are not necessarily identical. Each may be tuned as appropriate for the specific wavelength set and polarization being imaged. Baffling between the lenslets is not critical since a field stop can be used to limit image size, thereby preventing image overlap. Spectral filters are not required, but may be used to sharpen band edges or reject unwanted diffraction orders. 
     The technical approach to spectral imaging is to displace the image of the stop in parallel space and then form multiple images by using a lens array. Spectral displacement is achieved by a set of dispersers (for example, a pair of gratings, a pair of prisms or a pair of grisms) comprising first and second optically-dispersive elements that are spatially separated and arranged (oriented) so that dispersion introduced by the first disperser is canceled by the second disperser. The first disperser spectrally deviates the initial polychromatic beam. The spectral beams diverge as they propagate away from the first disperser, and the angular deviation is “corrected” at the second disperser, leaving a net spatial displacement as a function of wavelength. 
     The collimated light beams—now spatially separated based on wavelength—incident on the lenslet array are focused to a common shared) imaging-sensor array, which can be used for simultaneous sampling of the plural wavelength-specific images. Since dispersion is canceled by the optically-dispersive elements, residual color is minimized, if not eliminated, in the images projected by the lenslet array onto the imaging-sensor array. The spectral response of each of a plurality of sensor channels is given by the integration of the spectrally dependent pupil function with the aperture of the lenslet array. The response when the image of the stop is the same size as the lenslet aperture is shown for a five-aperture linear array in the left panel of  FIG. 4 . The response when the image of the stop just fills a ten-aperture dose-pack array is shown on the right of  FIG. 4 . The linear configuration has less channel to channel crosstalk and higher ideal peak efficiency, since the image of the stop matches to the lenslet aperture at one wavelength; however, the dose-pack configuration has higher average efficiency, since there is less dead space at the aperture array. The ideal efficiency for a close-pack array of infinite extent is π·cos(30)/3=0.9069, which is the upper limit of the efficiency. In practice the spectral response will be limited by pupil aberrations. 
     Stray light and crosstalk can be controlled by placing a field stop at the image plane of the telescope. The crosstalk of each configuration can be improved by incorporating spectral filters at the lenslet array apertures. These filters could also be used to reject higher diffraction orders; however, with some loss in efficiency. 
     Alternative implementations apply to polar imaging an approach that is analogous to that described relative to spectral imaging in that the incident optical image beam is displaced based on polarization and a plurality of focusing elements (e.g., lenslets) is used to from multiple images. In various illustrative configurations, polar displacement is achieved using a birefringent crystal (e.g., calcite, quartz or sapphire) in place of the disperser pair in the spectral imager. A collimated input beam leads to two collimated, linearly polarized output beams that are displaced from one another. A two-element lenslet array then forms images for each beam.  FIGS. 5 and 6  illustrate beam displacement through birefringent materials based on polarization. The polar separation and resolution are distinct from their spectral counterparts, since there are only two linear polarization states. There is no fundamental reason that the polar separation can&#39;t be complete. 
     As previously indicated, various embodiments combine spectral and polar beam displacement techniques in a single instrument. In one illustrative implementation, spectral-polar imaging is achieved by fabricating transmission gratings on the opposing faces of a parallel plate made of birefringent material where the optical axis is selected so the extraordinary rays are displaced in the direction perpendicular to the dispersion direction of the grating. In one such embodiment, the lens array consists of two spectral arrays situated in a side-by-side relationship, as shown, for example, in  FIG. 7 . An analogous configuration implementing a close-pack array, such as that in  FIG. 3 , is also envisioned within the scope and contemplation of the invention. 
     An important enabling element of various embodiments is a collimator lens, which performs three key functions: 1) collimating the image beam outputted from the telescope, 2) imaging the pupil aperture onto the lenslet array, and 3) providing the space for the beam displacer. The collimator should be chromatically well-corrected, distortion free and diffraction limited. The focal length of the collimator is given by the diameter of the lenslet aperture and the f-number of the telescope. By way of specific, non-limiting example, for the MRO telescope with a large-format visible FPA, the “perfectly-coupled” collimator will have a focal length of about 30 mm at f/8.7 with a 120 degree FOV. In this specific case, in order to provide space for the beam displacer, the stop must be placed approximately 30 mm from the last lens element. At the same time, the last lens element must encompass the required 120 degree FOV. The result is a 30 mm focal length lens with a minimum clear aperture diameter of 100 mm. 
     An alternative method of performing spectral imaging by optically coupling a telescope to a lenslet array is envisioned. The approach disperses the image of the pupil, then forms parallel images with the lenslet array. This approach promises superior efficiency, which may add value in low light conditions. However, the field of view that can be obtained may be limited. This latter approach also promises a relatively straightforward extension to polar and spectral polar imaging. 
     Representative, non-limiting implementations are more completely described and depicted in the following detailed description and the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically depicts the architecture of an illustrative spectral analysis system and the action of a beam displacer that is incorporated within the spectral analysis system and displaces wavelength sets within a polychromatic beam into discrete beams based on wavelength; 
         FIG. 2  shows an illustrative linearly configured lenslet array for use in association with a spectral analysis system such as the system of  FIG. 1 ; 
         FIG. 3  depicts a lenslet array with a “close-pack” configuration in association with a spectral analysis system such as the system of  FIG. 1 ; 
         FIG. 4  includes comparative graphical representations of the filter response of a linear lenslet array versus a close-pack lenslet array such as those shown in, respectively,  FIGS. 2 and 3 ; 
         FIG. 5  is a schematic representation in which a light beam is split by a birefringent material (e.g. crystal) into two displaced light beams (a light beam with two orthogonal field components) based on polarization instead of wavelength band; 
         FIG. 6  shows the two beams of  FIG. 5  being focused onto an imaging-sensor array; 
         FIG. 7  shows an illustrative lenslet array for use in association with an imaging system that splits an inputted polychromatic light beam into discrete beams based on both wavelength (spectral separation is vertical) and polarization (polar separation is horizontal) of the constituent light within the input beam; and 
         FIG. 8  schematically depicts the optical coupling of a collimator with a telescope. 
     
    
    
     DETAILED DESCRIPTION 
     The following description of apparatus for and methods of analyzing the spectral signature of an imaged scene is illustrative in nature and is therefore not intended to limit the scope of the invention or its application of uses. 
     Referring to  FIG. 1 , the architecture of an illustrative spectral analysis system  100  is schematically represented and includes (i) a data processing system  200 ; (ii) an imaging-sensor array  300  communicatively linked to the data processing system  200  and (iii) an optical system  400  configured for imaging electromagnetic energy emitted from a scene or source event object source event (not shown) external to the optical system  400  onto the imaging-sensor array  300 . 
     The data processing system includes a central processor  210  and a memory  215  and is programmed to execute spectral analysis algorithms  220  as described in more detail further in this description. Alternative implementations incorporate any of a variety of conventional imaging sensor arrays  300  configured to detect wavelengths over a predetermined range of electromagnetic wavelengths. An illustrative non-limiting example of an imaging-sensor array  300  suitable to the illustrative implementation under consideration includes arrayed detectors  310 . 
     The optical system  400  schematically represented in the illustrative implementation of  FIG. 1  includes front-end optics  405  optically aligned with an optical beam displacer  420 . The front-end optics  405  are configured for outputting a polychromatic, collimated image I C  of a selected scene. In the version of  FIG. 1 , the front-end optics  405  are schematically represented as a telescope  410  only by way of non-limiting example; alternative arrangements fulfilling the same collimating function are within the scope and contemplation of the invention. 
     The optical beam displacer  420  in the particular version of  FIG. 1  includes first and second optically-dispersive elements  422  and  426 . The first optically-dispersive element  422  is spatially situated for receiving the collimated image I C  and angularly dispersing constituent wavelength sets within the received collimated image I C  such that the wavelength sets angularly diverge away from the first optically-dispersive element  422  toward the second optically-dispersive element  426 . For purposes of explanation, four illustrative wavelength sets (i.e., λ A , λ B , λ C , and λ D ) are considered. Moreover, the path of each of wavelength sets λ A , λ B , λ C , and λ D  is represented schematically by appropriately labeled ray trace lines. 
     Referring still to  FIG. 1 , the second optically-dispersive element  426  is spatially situated for receiving the wavelength sets λ A , λ B , λ C , and λ D  dispersed by the first optically-dispersive element  422  and reducing the degree of mutual angular divergence, while maintaining mutual spatial displacement, among the wavelength sets λ A , λ B , λ C , and λ D . More specifically; the first optically-dispersive element  422  introduces desired separation of the polychromatic light within the collimated image I C  into spatially-separated, discrete wavelength sets. However, if these wavelength sets impinge upon the plane of the imaging-sensor array  300  at angles substantially deviant from perpendicular, an unacceptably high degree of chromatic blur will be introduced into the resultant imagery. Accordingly, the second optically-dispersive element  426  serves to “undo” or “cancel” as much as practicable the angular deviation introduced by the first optically-dispersive element  422 . In an ideal embodiment, the wavelength sets λ A , λ B , λ C , and λ D  would propagate toward the imaging-sensor array  300  along mutually parallel axes (ray trace lines), and impinge orthogonally upon the imaging-sensor array  300 . In this sense, the light, while divided into discrete wavelength sets λ A , λ B , λ C , and λ D  is re-collimated. 
     It will be readily understood that the beam displacer  420  can achieve beam displacement with alternative optical elements, and that the labeled “boxes” in the drawings are merely schematic representations of optically-dispersive elements  422  and  426 . Illustrative examples of alternative optically-dispersive elements suitable for implementation within the beam displacer  420  include prisms, diffraction gratings and “prisms,” the latter being hybrid elements having both diffractive and refractive properties. 
     The illustrative embodiment of  FIG. 1  further includes a plurality of back-end optical focusing elements  460 . Each focusing element  460  is optically aligned for projecting (focusing) upon a corresponding unique location  320  along the imaging-sensor array  300  the electromagnetic wavelengths associated with one of the spatially displaced wavelength sets λ A , λ B , λ C , and λ D . For clarity in the description and associated drawings (including  FIG. 1 ), each optical focusing element is identified by a reference character including the number “ 460 ” that identifies it generally as an optical focusing element, followed by a capital letter (e.g., A, B, C . . . ) identifying it more specifically as being associated with the projection of a particular wavelength set (e.g., λ A , λ B , λ C , etc.) upon a corresponding location (or section)  320  of the imaging-sensor array  300 . Similarly, each imaging-array section  320  upon which the wavelengths of a wavelength set λ A , λ B , λ C  or λ D  are focused is referenced by a set of characters including the number “ 320 ,” identifying it as an imaging-array section, followed by a capital letter (e.g., A, B, C . . . ) identifying it more specifically as being associated with the detection of a particular one of wavelength sets λ A , λ B , λ C , and λ D . In  FIG. 1 , imaging-array sections  320  are delineated from one another by dashed boundaries. When referred to collectively or non-specifically in this description, the imaging-array sections  320  and focusing elements  460  are referenced without alphabetic characters, despite the fact that no such “collective” or “generalized’ reference character is used in the drawings. 
     The optical focusing elements  460  of various versions are alternatively arranged and “packaged.” However, the use of a lenslet array is advantageous because the focusing elements  460  are then incorporated into a relatively robust unitary structure. The focusing elements  460  of  FIG. 1  are illustratively incorporated as part of a lenslet array  470  schematically indicated by a generic dashed-line box encompassing the focusing elements  460 . The lenslet array  470  can be alternatively configured; an illustrative linear configuration is shown in  FIG. 2 , while a “close-pack” configuration is shown in  FIG. 3 . 
     The foregoing is considered to be illustrative of the principles of the invention. Furthermore, since modifications and changes to various aspects and implementations will occur to those skilled in the art without departing from the scope and spirit of the invention, it is to be understood that the foregoing does not limit the invention as expressed in the appended claims to the exact construction, implementations and versions shown and described.