OPTICALLY SPARSE PRIMARY APERTURE FOR HIGH SPATIAL RESOLUTION IMAGING

Description

CROSS-REFERENCE TO RELATED APPLICATIONS AND PRIORITY

[0001] The present application claims priority to Indian application no. 202321046439, filed on July 11, 2023.TECHNICAL FIELD

[0002] The disclosure herein generally relates to telescopes, and, more particularly, to optically sparse primary aperture mirrors for space-based earth-observation telescopes.BACKGROUND

[0003] Scientific objectives from earth observation to astronomy require high-resolution observations from space-based platforms. However, designing space telescopes with large primary apertures to achieve high-resolution and high Signal-to-Noise Ratio observations, especially for those operating in longer wavelengths (like Thermal Infrared, TIR), is not feasible due to difficulties in manufacturing, launching, and post-deployment stabilizing.

[0004] The most common technique for achieving high spatial resolution Earth-observation images from space-based telescopes is to increase the collecting area viz: the primary aperture size. At a given wavelength (say, 10 µm), as the primary aperture diameter is increased from a few centimeters to a few tens of meters, the corresponding PSF becomes sharper, and the image resolution and contrast improve. An alternative to the challenge of deploying large mirrors in space is the use of a space-deployable optically sparse mirror with smaller segments. Such kind of OSA mirrors can be easily manufactured and reduces the mass budget for space-missions. A conventional partially filled primary aperture has the limitation of sidelobes that keep on increasing as the number of sub-apertures are increased, gradually obliterating the finer features and low contrast images. With increasing number of sub-apertures, the number, and amplitudes of the sidelobes increase along with that of the primary maxima, resulting in a trade-off of the imaging quality. Document LI LIU ET AL: "Novel array configuration and its optimization for sparse aperture imaging systems" discloses an array configuration composed of multiple concentric circles is proposed for sparse aperture imaging systems. To get better image quality, one property of the modulation transfer function, called the practical resolution limit, is chosen as the optimization criterion. The optimized array configurations with aperture numbers of 6, 7, 9, 10, and 12 are given by using the simulated annealing algorithm. The comparisons of modulation transfer function with Golay and circle arrays with an equivalent sub-aperture number are implemented. Document JING GUO ET AL: "Study on Optical Synthetic Aperture Imaging System with Seven Sub-Apertures" discloses a space optical technique. Along with the development of earth observation and space exploration, requirements of observation accuracy and resolution capability is becoming higher and stricter. Traditional optical systems could hardly satisfy the demands of modern optical remote sensing. Consequently, various countries begin their research on optical synthetic aperture imaging systems. However, many of these systems are made up of three, six or nine sub-apertures. Until now, some configurations like seven sub-apertures haven't been particularly discussed. Compared with three or six sub-apertures, seven sub-apertures have a larger light collection area, which will lead to higher SNR, and it is benefit for imaging. And for composed by less quantity of sub-apertures, it is easier to achieve co-phasing than nine sub-apertures system. Further, four typical configurations of seven sub-apertures are modeled, and then imaging simulations of these configurations are performed. In order to evaluate imaging performance of different structures, a satellite remote sensing image is used as an imaging target, and then corresponding output images quality of four structures are compared with each other. Document ZHENGUO WAN ET AL: "The Simulated Remote-Sensing Image Acquisition And Restoration Based on Optical Sparse Aperture System" discloses that, in spatial remote-sensing observation to the earth, the optical system aperture of satellite is becoming larger and larger. But the larger aperture lead to the more limits constrained by manufacture costs and system loading. Optical sparse aperture imaging systems are composed of several smaller sub-apertures which are arrayed in some rules. But, because of the aperture of optical sparse aperture imaging system is just partial filling for the equivalent single large one, the system point spread function could be a certain spread and the response to middle-and-low spatial frequency is reduced. Consequently, the resolution of obtained images is blurred. So, the obtained images should be restored to improve image resolution. Further, takes advantage of an optical sparse aperture system established in the laboratory, imaging on an aviation remote-sensing negative, and then obtain the simulated optical sparse aperture remote-sensing images. Resolution of simulated sparse aperture remote-sensing images is greatly increased through the method presented, which incorporates the Laplacian factor in the increment Wiener filter. Through this image restoration process, the limitation of system itself is well compensated. Experiment results show that algorithm of this paper is proper to many kind of sparse system with different array structure and filling factor, the simulated remote-sensing images resolution and SNR (Signal and Noise Ratio) could be improved greatly. Document CN 101 315 466 B discloses relates to the high-resolution optical imaging technology field and to a pupil filtering and synthetic aperture optical ultrahigh-resolution imaging method. The method adopts the pupil filtering technology and the synthetic aperture optical imaging technology to conduct imaging detection to subjects, augment the vision field of an imaging system to achieve large-aperture or large working distance measurement through the synthetic aperture imaging technology, raise the resolution of a sub-aperture imaging system with an ultra-resolution pupil filter, and finally achieve the ultra-resolution imaging detection by a large-aperture optical system. The method can improve the resolving power of equivalent full-aperture imaging systems and reduce the number of sub-apertures in optical systems and the distance between the sub-apertures, so as to reduce the volume and the complexity of the systems. The method is applicable to high-resolution, large field angle, large working distance and large-aperture optical imaging, particularly to technology fields of space remote sensing, earth observation, large working distance microscopic imaging, etc. Document WATSON SMET AL: "SIDELOBE REDUCTION VIA MULTIAPERTURE OPTICAL SYSTEMS" relates to the fact that impulse responses of multi aperture optical systems can generate large side lobe irradiances. The document discloses a design of multi aperture systems with impulse responses that exhibit sidelobe irradiances less than that of the Airy pattern and central lobe widths no greater than that of a single large aperture of an equivalent diameter. Multi aperture systems composed of 19, 37, 61, and 91 apertures satisfy these performance criteria. However, the amount of energy in the central lobes of the multi aperture systems was less than that of a single large aperture. Document FR2858425A1, discloses an instrument which has a unit to move pupils (2, 4) with respect to a distant object such that a detector records successively n images corresponding to n successive positions of the pupils. A calculation unit employs an algorithm for processing the images to deconvolute the combination of the images and restore an image having a spectrum that is comparable to an image that would be obtained by a telescope of diameter equal to a circle in which the pupils move. An independent claim is also included for the method of observing a remote object.SUMMARY

[0005] Embodiments of the present disclosure present technological improvements as solutions to one or more of the above-mentioned technical problems recognized by the inventors in conventional systems. The invention is set out in appended set of claims.

[0006] It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.BRIEF DESCRIPTION OF THE DRAWINGS

[0007] The accompanying drawings, which are incorporated in and constitute a part of this disclosure, illustrate exemplary embodiments and, together with the description, serve to explain the disclosed principles: FIG. 1 illustrates a first design of an optically sparse primary aperture according to some embodiments of the present disclosure. FIG. 2 illustrates a second design of the optically sparse primary aperture according to some embodiments of the present disclosure. FIG. 3 illustrates a third design of the optically sparse primary aperture according to some embodiments of the present disclosure. FIG. 4 illustrates example results obtained by the first design, the second design, and the third design, of the optically sparse primary aperture, according to some embodiments of the present disclosure. DETAILED DESCRIPTION OF EMBODIMENTS

[0008] Exemplary embodiments are described with reference to the accompanying drawings. In the figures, the left-most digit(s) of a reference number identifies the figure in which the reference number first appears. Wherever convenient, the same reference numbers are used throughout the drawings to refer to the same or like parts. While examples and features of disclosed principles are described herein, modifications, adaptations, and other implementations are possible without departing from the scope of the disclosed embodiments.

[0009] Most common technique for achieving high spatial resolution Earth-observation images from space-based telescopes is to increase the collecting area viz; the primary aperture size. At a given wavelength (say, 10 µm), as the primary aperture diameter is increased from a few centimeters to a few tens of meters, the corresponding PSF becomes sharper, and the image resolution and contrast improve. An alternative to the challenge of deploying large mirrors in space is the use of a space-deployable optically sparse mirror with smaller segments. Such kind of OSA mirrors can be easily manufactured and reduces the mass budget for space-missions. A conventional partially filled primary aperture has the limitation of sidelobes that keep on increasing as the number of sub-apertures are increased, gradually obliterating the finer features and low contrast images. With increasing number of sub-apertures, the number, and amplitudes of the sidelobes increase along with that of the primary maxima, resulting in a trade-off of the imaging quality.

[0010] In order to address these challenges, the embodiments disclosed herein provide an optically sparse primary aperture for high spatial resolution imaging. The optically sparse primary aperture includes a central sub-aperture, and a plurality of peripheral sub-apertures encompassing the central sub-aperture. Size of the central sub-aperture and the plurality of peripheral sub-apertures is in a proportionate relationship. Further, the plurality of the peripheral sub-apertures forms at least two concentric zones, wherein each concentric zone has equal number peripheral sub-apertures from among the plurality of peripheral sub-apertures, and the sizes of the peripheral sub-apertures in each two adjacent concentric zones have a proportionate relationship. Three different designs / configurations of the optically sparse primary aperture have been disclosed herein, which have been found to be facilitating quality imaging while remaining light-weight, and configurable.

[0011] Referring now to the drawings, and more particularly to FIG. 1 through FIG. 4, where similar reference characters denote corresponding features consistently throughout the figures, there are shown preferred embodiments and these embodiments are described in the context of the following exemplary system and / or method.

[0012] The optically sparse primary aperture includes a central sub-aperture 101, and a plurality of peripheral sub-apertures 102 encompassing the central sub-aperture. Size of the central sub-aperture and the plurality of peripheral sub-apertures is in a proportionate relationship. The plurality of the peripheral sub-apertures forms at least two concentric zones. Each concentric zone has equal number of peripheral sub-apertures from among the plurality of peripheral sub-apertures, and the sizes of the peripheral sub-apertures in each two adjacent concentric zones have a proportionate relationship. Three different designs or configurations of the optically sparse primary aperture are depicted in FIGS. 1 through 3.

[0013] The configuration in FIG. 1, also referred to as a first design, depicts that the plurality of peripheral sub-apertures 102 is equally distributed in three concentric zones. A first concentric zone of the three concentric zones has the peripheral sub-apertures with diameter one third of that of the diameter of the central sub-aperture, a second concentric zone of the three concentric zones has the peripheral sub-apertures with diameter one third of that of the diameter of the central sub-aperture, and a third concentric zone of the three concentric zones has the peripheral sub-apertures with diameter equal to the diameter of the central sub-aperture. In this configuration, the central sub-aperture has a diameter of 0.006m. A Point Spread Function (PSF) profile of this configuration indicated that an associated primary maxima coincides with that of a standard design with significant sidelobe suppression than any of the equal-sized sub-aperture designs. A fill-factor for this configuration with respect to that of the standard OSA is defined as: Fill Factor = N sub * d sub / N standard * d standard , Where, N sub and d sub represent the number of segments and diamter of each segment for the design. N standard and d standard represent the same for the standard configuration.

[0014] Considering N standard = 12 with d standard = 0.01 m, the fill factor for the configuration in FIG. 1 is ~33%. With an assumption that the sub-apertures in both these configurations have unit thickness and made of the same material, the corresponding mass ratio was found to be about 1 / 3rd of the standard design. Also, in comparison with a standard configuration, weight of support framework required for the configuration in FIG. 1 was found to be less by 70%. This is with an assumption that the cross-section and material of the supporting rods are same in both the configurations.

[0015] The configuration in FIG. 2, also referred to as a second design, depicts that the plurality of peripheral sub-apertures is equally distributed in three concentric zones. Size of peripheral sub-aperture in each concentric zone is reduced based on one or more coefficients of the Taylor expansion series, for ln, such that, the peripheral sub-apertures in a first concentric zone among the three concentric zones have a diameter half that of the central sub-aperture, the peripheral sub-apertures in a second concentric zone among the three concentric zones have the diameter one third that of the central sub-aperture, and the peripheral sub-apertures in a third concentric zone among the three concentric zones have the diameter one-fourth that of the central sub-aperture.

[0016] The configuration in FIG. 2 was found to be resulting in a mass (and, fill factor) reduction to 37% of that of the standard configuration, and could be implemented using the same number of peripheral apertures as in the configuration in FIG. 1, with the central aperture of size 0.01 m. The total number of peripheral apertures is 25, where 24 apertures are arranged in sets of 8 around the central aperture and at the same coordinates as in the configuration in FIG. 1.

[0017] The PSF profile of the configuration in FIG. 2 indicated that it is very close to that of a 12 sub-aperture configuration where each segment is 0.01 m. However, the PSF is broader with the sidelobes shifted significantly away from the peak. However, the mass of the support framework for this configuration is ~80% to that of the standard configuration.

[0018] The configuration in FIG. 3, also referred to as third design, depicts that the plurality of peripheral sub-apertures is equally distributed in three concentric zones and size of the plurality of peripheral sub-apertures in each of the three concentric zones is reduced as per one or more coefficients of a Taylor expansion series, for inverse of tan function, such that, the peripheral sub-apertures in a first concentric zone among the three concentric zones have a diameter equal to one-third of the central sub-aperture, the peripheral sub-apertures in a second concentric zone among the three concentric zones have the diameter equal to one-fifth that of the central sub-aperture, and the peripheral sub-apertures in a third concentric zone among the three concentric zones have the diameter equal to one-seventh that of the central sub-aperture.

[0019] In this configuration, the diameter of the peripheral sub-apertures reduces from the central sub-aperture according to the coefficients of the Taylor expansion series for tan -1< (x) i.e., 1 / 3,1 / 5,1 / 7. The central sub-aperture has a diameter of 0.02 meter and the peripheral ones steadily decrease with increasing distance from the central aperture. The PSF profile of this design, however, indicated that the primary maximum is much stronger (lying somewhere between the PSFs of a 17 sub-apertures and a 20 sub-apertures model) along with maximum smoothing out of the sidelobes. The configuration in FIG. 3 is found to have a resultant fill factor ~80% of that of the 12-aperture standard configuration. The corresponding mass of the support framework is found to be nearly equal to that of the standard configuration.Experimental data and imaging performance:

[0020] The configurations in FIGS. 1 through 3 have less collecting area than the standard configuration. To do a comparative study of their imaging quality, an input panchromatic image from the Skysat data archive of the Birmingham airport scene on 9th April 2020 with a ground spatial distance of 81 cm (https: / / www.satimagingcorp.com / satellitesensors / skysat-1 / ) was used. This input image was rescaled to a coarser resolution matching an imaging system with pixel size 10 µm, focal length 0.1 m and operating at 500 km. In the top panels of FIG. 4 (i.e., panels o1, o2, o3 and o4), the image observed (using convolution with the PSF) by the standard configuration, and the configurations in FIGS. 1 through 3, respectively are depicted. The observed images were found to lack clarity for low collecting area designs.

[0021] As a part of the image reconstruction technique (with deconvolution), a standard approach considered demonstrated that the quality of an image captured by a partially filled system can be improved at the post-processing level by incorporating the PSF of a large monolithic mirror. Taking this into consideration, the restored images are shown in panels r1, r2, r3 and r4. It was seen that in all these three configurations, most features have been reconstructed back with equally good contrast and resolution as the standard configuration. It was also noted that the Peak-Signal-to-Noise-Ratio (PSNR) for the standard configuration is 38.95 dB, which reduced to about 38.60, 38.73 and 38.86 dB for the configurations in FIG. 1, FIG. 2, and FIG. 3 respectively. The Structural Similarity Index Measure (SSIM) also was found to be reducing gradually from 0.994 (for standard design) to 0.982, 0.985 and 0.991 for the configurations in FIG. 1, FIG. 2, and FIG. 3 respectively.

[0022] Bottom panels (d1, d2, d3 and d4) plot the difference of the restored images with that of the input image. From the results, it can be observed that the performance deterioration is marginal while providing a considerable gain in terms of the overall weight.

[0023] The three designs / configurations may be used for high spatial resolution imaging or any other similar imaging applications. For example, the designs maybe of a telescope that is used for the high spatial resolution imaging.

[0024] The written description describes the subject matter herein to enable any person skilled in the art to make and use the embodiments. The scope of the subject matter embodiments is defined by the claims and may include other modifications that occur to those skilled in the art. Such other modifications are intended to be within the scope of the claims if they have similar elements that do not differ from the literal language of the claims.

[0025] The embodiments of present disclosure herein address unresolved problem of large aperture requirements of telescopes for intended performance. The embodiment thus provides three configurations for optimum imaging performance of telescopes.

[0026] It is to be understood that computer-readable storage means may contain program-code means for implementation of one or more steps of the method, when the program runs on a server or mobile device or any suitable programmable device. The hardware device can be any kind of device which can be programmed including e.g., any kind of computer like a server or a personal computer, or the like, or any combination thereof. The device may also include means which could be e.g., hardware means like e.g., an application-specific integrated circuit (ASIC), a field-programmable gate array (FPGA), or a combination of hardware and software means, e.g., an ASIC and an FPGA, or at least one microprocessor and at least one memory with software processing components located therein. Thus, the means can include both hardware means and software means. The method embodiments described herein could be implemented in hardware and software. The device may also include software means. Alternatively, the embodiments may be implemented on different hardware devices, e.g., using a plurality of CPUs.

[0027] The embodiments herein can comprise hardware and software elements. The embodiments that are implemented in software include but are not limited to, firmware, resident software, microcode, etc. The functions performed by various components described herein may be implemented in other components or combinations of other components. For the purposes of this description, a computer- usable or computer readable medium can be any apparatus that can comprise, store, communicate, propagate, or transport the program for use by or in connection with the instruction execution system, apparatus, or device.

[0028] The illustrated steps are set out to explain the exemplary embodiments shown, and it should be anticipated that ongoing technological development will change the manner in which particular functions are performed. These examples are presented herein for purposes of illustration, and not limitation. Further, the boundaries of the functional building blocks have been arbitrarily defined herein for the convenience of the description. Alternative boundaries can be defined so long as the specified functions and relationships thereof are appropriately performed. Alternatives (including equivalents, extensions, variations, deviations, etc., of those described herein) will be apparent to persons skilled in the relevant art(s) based on the teachings contained herein. Such alternatives fall within the scope of the disclosed embodiments. Also, the words "comprising," "having," "containing," and "including," and other similar forms are intended to be equivalent in meaning and be open ended in that an item or items following any one of these words is not meant to be an exhaustive listing of such item or items, or meant to be limited to only the listed item or items. It must also be noted that as used herein and in the appended claims, the singular forms "a," "an," and "the" include plural references unless the context clearly dictates otherwise.

[0029] Furthermore, one or more computer-readable storage media may be utilized in implementing embodiments consistent with the present disclosure. A computer-readable storage medium refers to any type of physical memory on which information or data readable by a processor may be stored. Thus, a computer-readable storage medium may store instructions for execution by one or more processors, including instructions for causing the processor(s) to perform steps or stages consistent with the embodiments described herein. The term "computer-readable medium" should be understood to include tangible items and exclude carrier waves and transient signals, i.e., be non-transitory. Examples include random access memory (RAM), read-only memory (ROM), volatile memory, nonvolatile memory, hard drives, CD ROMs, DVDs, flash drives, disks, and any other known physical storage media.

[0030] It is intended that the disclosure and examples be considered as exemplary only, with a true scope of disclosed embodiments being indicated by the following claims.

Claims

1. An optically sparse primary aperture for high spatial resolution imaging, comprising: a central sub-aperture (101); and a plurality of peripheral sub-apertures (102) encompassing the central sub-aperture (101), wherein, size of the central sub-aperture (101) and the plurality of peripheral sub-apertures (102) is in a proportionate relationship, and the plurality of the peripheral sub-apertures (102) forms at least two concentric zones, wherein each concentric zone has equal number of peripheral sub-apertures (102) from among the plurality of peripheral sub-apertures (102), and the sizes of the peripheral sub-apertures (102) in each two adjacent concentric zones have a proportionate relationship, wherein the optically sparse primary aperture aids in quality imaging while remaining light-weight and wherein the optically sparse primary aperture is configured in any one of configurations comprising: i) the plurality of peripheral sub-apertures (102) equally distributed in three concentric zones, comprising: a first concentric zone having the peripheral sub-apertures (102) with diameter one third of that of the diameter of the central sub-aperture (101); a second concentric zone having the peripheral sub-apertures (102) with diameter one third of that of the diameter of the central sub-aperture (101); and a third concentric zone having the peripheral sub-apertures (102) with diameter equal to the diameter of the central sub-aperture (101), ii) the plurality of peripheral sub-apertures (102) equally distributed in three concentric zones and size of peripheral sub-aperture (102) in each concentric zone is reduced based on one or more coefficients of the Taylor expansion series, for ln, such that, the peripheral sub-apertures (102) in a first concentric zone among the three concentric zones have a diameter half that of the central sub-aperture (101), the peripheral sub-apertures (102) in a second concentric zone among the three concentric zones have the diameter one third that of the central sub-aperture (101), and the peripheral sub-apertures (102) in a third concentric zone among the three concentric zones have the diameter one-fourth that of the central sub-aperture (101), iii) the plurality of peripheral sub-apertures equally distributed in three concentric zones and size of the plurality of peripheral sub-apertures in each of the three concentric zones is reduced as per one or more coefficients of a Taylor expansion series, for inverse of tan function, such that, the peripheral sub-apertures (102) in a first concentric zone among the three concentric zones have a diameter equal to one-third of the central sub-aperture (101), the peripheral sub-apertures (102) in a second concentric zone among the three concentric zones have the diameter equal to one-fifth that of the central sub-aperture (101), and the peripheral sub-apertures (102) in a third concentric zone among the three concentric zones have the diameter equal to one-seventh that of the central sub-aperture (101).