Ring optical interferometer

A multi-aperture interferometric optical system collects light propagating from a source of light and develops overlapping diffraction patterns on an optical detector that produces output signals for processing to form an image corresponding to the diffraction patterns. A preferred embodiment of the invention is a large aperture orbiting, earth-watching ring interferometric optical system configured such that there is no macro-structure pointing. Four mirror-ring structures direct incoming light to a multi-spectral primary optical detector that acquires light-pattern information from which an image can be derived.

TECHNICAL FIELD

This invention relates to large aperture optical systems adapted to form high resolution images of distant extended scenes and, in particular, to a multi-aperture interferometric optical system that collects light propagating from a source of light and develops a diffraction pattern on an optical detector that produces output signals for processing to form an image corresponding to the diffraction pattern.

BACKGROUND OF THE INVENTION

The theoretical advantages of placing telescopes above the distorting atmosphere have been well known and practically pursued for about four or more decades. Briefly stated, these advantages include sharper images and accessibility to a broader range of wavelengths. The Hubble Space Telescope and NASA's upcoming NGST (Next Generation Space Telescope) are particularly well known examples of spaceborne telescopes. Remote sensing satellites beginning with Landsat and Spot, and more recently systems launched and operated by Space Imaging, Digital Globe, and Orbimage, represent earth-pointing examples of telescopes, known to skilled persons as “large aperture cameras.” There are, likewise, but slightly less well known, similar advantages to placing optical interferometers into space. Examples of such systems include NASA's SIM (Space Interferometry System) and SIRTF (Space Infrared Telescope Facility).

In many respects, telescopes and optical interferometers are designed with the same result in mind, namely, to measure the optical energy distribution of a spatial “scene” or of some “object.” Telescopes do so by forming a single image of an object or a scene, whereas optical interferometers explicitly measure the amplitude and phase of specific spatial frequencies of an object or a scene. Both devices can do so across a range of bands in the spectral dimension. By post-processing images derived from telescopes, one can readily obtain interferometer-like spatial frequency measurements; and by post-processing data from an optical interferometer, one can readily obtain telescope-like images, especially if a complete set of spatial frequencies has been measured.

A form of telescope implemented with non-full apertures was introduced and practically pursued before, but achieved popularity during, the 1980s. Such telescopes are referred to as “sparse array,” “phased array,” or “multi-aperture” telescopes. The basic notion of sparse array telescope design is to “coherently combine” several smaller telescopes, or sub-apertures, to achieve the resolving capabilities of a much larger telescope. An example of a multi-aperture imaging system is described in U.S. Pat. No. 6,905,591 for Multi-Aperture Imaging System. The premise underlying the operation of sparse array telescopes is that the spatial autocorrelation function of any given mirror configuration containing no drop-out points (“nulls” in spatial frequency space) achieves telescopic “imaging” or “full-coverage spatial frequency” optical interferometry in the absence of monolithic (or pseudo-monolithic, segmented) mirrors. Such a mirror configuration reduces cost and complexity. The accepted cost of implementing this relatively inexpensive approach is a reduction in light gathering capability, hence resulting in higher effective f/numbers and longer exposure times. The intended result is that much larger telescopes could be contemplated and built, thereby increasing the resolution of state of the art systems within acceptable cost budgets dictated by public security concerns and scientific endeavor priorities.

The cost virtues of sparse array telescopes have been and are now duly extolled and elucidated. At the same time, a number of various specific designs that attend to the unique design challenges presented by very large, space-based structures have been presented and sometimes implemented, at least in simulations. Noteworthy among these challenges is the need to position many optical mirrors to accuracies initially approaching and usually much finer than the wavelengths of visible light. This challenge has been referred to as “phasing” or what most people would call “maintaining focus.” Moreover, the long-established optical interferometric principle of pointing only the sub-apertures (i.e., not the whole structure) and allowing delay lines to maintain coherence is a clear design requirement for most, if not all, realistic approaches to 10-meter and larger outside-aperture class systems. In addition to the generic and given requirements for a sparse array telescope, various provisions have been envisioned, built, and tested in structures that are to be initially compactly stowed in a given structure for launch and later deployed into an operational configuration.

All of the foregoing basic requirements were well described in the 1980s, and a wide variety of specific design implementations approaching these requirements have ensued. With only a few exceptions, which tend to be classic optical interferometers in character, the sheer cost and complexity of actually building, testing, launching, and operating sparse array telescopes have, to date, permitted production of no known operational system. It has generally been found that actual structural implementation of these conceptual designs is far more difficult than simply describing the now well-understood theoretical requirements that the work of the 1980s and 1990s outlined.

SUMMARY OF THE INVENTION

The present invention embodies an overall functional approach to the design and construction of very large sparse array telescopes in the form of a multi-aperture interferometric optical system. The invention implements an approach that explicitly negates the classic telescope design notion of forming a singular image through coherent beam combination, which works well in a laboratory at smaller scales. The invention implements a field-variant object-space sampling approach much more akin to wide-field wavefront-estimate-assisted speckle imaging through a turbulent atmosphere, as described in U.S. Pat. No. 6,084,227 for Method and Apparatus for Wide Field Distortion Compensated Imaging ('227 patent), on which applicant is named as inventor. This technique treats a very large structure as though it were just another kind of atmosphere, albeit an “atmosphere” that is in general a little better behaved than the one earth-bound astronomers have been considering for centuries.

Stated in simpler terms, the invention does not implement an attempt to beam-combine a singular image onto an electronic sensor or light sensitive film under the expensive assumption that one has physically forced a large structure to maintain nanometer-scale intra-positional accuracies. The invention instead expects and accepts major perturbations in the large physical structure that collects optical energy from an object or a scene and interprets the electronically sensed data (what classic telescope principles might term “gross distortions,” and what classic optical interferometry might term “very complicated biasing in time, space, spatial amplitude and phase”) with reference to its known errors (or knowable biases). The price for this approach is the same as that paid to examine sparse array designs, namely, exposure time and the time-efficiency of imaging static and dynamic objects. The objective of this approach is also the same, which is the design of a large system that is practicable and operable within acceptable budgets.

A preferred embodiment of the invention is a large aperture orbiting, earth-watching ring interferometric optical system using 23 nominally identical commercial-off-the-shelf (COTS)-grade convex primary mirrors. The nominal orbit of the optical is geo-synchronous, with designed ground resolution of approximately 0.8 meter at 500 nanometers. The optical system is configured such that there is no macro-structure pointing; the primary mirrors are responsible for gross target pointing. Secondary, tertiary, and quaternary mirror ring structures receive light reflected by the primary mirrors and steer the reflected light to a detector plane, where a multi-spectral primary optical detector is positioned. There are 23 mirror arms defined by a primary mirror and corresponding secondary, tertiary, and quaternary mirrors that direct the incoming light along a path to the primary optical detector. The design approach of the preferred embodiment described uses a linear piston system in association with the tertiary mirrors to perform most of the optical path length (OPL) equalization and tip/tilt devices in association with the secondary and quaternary mirrors to perform most of the equalization to make common the effective focal lengths and focal planes of each of the mirror arms of the optical system. An alternative embodiment uses a single quaternary mirror in association with tip/tilt and linear piston positioning systems as a trade-off for more complicated positioning systems for the secondary and tertiary ring structures.

Initial and ongoing phasing is achieved through optical detector feedback loops into either the secondary or tertiary mirrors. The multi-spectral primary optical detector includes four detector elements, each viewing a 20 nanometer-50 nanometer bandpass image, tunable over the visible through near-IR spectrum. The overall “instantaneous but rapidly changing” wavefront error budget is on the order of one wavelength (or even worse), rather than the more traditional one-tenth wavelength.

Additional objects and advantages of this invention will be apparent from the following detailed description of a preferred embodiment thereof, which proceeds with reference to the accompanying drawings.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

FIGS. 1 and 2show an isometric view and an enlarged, fragmentary rearward view, respectively, of a preferred embodiment of an orbiting multi-aperture interferometric optical system10constructed in accordance with the invention. With reference toFIGS. 1 and 2, optical system10is composed of a main body in the form of a central hub12and a secondary assembly in the form of a plate structure14spaced apart from each other by connection to the opposite ends of three cylindrical columns16. Central hub12is composed of two separate cylindrical structures18and19, the larger structure18containing most of the satellite equipment and the smaller structure19providing an anchor for a primary mirror structure20and containing a multi-spectral primary optical detector. Four mirror ring structures, two each connected to central hub12and plate structure14, direct incoming light to the multi-spectral primary optical detector positioned at a detector plane on central hub12.

FIG. 1shows that central hub12is connected to primary mirror structure20by five cylindrical support rods22radially extending from a main fairing24of cylindrical structure19of central hub12to a ring truss26. Ring truss26supports23primary mirrors28, each set within a cup-shaped fairing30.FIG. 2shows an inner surface40of plate structure14encircled by an annular secondary fairing42and supporting23secondary mirrors44spaced apart along the periphery of plate structure14. Secondary mirrors44are positioned such that their light reflecting surfaces confront those of primary mirrors28.

FIG. 1shows that a recessed top surface50of cylindrical structure18of central hub12supports23tertiary mirrors52positioned spaced apart along the periphery of top surface50near main fairing24.FIG. 2shows that a periphery56of an open-ended conical support member58attached to inner surface40of plate structure14supports23quaternary mirrors60. Skilled persons will appreciate that light baffles and shields for the mirrors would be incorporated in an on-orbit optical system. Such baffles and shields would obscure the mirrors shown in the drawings and, therefore, are omitted from them.

FIG. 1shows a box containing the multi-spectral primary optical detector70, which is preferably component of a camera system, mounted at the center of top surface50of central hub12. A tube72extending outwardly from the box toward inner surface40of plate structure14functions as a light baffle for camera system70. A high gain communication antenna74for transmitting and receiving data and control signals from a remote station is mounted on a bottom surface76of plate structure14. Two solar power panels78extend radially outwardly from a support member80of a bottom surface82of cylindrical structure19.

FIGS. 3 and 4show a preferred embodiment of multi-aperture interferometric optical system10that is formed of four ring structures, each including a set of mirrors cooperating to steer incoming light to form a coherent diffraction pattern on a light receiving surface of camera system70. The overall effective focal length of this embodiment of optical system10is 900 meters. With particular reference toFIGS. 1 and 3, an outermost, primary ring structure20supports 23 1-meter diameter primary mirrors28mutually spaced apart along a 25-meter diameter ring truss26, which defines a periphery around which primary mirrors28are arranged. Primary mirrors28are preferably positioned on ring truss26in a slightly asymmetric manner in which certain neighboring pairs of primary mirrors are spaced apart by center-to-center distances of either greater than or less than 2 meters. This asymmetric spacing tends to produce more uniform UV plane signal coverage.

With particular reference toFIGS. 2 and 4, 23 15-centimeter diameter secondary mirrors44are mutually spaced apart along a 3.5-meter diameter plate structure14; 23 tertiary mirrors52of between about 8 centimeters and 9 centimeters in diameter are mutually spaced apart along a 3.5-meter diameter top surface50of cylindrical structure18; and 23 quaternary mirrors60of between about 1 centimeter and 2 centimeters in diameter are mutually spaced apart along periphery56of a 0.4-meter diameter conical support member58. (The ranges indicated for the diameters of the tertiary and quaternary mirrors provide for an optical designer flexibility to optimize the overall system design.) Plate structure14, top surface50, and conical support member58define the peripheries around which secondary mirrors44, tertiary mirrors52, and quaternary mirrors60, respectively, are arranged. The secondary, tertiary, and quaternary mirrors are positioned to receive the light reflected by the primary mirrors and steer it for incidence on camera system70located at a detector plane90.

FIG. 5shows that camera system70includes a dichroic optical element92that splits the incident light into two components and directs them to a primary camera94and a wavefront sensing system96. Camera system70can be implemented with a primary camera94having a beam splitter that further divides the incident beam component into, for example, four components that each strike a different one of four detector elements. (The term “detector” as used herein refers to either a single detector element or multiple detector elements.) A preferred detector element is a charge-coupled device (CCD) of a channel amplification type, which enables high frame rate (i.e., at least 5 frames/second and nominally up to 100 frames/second) light pattern information acquisition. A suitable CCD detector element is an L3Vision CCD87512×512 array with in-channel amplification, manufactured by e2 v technologies. Each detector element preferably views a 20 nanometer-50 nanometer bandpass image that is tunable over the visible through near-IR spectrum. A primary camera94that is capable of imaging multiple spectral bands enables target recognition and other color imaging applications. Wavefront sensing system96provides information for optical system focusing and forming an image from the diffraction pattern.

The choices of the numbers of mirrors, distances between adjacent mirrors, mirror sizes, and related optical parameters for this embodiment were governed by a desire for minimal, efficient achievement of full and pseudo-uniform interferometric spatial frequency coverage.

FIG. 6shows a mirror arm100defined by a primary mirror28and corresponding secondary mirror44, tertiary mirror52, and quaternary mirror60. Mirror arm100has an optical path length segment and an effective focal length segment that are adjustable, as will be described below. With reference toFIG. 6, each mirror in mirror arm100is given its nominal location in a Cartesian coordinate system in which primary mirror28is located at the origin (0,0) and detector plane90is located 12 meters to the right at (12,0). Incoming light propagates through an arbitrary entrance and is reflected by primary mirror28and intermediate mirrors44,52, and60to camera system70positioned at detector plane90.

More specifically, primary mirror28receives incident incoming light and first reflects it to secondary mirror44, at location (10.5,10). Secondary mirror44reflects the light to tertiary mirror52at location (10.25,0), which reflects the light to quaternary mirror60at location (11.8,8), which then reflects the light to camera system70. Skilled persons will appreciate that each primary mirror has a corresponding secondary, tertiary, and quaternary mirror, thereby providing a total of 23 mirror arms configured as depicted inFIG. 6, forming generally concentric rings. Although they are generally concentric, the rings are not all coplanar. As will be evident from the following description, certain of the rings may be tilted, so they need not lie in parallel planes. Moreover, the rings need not be circular. The “generally” concentric qualifier stems for possible small distance lateral shifting of the component rings, resulting in eccentric nesting of the different rings.

The light reflecting surfaces of the primary mirrors are of parabolic shape; and the light reflecting surfaces of the secondary, tertiary, and quaternary mirrors are of pseudo-conic (i.e., aspheric) shape. The mirror element prescription set forth in the table below presents the optical parameters of the mirror elements in each mirror arm100.

Steering of optical system10to point its optical axis102at an angle relative to nadir is effected by reorienting primary mirrors28, each of which can be reoriented, e.g., ±10 degrees, in all directions from its nominal position. Mirror arms100meet three conditions to form a coherent diffraction pattern at detector plane90. These three conditions include nearly identical focal planes, common optical path lengths, and common effective focal lengths. Nearly identical focal planes for mirror arms100cause the bundles of light rays to converge to a single, shared three-dimensional spot for the system optical axis. A common optical path length maintains coherence of the light rays at detector plane90to provide high frequency detail in the diffraction pattern. A common effective focal length maintains constant lateral magnification.

To simultaneously meet these three conditions, three independent variables are controlled, namely, the positions of the secondary, tertiary, and quaternary mirrors. The secondary and quaternary mirrors are associated with positioning systems permitting lateral motions up to about ±20 centimeters and about ±5 centimeters, respectively. The tertiary mirrors are associated with positioning systems permitting lateral motions of up to ±1 meter. In addition to the lateral movements of the secondary, tertiary, and quaternary mirrors, the plane of the secondary mirror ring structure can be tilted by a mechanism that moves inner surface40of plate structure14supporting secondary mirrors44. The plane of the tertiary mirror ring structure can similarly be tilted by a separate mechanism that moves top surface50of a ring-shaped plate (not shown) supporting tertiary mirrors52. Whether the plane of quaternary mirror ring structure is equipped with a tilt mechanism would be a mechanical design option.

In the illustrated preferred arrangement of optical system10, central hub12may have a radius of 1 meter, and tertiary mirrors52form a ring with a nominal radius of 1.75 meters. The tertiary mirrors mounted on top surface50of a ring-shaped plate can be moved laterally relative to hub12and thereby permit eccentric placement of the ring of tertiary mirrors52around the hub12, and be moved relative to the hub by tip and/or tilt motion. The lateral and tip/tilt movements of secondary mirrors44can be achieved by similar arrangements. Thus, a ring supporting structure can provide coordinated movement of all of the mirrors of a ring in lateral and tip/tilt directions.. (In some arrangements, tip/tilt control of the secondary mirrors can be omitted).

Tertiary mirrors52in one implementation are mounted on a linear, piston-driven actuator that permits ±1 meter of vertical movement of top surface50. In this arrangement, the single piston moves all 23 of tertiary mirrors52. Each tertiary mirror52can also be provided with small scale positioning systems for precise vertical positioning from the baseline provided by the shared piston. Each tertiary mirror52on the shared piston also has separate tip/tilt control. Alternatively, instead of employing a single, shared vertical positioning system (with fine individual adjustments), each tertiary mirror52can be mounted on its own linear actuator, thereby permitting independent movement.

Quaternary mirrors60are set to positions dictated by the particular placement and orientation of the primary, secondary, and tertiary mirrors to which each quaternary mirror60corresponds. As such, the quaternary mirrors60typically use highly customized positioning. While a movable ring support structure as described for the secondary and tertiary mirrors may be employed for gross positioning, the position of each quaternary mirror60may be sufficiently independent such that coordinated movement of the 23 mirrors by a movable support ring offers little benefit. In the depicted arrangement, the support structure for the quaternary mirrors60is static and each quaternary mirror60is provided with its own positioners. The quaternary mirrors are generally associated with a position configured to adjust focus only.

FIG. 7shows with reference to primary mirrors28(with fairings30removed for clarity) mounted on ring truss26an exemplary positioning system implementation that can accomplish individual tip/tilt and linear adjustment for each mirror. Although it is shown with respect to primary mirrors28, this positioning system can also be implemented with the secondary, tertiary, and quaternary mirror ring structures. With reference toFIG. 7, each of three extensible pistons110a,110b, and110chas one end mounted in a spherical bearing joint on the bottom side of fairing30of mirror28and the other end fixed to a plate112mounted on ring truss26. Changing the lengths of extensible pistons110a,110b, and110cby equal amounts linearly adjusts the vertical distance between the mirror28connected to them and ring truss26and thereby adjusts the optical path length of the mirror arm100to which mirror28is associated. Changing the lengths of extensible pistons110a,110b, and110cby different amounts accomplishes tip/tilt positioning of mirror28connected to them. (In the preferred embodiment described, primary mirrors28undergo only tip/tilt positioning.) Skilled persons will appreciate that the tip/tilt positioning of primary mirrors28contributes significantly to an angular momentum vector that would tend to tip the mass of the entire structure of optical system10. Ensuring the conservation of angular momentum would maintain the pointing direction stability of optical system10under such conditions. This can be accomplished by the use of countermasses producing an angular momentum vector of opposite direction to that produced by primary mirror movement and other sources of residual angular momentum offset. In other words, these same principles apply to all masses that move and generate an angular momentum vector.

It will be appreciated that positions of the secondary mirrors depend on position of the primary mirrors, and that the positions of the tertiary mirrors depend on the positions of the primary and secondary mirrors. Control arrangements may be devised that mechanically couple movement of secondary mirrors52to primary mirrors28to achieve at least gross positioning. Such control systems are simplified if the relationship of the movements of the components are linear. Nonlinear relationships can be addressed by screw driven cams and similar types of positioning mechanisms.

FIG. 8is a graph showing the optical path length of optical system10relative to nadir and the corresponding movements of secondary mirror44, tertiary mirror52, and quaternary mirror60required to adjust their corresponding optical path length contributions to their associated mirror arm100in response. The abscissa represents the position value of secondary mirror44, and the ordinate indicates the position value of each of tertiary mirror52and quaternary mirror60. (Primary mirror28performs only tip/tilt movement in this example.) Curve114and curve116represent the corresponding movements of tertiary mirror52and quaternary mirror60, respectively, in response to movement by secondary mirror52. Curve118represents the optical path length of the mirror arm100to which the primary, secondary, tertiary, and quaternary mirrors are associated. For example, if there is a command to point system optical axis1025-degrees from nadir, optical system10specifies from memory storage the corresponding optical path length, individually for each of the 23 mirror arms100.FIG. 8provides motion curves that specify the position values of each of secondary, tertiary, and quaternary mirrors to adjust their corresponding optical path length contributions to their associated mirror arm100in response to the command.

Primary mirrors28are mounted on tip/tilt positioners to provide their steering capability. Redundant positioning systems may be employed in anticipation of failures of certain systems in long-term space environments. Thus, a tip-controlling motor may be mounted on a stage controlled by a tilt-controlling motor, which in turn is mounted on top of a second tip-controlling motor, which in turn is mounted on a third tilt-controlling motor. The last two motors would generally remain unused; however, if the first tip/tilt motors fail, the underlying tip/tilt motors can be used to preserve complete operation. In addition, or in the alternative, the motors that control the tip/tilt motions (e.g., through worm gears) can employ redundant motor windings, so that if one motor winding fails, the control system can switch to the backup winding. Loss of mirrors is akin to shuttering small parts of the aperture of the optical interferometer. Resolution would be somewhat impaired if many adjoining mirrors are lost, but overall optical system performance would not be seriously degraded until about 15 of the mirror arm paths are in operation. If operating in a motion target indicator mode, in which moving objects are detected (i.e., no image formation), optical system10can function with as few as 10 mirror arm paths in operation.

When the beams from the 23 primary mirrors are superimposed on the optical detector, a diffraction pattern results in the form of a complicated point spread function. Slight imperfections in the tip/tilt and optical path length adjustment mechanisms characterizing the state of the optical system also complicate the point spread function. This pattern is characterized and compensated-for to yield a final image by practice of known techniques taught by applicant's '227 patent.FIG. 9shows an exemplary diffraction pattern119formed on the light receiving surface of optical detector70. Wavefront sensing system96(FIG. 5) detects biases and misalignments of optical system10, and the '227 patent teaches techniques for measuring the biases and misalignments and how to correct for them. The '227 patent particularly concentrates on the wavefront sensing of low contrast, extended scenes and their notorious difficulties in achieving focus.

From geostationary orbit, it appears that the interferometric optical system described above can achieve an imaging resolution on the order of approximately 0.8 meter. If placed ten times closer to Earth, a resolution ten times greater could be achieved.

Skilled persons will appreciate that, although the preferred embodiment described is implemented with certain degrees of movement (tip/tilt/lateral/vertical) for different mirrors, other embodiments can employ different combinations of movements (including movement of the detector). More generally, while the detailed arrangement employs four sets of mirror (primary, secondary, tertiary, and quaternary), other embodiments can employ more or fewer mirror sets.

An implementation using a single quaternary mirror60with a tip/tilt positioning device (possibly in association with a linear piston (focusing) positioning device) may be substituted for a mirror ring to reduce cost. The effect would be to constrain the quaternary mirror to linear motion at the expense of complicating the shapes of curves114and116representing the positioning of the tertiary mirror52and quaternary mirror60, respectively.

The invention described above was in the context of an orbiting earth-imaging optical interferometer; however, the same arrangement can be pointed to image astronomical subjects. Such an optical interferometer can also be terrestrially based and used to image subjects in and beyond the atmosphere.

FIG. 10is a synoptic flow diagram that summarizes the primary information and control processes underpinning the dynamic function of the invention. The corresponding process blocks are noted in parentheses in the following description.

Instead of taking single images by a “dwelling” detector, as is typical of prior art sparse array designs, primary data are gathered using a high frame rate stream of primary detector interferometric data from multiple spectral bands (120). Attempting to form a single image would produce the same blurry image typical of ground-based astronomy looking through the atmosphere. The primary data are first decomposed into wavelet-like, spatial-spectral data (122), which are then delivered to three cascaded data processing units. They include a forward and backward physical state estimation unit (124), which drives a physical figure (i.e., overall mirror position) control “phasing” system (126). The phasing system not only supports actual figure control but also provides time-delayed “best estimate” physical state information to an image synthesis processing unit (128). The image synthesis processing unit references raw data from the primary sensors against “best estimate biases” synthesized from the raw information of the figure control phasing system and physical state estimation unit, ultimately forming a single high-resolution image of the object/scene (130).

The inclusion of independent wavefront sensors or on-board position sensors (132,134), together with pointing instructions (136), provide data that improve the physical figure control system and/or the best estimate biases. This facilitates constant correction of the satellite and thereby contributes to calculation of mirror actuator commands (138) for the desired overall mirror movement (140). Wavefront sensing system96ofFIG. 5(operating in accordance with teachings of the '227 patent) contributes to the mirror position instrumentation indicated in process block132of FIG.10. Other potential possible contributors to the mirror position instrumentation include on-board laser positioning devices.