Novel ground distributed optical synthetic aperture direct imaging method and device

Abstract

The invention discloses a novel ground distributed optical synthetic aperture direct imaging method and device. The device comprises a plurality of sub-telescopes, a K mirror, a delay line, an entrance pupil mechanism and a light combination telescope. According to the invention, direct imaging is realized by ensuring the pupil mapping relation, and the imaging field of view is larger than that of a non-common-frame Michelson interference array; more complete spatial frequency sampling is carried out on a celestial body target by means of earth rotation and using a small number of small-caliber telescopes, the frequency signal acquisition efficiency is higher than that of a non-common-rack Michelson interference array, and imaging can be carried out on a complex surface source. According to the invention, by adopting the layout that the sub-telescopes are distributed on the ground, compared with a common-frame Fizeau interference array, the length of a base line is not limited by the size of a frame, and higher-resolution imaging can be realized.

Description

Technical Field

[0001] This invention belongs to the field of optical synthetic aperture technology in the field of astronomical observation technology, and particularly relates to a novel ground-based distributed optical synthetic aperture direct imaging method and device. Background Technology

[0002] Optical synthetic aperture technology is an advanced astronomical observation method that aims to simulate a larger-aperture virtual telescope by having multiple independent optical telescopes work together. This overcomes the diffraction limit of a single-aperture telescope and significantly improves angular resolution. Representative facilities include the Very Large Telescope Interferometer (VLTI) at the European Southern Observatory, the Large Binoculars (LBT) and the Keck Interferometer in the United States. In traditional astronomical observation, single-aperture telescopes or mosaic mirror telescopes are limited by their physical size and cannot resolve the fine structures of distant celestial objects. Synthetic aperture technology solves this problem through interferometry. Since Michelson used an astronomical optical interferometer to measure the angular diameter of stars in the early 20th century, this technology has become one of the core tools of modern astronomical observation.

[0003] Currently available optical synthetic aperture imaging technologies can be divided into non-co-rack Michelson interferometry and co-rack Fizeau interferometry.

[0004] Non-co-mounted Michelson interferometry is an indirect imaging technique. It uses multiple ground-based telescopes, each with its own independent tracking rig, to create multiple baselines. Through multi-baseline interferometry, it samples the Fourier spatial information of the target and then reconstructs a two-dimensional image via inverse transform. A schematic diagram of a non-co-mounted Michelson interferometer array consisting of two telescopes is shown below. Figure 1 As shown. This method requires that different sub-telescopes point to the same target and have the same optical path to the beam combiner to ensure beam coherence, but it does not require that the beam arrangement entering the beam combiner be similar to the baseline arrangement.

[0005] This method involves a series of steps that have been validated in systems such as VLTI. The process is detailed below:

[0006] Step 1: Telescope Array Design and Layout

[0007] The spatial arrangement of the telescopes (i.e., baseline configuration) is designed based on the location of the target celestial object and the required resolution. The baseline length determines the maximum resolution of the system. Typically, the array consists of multiple fixed or movable telescopes (such as VLTI's four 8.2-meter main telescopes and several auxiliary telescopes) to cover different spatial frequencies. More intensive spatial frequency sampling can be achieved by utilizing the Earth's rotation.

[0008] Step 2: Beam Collection and Collimation

[0009] Each telescope independently receives incident light waves from the same celestial body and collimates and focuses them using optical systems (such as mirrors or lenses) to ensure the beam remains coherent. This process must take into account the effects of atmospheric turbulence and is typically equipped with adaptive optics systems for real-time correction.

[0010] Step 3: Beam transmission and delay line compensation

[0011] The beams collected by each telescope are transmitted to a beam combiner via vacuum tubes or optical fibers. Because optical path differences can disrupt interference conditions, a delay line system is used to dynamically adjust the optical path length to compensate for changes in optical path caused by the Earth's rotation and baseline geometry. For example, VLTI employs moving mirrors to achieve nanometer-level precision optical path matching.

[0012] Step 4: Interference Fringe Formation and Detection

[0013] In a beam combiner, multiple beams are superimposed to produce an interference pattern (i.e., fringes). The intensity and phase information of the fringes are recorded by a beam splitter and a detector (such as a CCD or infrared camera). The visibility function of the interference fringes is directly related to the brightness distribution of celestial objects and is a key input for image reconstruction.

[0014] Step 5: Data Acquisition and Visibility Function Extraction

[0015] Multiple samples were taken of the interference fringes to obtain visibility function data at different baselines and wavelengths. These data correspond to the Fourier components of the celestial brightness distribution.

[0016] Step Six: Image Reconstruction and Fourier Inversion

[0017] Using the acquired visibility function, a two-dimensional image of the celestial object is reconstructed via inverse Fourier transform. This step relies on sufficient UV plane coverage (i.e., sampling in different baseline directions) to avoid image distortion.

[0018] Step 7: Calibration and Post-processing

[0019] The reconstructed image undergoes phase calibration and deconvolution processing (such as CLEAN or maximum entropy methods) to eliminate errors introduced by atmospheric disturbances, instrument sampling, and other factors. The final output is a high-resolution celestial image or model.

[0020] In the field of optical synthetic aperture, shared-frame Fizeau interferometry is another important branch developing alongside non-shared-frame Michelson interferometry. This technology originates from Fizeau's interference concept, and its core characteristics are: first, multiple sub-apertures share the same telescope frame and optical support structure (i.e., "shared frame"), ensuring that the optical path from the target to different sub-apertures is the same; second, the sub-aperture array maintains strict geometric similarity with the beam-shrinking array, a necessary condition for direct imaging. Unlike Michelson interferometers (such as VLTI) that require reconstructing the image through the visibility function of interference fringes, the Fizeau interferometer images by directly forming interference fringe patterns on the focal plane. Large binoculars (LBTs) are representative of this technology, mounting two 8.4-meter primary mirrors on the same frame. The interferogram produced by point-source Fizeau interferometry is the product of the diffraction-limited image of the sub-telescopes and the interference modulation, exhibiting spatial invariance within a certain field of view, making it a direct imaging method. Its principle diagram is shown below, with sub-aperture spacing... Beam spacing after beam contraction Sub-aperture size and the size of the sub-aperture after beam contraction The following geometric relationship needs to be satisfied:

[0021] .

[0022] That is, the beam arrangement entering the beam combiner must satisfy a similarity relationship with the baseline arrangement, and the similarity ratio is equal to the optical magnification of the sub-aperture. .

[0023] Given the imaging characteristics of Fizeau interferometry, the LBT (Low Bit Transformer) will be used as an example to explain in detail its direct imaging process. The structural diagram is shown below. Figure 2 As shown:

[0024] Step 1: Construction of the Common Rack Aperture System

[0025] Two or more separate optical sub-apertures are constructed on a rigid telescope rig. In the LBT, these two sub-apertures are its two independent 8.4-meter primary mirrors. During observation, their relative positions are fixed, forming one or more fixed long baselines (thanks to the Earth's rotation).

[0026] Step 2: Beam Collection and Active / Adaptive Optical Correction

[0027] Each sub-aperture collects light waves from celestial bodies synchronously. The key technology lies in equipping each sub-aperture with an independent adaptive optics system to correct wavefront distortions introduced by atmospheric turbulence in real time. This step is crucial to ensure the wavefront quality of each sub-beam at the point of convergence, resulting in stable and high-contrast interference fringes.

[0028] Step 3: Beam transmission and internal optical path difference compensation

[0029] Each beam is guided through an internal optical path to a shared beam combiner. While the shared frame has no external optical path difference, minute thermal deformation and mechanical errors can still cause internal optical path differences. Therefore, the system requires an internal optical path difference compensation mechanism to dynamically adjust the length of each optical path with nanometer-level precision, ensuring they reach near-zero optical path difference at the combiner, thereby achieving interference.

[0030] Step 4: Direct Imaging of the Interferogram

[0031] The combined and optically path-compensated light beams are focused onto a single detector (such as a CCD / infrared camera). Here, the light from different sub-apertures superimposes, directly forming an image containing high-frequency interference fringes on the focal plane.

[0032] Step 5: Image deconvolution

[0033] An image containing high-frequency interference fringes is a convolution of the target's true brightness distribution with a complex interference point spread function. To obtain an image that most closely resembles the real celestial object, deconvolution algorithms (such as blind deconvolution or Wiener filtering based on the point spread function) are needed to process the original interference image, suppress noise, and enhance the resolved details.

[0034] Non-co-mounted Michelson interferometers are based on the Michelson interferometry principle. They use beams from discrete telescopes to interfere at a beam combiner, and then reconstruct the image through Fourier inversion. Their drawbacks stem from the optical path geometry of their discrete baselines and the signal processing methods, resulting in a small field of view, low observation efficiency, and poor quality of the reconstructed image.

[0035] 1. Small field of view

[0036] Because non-co-mounted Michelson interferometers (VLTIs) do not require similarity between their entrance pupils and exit pupils, the exit pupils of the sub-telescopes typically coincide (interference occurs on the pupil plane) or the exit pupil positions of the sub-telescopes are fixed (interference occurs on the image plane). Interference fringes can only be formed when light waves from celestial objects meet strict optical path difference conditions. This results in the system's coherent field of view (FoV) being limited by the diffraction limit (λ / D) of a single telescope. The narrow field of view makes it impossible to simultaneously perform high-resolution imaging of extended celestial objects or large areas of the sky. For example, in VLTIs, the field of view is typically only tens of milliarcseconds, and the small observation field of view limits observation efficiency.

[0037] 2. Sparse UV coverage leads to low efficiency in frequency domain signal acquisition.

[0038] Because each baseline corresponds to one UV point, UV coverage is sparse in a single observation. In a single observation, N telescopes can only produce N(N-1) / 2 baselines. Although the projection of the telescope baselines varies with the Earth's rotation, which can increase the density of UV points, further improving image quality requires a significant increase in the number of telescopes. Even then, the imaging quality is still difficult to compare with that of direct imaging via Fizeau interferometry. For example, the CHARA telescope array has 15 baselines. Even after increasing the UV sampling points using the Earth's rotation, the resulting UV coverage is still relatively sparse compared to a single-aperture array. It can image simple two-point and multi-point sources, but its ability to image complex surface sources is insufficient.

[0039] The shared-rack Fizeau interferometer array, based on the Fizeau interference principle, uses a shared rack for sub-apertures, directly forming an interference image on the focal plane, resulting in a large imaging field of view and high UV coverage efficiency. Its main drawback stems from the inherent limitations of its mechanical structure, making it difficult to improve resolution.

[0040] Increasing the baseline is costly, and the frame size limits its upper limit. Since all sub-apertures are fixed on the same frame, the maximum baseline length is directly determined by the frame size. Increasing the baseline requires redesigning and building a larger frame structure, which results in significant structural deformation. This deformation is fatal for interferometric imaging, which requires high pose accuracy. Therefore, this technical solution has a baseline limit and is difficult to achieve at the hundred-meter level.

[0041] Existing non-co-mounted Michelson interferometry and co-mounted Fizeau interferometry in optical synthetic aperture arrays both have drawbacks. The imaging field of view of non-co-mounted Michelson interferometry is limited by the diffraction limit of a single telescope, resulting in a narrow field of view. This invention, through a Fizeau-style focal plane interference design, allows for coherence over a wider angular range while controlling pupil mapping errors, achieving an effective field of view far greater than that of non-co-mounted Michelson interferometry. Non-co-mounted Michelson interferometry employs an indirect imaging method, where the measurement signal of a baseline is ultimately converted into a point on the UV plane. This means that even with a large number of telescopes and the Earth's rotation, it can only image a simple image composed of point sources. This invention employs a direct imaging method, where the UV sampling acquired from a baseline corresponds to three circular regions on the UV plane (for monochromatic light). Through a reasonable array arrangement and utilizing the Earth's rotation, fewer telescopes can be used for full sampling in the frequency domain, resulting in an order-of-magnitude improvement in imaging sampling efficiency compared to non-co-mounted Michelson interferometry.

[0042] There are technical limitations to maximizing the baseline of a shared-rack Fizeau interferometer array. This invention employs distributed sub-telescopes that are not limited by rack size, making it easier to achieve high-resolution imaging. It also retains the advantage of a large field of view inherent in the Fizeau interferometer array.

[0043] The information disclosed in this background section is intended only to enhance the understanding of the general background of the invention and should not be construed as an admission or in any way implying that the information constitutes prior art known to those skilled in the art. Summary of the Invention

[0044] To address the problems existing in the prior art, the present invention aims to provide a novel ground-based distributed optical synthetic aperture direct imaging method and device. This invention employs flexible optical path design and dynamic adjustment, as well as image reconstruction, to achieve high-resolution direct imaging of astronomical targets such as stars. This invention can be widely applied to high-resolution astrophysical research, such as stellar surface imaging, exoplanet imaging, and observation of active galactic nuclei.

[0045] To achieve the above objectives, the present invention adopts the following technical solution:

[0046] A novel ground-based distributed optical synthetic aperture direct imaging method includes the following steps:

[0047] Step 1: Based on the right ascension and declination of the target, determine the observation time period and the position of each sub-telescope, and move the sub-telescopes to their respective positions; if the telescopes are fixed on the ground, only the observation time needs to be determined; the observation time periods are denoted as t1, t2, ..., tt n The start and end times of each observation period are denoted as t. is and t ie , i = 1 to n;

[0048] Step 2: at t is Beforehand, the azimuth and elevation angles of each sub-telescope are simultaneously adjusted to point at the observation target and track it; four light spots are observed on both the imaging camera and the image stabilization camera; the light spots on the imaging camera are sequentially translated to the same designated position, and the position of each light spot on the image stabilization camera is recorded; closed-loop control is activated to keep the positions of the four light spots on the image stabilization camera at the recorded positions;

[0049] Step 3: Based on the condition of equal optical path length, adjust the delay line length of each sub-telescope in real time to compensate for the optical path difference, so that the optical path detection camera can detect the interference fringes between the sub-telescopes. Within the adjustment range of the small plane mirror, continuously change the position of the small plane mirror to achieve co-phase interference of the light spot on the imaging camera, and record the state of the interference fringes between each sub-telescope on the optical path detection camera at this time; activate the optical path closed-loop control, and the calculation and control unit calculates the translation amount of the delay line small plane mirror in the optical path, drives the small plane mirror to translate and correct the optical path difference, and maintains the interference fringes on the optical path detection camera in the recorded state;

[0050] Step 4: According to the trajectory equation of the entrance pupil mechanism, adjust the entrance pupil of the combining telescope to the designated position and move it according to the relationship between position and time in the trajectory equation; according to the formula of the K-mirror angle at the current time, rotate the K-mirror to the corresponding angle and maintain the rotation so that the north celestial pole direction behind the K-mirror always coincides with the north direction.

[0051] Step 5: At t is The moment begins to be exposed, until t ie The exposure ends at a certain point, and a set of baseline interferometric images is acquired;

[0052] Step 6: Repeat steps 2-5 until all baseline interferometric images have been acquired;

[0053] Step 7: Register and accumulate all the images to complete image reconstruction, and then perform image deconvolution to obtain a clear, high-resolution image.

[0054] A novel ground-based distributed optical synthetic aperture direct imaging device includes several sub-telescopes, a K-mirror, a delay line, an entrance pupil mechanism, and a beam combining telescope; wherein...

[0055] Each of the sub-telescopes is distributed at a designated position on the same horizontal plane, fixed on the ground or in a track. Each sub-telescope uses an independent horizontal gantry. During the Earth's rotation, each sub-telescope can point to the same target and conduct long-term tracking observations of celestial bodies. The parallel beam of light from the celestial body passes sequentially through the primary mirror, secondary mirror, and folding mirror group of the sub-telescope. After passing through the folding mirror group, the beam direction is vertically downward and the position is stationary relative to the gantry. Each sub-telescope has the same beam compression ratio.

[0056] Each sub-telescope is equipped with a K-mirror behind its folding mirror group. Each K-mirror rotates independently around its own optical axis to change the direction of the target imaging field of view. The rotation axis of the K-mirror coincides with the center of the beam to ensure that the beam does not translate during rotation. The rotation angle of the K-mirror ensures in real time that during the imaging process, before the beam enters the combining telescope for imaging, the U direction in the UVW coordinate system of the target field of view is rotated to always be aligned with the east direction in the horizontal coordinate system, and the V direction in the UVW coordinate system of the target field of view is rotated to always be aligned with the north direction in the horizontal coordinate system.

[0057] The beam emitted from the K-mirror is reflected by a plane mirror fixed to the sub-telescope, turning it into a horizontal propagation. The beam then passes through the delay line, which consists of a plane mirror fixed to the ground at a 45° angle to the incident light, a parabolic mirror, and a small plane mirror. The small plane mirror is fixed on a piezoelectric displacement stage and is used to translate back and forth in front of the focal plane of the parabolic mirror for high-frequency optical path adjustment. The parabolic mirror and the small plane mirror are fixed as a whole on a motion stage to change the optical path over a wide range, ensuring that the optical path of the beams passing through different sub-telescopes to the focal plane of the combining telescope is equal.

[0058] The light rays emitted from the delay line enter the entrance pupil mechanism of the beam combining telescope. Each entrance pupil mechanism consists of two plane mirrors. The normal of the first plane mirror is horizontal and forms a 45° angle with the incident light. The reflected light is perpendicular to the incident light and propagates along the horizontal plane. The second plane mirror converts the horizontal incident light into vertically downward propagating reflected light, which then enters the beam combining telescope. The first plane mirror performs a one-dimensional translation along the direction of its incident light. In addition to translating synchronously with the first plane mirror in this direction, the second plane mirror also performs a horizontal translation in a direction perpendicular to it. Therefore, the projection of the beam emitted from the entrance pupil mechanism of the beam combining telescope onto the horizontal plane undergoes a two-dimensional movement within the horizontal plane. The translation of the entrance pupil mechanism ensures that during the imaging process, the projection of the beam emitted from the entrance pupil mechanism onto the horizontal plane is similar to the projection of the sub-telescope array onto the UV plane, with the similarity ratio being the reciprocal of the beam compression ratio of the sub-telescope.

[0059] The light beam enters the beam combining telescope through the entrance pupil mechanism, and the beam combining telescope remains stationary. Several beam splitters are set on the focal plane, and optical path detection cameras, pointing detection cameras, pupil detection cameras, and imaging cameras are placed on the focal planes corresponding to each beam splitter.

[0060] By adopting the above technical solution, the present invention has the following beneficial effects:

[0061] This invention achieves direct imaging by ensuring pupil mapping, resulting in a larger imaging field of view compared to non-shared-rack Michelson interferometers. Utilizing the Earth's rotation, it employs a smaller number of small-aperture telescopes to perform more complete spatial frequency sampling of celestial targets, achieving higher frequency signal acquisition efficiency than non-shared-rack Michelson interferometers and enabling imaging of complex surface sources. Furthermore, this invention employs a layout where sub-telescopes are distributed across the ground; compared to shared-rack Fizeau interferometers, the baseline length is not limited by rack size, enabling higher resolution imaging. Attached Figure Description

[0062] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0063] Figure 1 This is a schematic diagram of a non-co-rack Michelson interferometer array in the prior art.

[0064] Figure 2 This is a schematic diagram of a common-rack Fizeau interferometer array in the prior art.

[0065] Figure 3This is a schematic diagram of the structure of the ground-based distributed optical integrated aperture direct imaging device provided by the present invention.

[0066] Figure 4 This is a schematic diagram of another perspective of the ground-based distributed optical integrated aperture direct imaging device provided by the present invention.

[0067] Figure 5 This is a schematic diagram illustrating the connection relationship between the folding-axis mirror assembly, the K-mirror, and the delay line provided by the present invention.

[0068] Figure 6 This is a schematic diagram of the array projection with a time angle of 0° and the MTF provided by the present invention.

[0069] Figure 7 This is a schematic diagram of the array projection and MTF superposition at different times provided by the present invention. Detailed Implementation

[0070] The technical solution of the present invention will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0071] The specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are for illustration and explanation only and are not intended to limit the present invention.

[0072] First, the following explanations are provided for the technical terms used in this application:

[0073] Entrance pupil: The image formed in object space by the aperture stop through the optical group in front of it (the optical element closest to the object). It is the "effective entrance" for light to enter the optical system, directly limiting the size of the incident beam.

[0074] Exit pupil: The image formed in image space by the aperture stop through the optical group behind it (the optical element closest to the image side). It is the "effective exit" for light leaving the optical system, limiting the cross-sectional size of the outgoing beam.

[0075] K-mirror: An optical assembly consisting of three plane mirrors mounted on the same rotation axis (which is parallel to or coincides with the system's principal ray).

[0076] K-mirror principal section: a plane perpendicular to the common edge of the three mirrors and containing the principal optical axis.

[0077] Combination Figure 3-7As shown, this invention proposes a novel ground-based distributed optical synthetic aperture direct imaging method and apparatus; wherein, the specific structure of the apparatus includes several sub-telescopes 1, K-mirrors 2, delay lines 3, entrance pupil mechanisms 4, and combining telescopes 5; the positions and connections between the various parts of the apparatus are as follows:

[0078] 1) Each sub-telescope is located at a designated position on the same horizontal plane. This can be fixed to the ground or in orbit. Each sub-telescope uses an independent horizontal gantry. During the Earth's rotation, all sub-telescopes can point to the same target for long-term tracking observation of celestial bodies. The parallel beam of light from the celestial body passes sequentially through the primary mirror, secondary mirror, and folding mirror group of each sub-telescope. After passing through the folding mirror group, the beam direction is vertically downward, and the position is stationary relative to the gantry. Each sub-telescope has the same beam compression ratio.

[0079] 2) Each sub-telescope is equipped with a K-mirror behind its folding-axis mirror assembly. Each K-mirror can rotate independently around its own optical axis to change the direction of the target imaging field of view. The rotation axis of the K-mirror must coincide with the beam center to ensure that the beam does not translate during rotation. The rotation angle of the K-mirror is adjusted in real time to ensure that, during the imaging process, before the beam enters the combining telescope for imaging, the target field of view in the U direction of the UVW coordinate system is rotated to always align with the east direction in the horizontal coordinate system, and the target field of view in the V direction of the UVW coordinate system is rotated to always align with the north direction in the horizontal coordinate system.

[0080] 3) The beam emitted from mirror K is reflected by a plane mirror fixed to the sub-telescope, transforming it into a horizontal propagation path. The beam then passes through a delay line, which consists of a plane mirror fixed to the ground at a 45° angle to the incident light, a parabolic mirror, and a small plane mirror. The small plane mirror is fixed on a piezoelectric displacement stage and can be moved back and forth in front of the focal plane of the parabolic mirror for high-frequency optical path adjustment. The parabolic mirror and the small plane mirror are fixed as a whole on the motion stage, allowing for a wide range of optical path changes to ensure that the optical paths of the beams passing through different sub-telescopes to the focal plane of the combining telescope are equal.

[0081] 4) The light rays emitted from the delay line enter the entrance pupil mechanism of the beam combiner telescope. Each entrance pupil mechanism consists of two plane mirrors. The normal of the first plane mirror is horizontal and forms a 45° angle with the incident light. The reflected light is perpendicular to the incident light and propagates along the horizontal plane. The second plane mirror converts the horizontal incident light into vertically downward propagating reflected light, which then enters the beam combiner telescope. The first plane mirror can be translated one-dimensionally along its incident light direction. The second plane mirror can not only translate synchronously with the first plane mirror in this direction, but also translate horizontally in a direction perpendicular to it. Therefore, the projection of the beam emitted from the entrance pupil mechanism of the beam combiner telescope onto the horizontal plane can be moved two-dimensionally within the horizontal plane. The translation of the entrance pupil mechanism ensures that during imaging, the projection of the beam emitted from the entrance pupil mechanism onto the horizontal plane is similar to the projection of the sub-telescope array onto the UV plane, with the similarity ratio being the reciprocal of the beam compression ratio of the sub-telescope.

[0082] 5) The light beam passing through the entrance pupil mechanism of the beam combining telescope enters the center of the beam combining telescope, which remains stationary. Several beam splitters are set on the focal plane, and optical path detection cameras, pointing detection cameras, pupil detection cameras, and imaging cameras are placed on the focal planes corresponding to each beam splitter.

[0083] The working principle of the above device is as follows:

[0084] This method utilizes multiple ground-based small telescopes to track and observe the same target, achieving co-pointing. Co-phase is achieved by adjusting the optical path lengths of each telescope to the image plane using delay lines. The entrance pupil position of the combining telescope is adjusted using a K-mirror and a combining telescope entrance pupil mechanism, ensuring similarity between the entrance pupil of the combining telescope and the entrance pupils of the individual telescopes. Based on this, an instantaneous wide-field image is obtained. By observing at different times, complete or desired UV coverage is achieved, and a high-resolution image of the target is obtained through subsequent image reconstruction.

[0085] Let the coordinate system of the sub-telescope array be a horizontal coordinate system, with east as the x-axis, north as the y-axis, and upward as the z-axis. The origin and the entrance pupil center are in the same plane. Let the azimuth angle of the entrance pupil center of one of the sub-telescopes relative to the origin be A (north is 0°, east is positive), and the distance to the origin be D. Then the components of the vector from the origin to the entrance pupil center in the UVW coordinate system are:

[0086] ;

[0087] The UVW coordinate system is a coordinate system that follows the movement of the target celestial body. With the observed target as the origin, the U-axis points eastward along the declination circle, the V-axis points northward along the right ascension circle, and the W-axis points from the observer towards the target, i.e., the line of sight. In the formula, H is the hour angle of the celestial body. The hour angle is 0 when the celestial body passes through the local meridian, with westward being positive. If the observation is made at time t after the celestial body has passed through the local meridian, then the hour angle of the celestial body is H = wt, where w is the angular velocity of the Earth's rotation. φ represents the declination of a celestial body, with the origin at the equatorial plane and positive latitude towards the north; φ represents the latitude of the telescope array, with positive latitude in the Northern Hemisphere.

[0088] 1) Common pointing principle

[0089] Each sub-telescope uses the same rig to point at and track the same target. The telescope employs a horizontal rig, located at latitude φ, with an observation angle of H and a declination of [missing information]. When observing a celestial body, the telescope's altitude angle h T and azimuth A T They are respectively:

[0090] ;

[0091] Due to subtle differences between optical paths and atmospheric disturbances, the diffraction spots of each sub-telescope on the imaging camera will not perfectly coincide. A stabilized camera is positioned on the focal plane of the combining telescope. By splitting the beam or field of view, the diffraction spots of each sub-telescope are captured on the camera. Algorithms, either related to beam splitting or centroid splitting, monitor the position of the diffraction spots of each sub-telescope in real time. A computational control unit calculates the rotation angle of the oscillating mirror in the optical path and drives the oscillating mirror to rotate and correct the spot position, thus fixing the spot on the stabilized camera at the designated position and achieving the coincidence of the diffraction spots on the combining camera. Ideally, the oscillating mirror should be a plane mirror close to the exit pupil of the ion telescope, such as the M3 mirror of a sub-telescope, to minimize the change in the exit pupil caused by the rotation of the oscillating mirror.

[0092] 2) Co-phase principle

[0093] As the Earth rotates, the azimuth and elevation angles of the two sub-telescopes are constantly adjusted, and the external optical path difference between the two sub-telescopes (the optical path difference when the beam reaches the entrance pupil of the two sub-telescopes) also changes continuously. To ensure that the optical path difference caused by the adjustment of the entrance pupil mechanism of the combining telescope, which is used for pupil similarity mapping, also changes continuously, a delay line is needed to adjust the optical path of each sub-optical path so that the optical path reaching the focal plane of the combining telescope is the same. Assume that the entrance pupil centers of all sub-telescopes are in the same horizontal plane, and the projection of the center of the primary mirror of the combining telescope onto this plane is the origin. Let A be the azimuth angle of the entrance pupil center of a certain sub-telescope relative to the origin, and D be the distance to the origin. Assume there exists a sub-telescope whose entrance pupil center is at the origin of the array, and the optical path from its entrance pupil to the entrance pupil of the combining telescope is L. The other telescopes form a baseline with it, and the optical path from the entrance pupil to the exit pupil of each sub-telescope is L0. The external optical path delay of the sub-telescope relative to the hypothetical sub-telescope is the negative value of the W coordinate of the sub-telescope in the UVW coordinate system, i.e.:

[0094] ;

[0095] To ensure equal optical path lengths for all sub-telescopes, the optical path from the exit pupil of each sub-telescope to the entrance pupil of the combining telescope should be L + W - L0. Based on the trajectory equations of the entrance pupil mechanisms in each optical path (given below), the position of the delay line of each telescope at different times can be calculated. Overall movement of the delay line achieves optical path compensation, with a compensation error ranging from a few micrometers to tens of micrometers. For practical applications, an optimal value for L can be determined based on the calculated range of delay line lengths for different times and optical paths, minimizing the optical path length from the exit pupil of each sub-telescope to the entrance pupil of the combining telescope, thereby shortening the optical path and reducing costs.

[0096] Based on this, an optical path detection camera is arranged on the focal plane of the optical combining telescope to detect the residual optical path difference between the sub-telescopes using methods such as the dispersion fringe method. The calculation and control unit calculates the translation amount of the delay line plane mirror in the optical path and drives the plane mirror to translate and correct the optical path difference to achieve co-phase at the ten-nanometer level.

[0097] 3) Implementation of pupil similarity mapping relationship

[0098] Telescope arrays used for direct imaging need to satisfy a strict similarity relationship between the projection of the entrance pupil array of the sub-telescope in the line of sight and the projection of the entrance pupil of the combining telescope in the line of sight; otherwise, the co-phase condition will be destroyed as the field of view expands, thus reducing the imaging field of view.

[0099] The solution adopted in this invention is to adjust the entrance pupil position in real time by adjusting the entrance pupil mechanism of the combining telescope so that it maintains a similarity relationship with the projection of the entrance pupil array of the sub-telescope on the UV plane, and to achieve the coordinate system transformation by rotating the K-mirror.

[0100] During the tracking process, the projection of the telescope array onto the UV plane in the UVW coordinate system is constantly changing. When the entrance pupil center of the sub-telescope coincides with the fixed point of the frame, the azimuth angle A of the entrance pupil center of the sub-telescope relative to the origin and the distance D to the origin are constants, and the trajectory of the projection is an ellipse. In order to simplify the control of the exit pupil position, it is necessary to make the entrance pupil center of the sub-telescope coincide with the fixed point of the frame.

[0101] First, the entrance pupil mechanism of the combining telescope needs to be translated accordingly to change the exit pupil position to ensure similarity. The trajectory equation of the entrance pupil mechanism is as follows: ;

[0102] East is the x-axis, north is the y-axis, M is the optical magnification of the sub-telescope, and the x-axis and y-axis correspond to the U-axis and V-axis, respectively.

[0103] Furthermore, the entrance pupil of each sub-telescope has a direction. If the exit pupils are similar in shape to the entrance pupil array but all exit pupils are rotated by the same angle, the requirements cannot be met. Therefore, a K-lens is needed to rotate the field of view, ensuring that the target's V direction (north celestial pole direction) in the UVW coordinate system always coincides with the north direction in the horizontal coordinate system. The horizontal telescope's observation angle is H, and its declination is... When observing a celestial body, the rotation angle of the V direction relative to the north direction in front of the K-mirror can be calculated using the following formula:

[0104] ;

[0105] Looking towards the light, a counter-clockwise rotation is positive. Where P... O The angle (star position angle) is V relative to the instrument's north direction. For a horizontal telescope, the instrument's north is the zenith direction. O The expression is

[0106] ;

[0107] The angle between the instrument's north and north directions is due to the relative rotation of the mirror. The zenith distance of the observed target is related to the elevation angle as follows:

[0108]

[0109] Therefore, the rotation angle of the V direction relative to the north direction in front of the K-mirror can be calculated by the following formula:

[0110] ;

[0111] It can be seen that the rotation angle of the V direction relative to the north direction in front of the K mirror varies with the azimuth angle of the celestial body. and elevation angle Constantly changing, for a given geographical latitude and declination, P can be expressed as a function containing only time. The field of view rotation angle caused by rotating the K-mirror by an angle θ is -2θ, therefore the K-mirror only needs to rotate the principal section normal vector to an angle of north. Angle, so that the V direction behind the K mirror coincides with the north direction.

[0112] 4) UV coverage

[0113] For sparse telescope arrays, the system's optical transfer function (MTF) has many zero values ​​at a given moment, resulting in a significant loss of detail in the acquired images. The projection of an array located at 40° North latitude onto the UV plane and its MTF onto an object at 20° South latitude at an hour angle of 0° are shown below. Figure 6 As shown.

[0114] As the Earth rotates, the projection of the array onto the UV plane constantly changes, and the system transfer function (MTF) also changes continuously. The projection trajectory of any baseline onto the UV plane is an ellipse, and the parameters of the ellipse are related to the azimuth, length, latitude of the baseline, and declination of the observed target. By optimizing the array, determining the telescope's aperture, location, and observation time, and superimposing images from different times, followed by image inversion, a high-resolution image can be reconstructed. For example, when an array located at 40° North latitude observes a celestial body at 20° South latitude, and the hour angle changes from -50° to 50° in 20-degree intervals, the superposition results of the array's projection onto the UV plane and the MTF are as follows. Figure 7 As shown, the coverage of MTF in the mid-to-high frequency range is greatly improved.

[0115] The novel terrestrial distributed optical synthetic aperture direct imaging method of this application specifically includes the following steps:

[0116] 1) Based on the right ascension and declination of the target, determine the observation time period and the position of each sub-telescope, and move the sub-telescopes to their respective positions. If the telescopes are fixed on the ground, only the observation time needs to be determined. The observation time periods are denoted as t1, t2, ..., t... n The start and end times of each observation period are denoted as t. is and t ie (i=1~n).

[0117] 2) At t is Beforehand, the azimuth and elevation angles of each sub-telescope are simultaneously adjusted to point at the observation target and track it. Four light spots can be observed on both the imaging camera and the stabilized camera. The light spots on the imaging camera are sequentially translated to the same designated position, and the position of each light spot on the stabilized camera is recorded. Closed-loop control is then activated to maintain the positions of the four light spots on the stabilized camera at the recorded positions.

[0118] 3) Based on the condition of equal optical path length, the delay line length of each sub-telescope is adjusted in real time to compensate for the optical path difference, enabling the optical path detection camera to detect the interference fringes between the sub-telescopes. Within the adjustment range of the small plane mirror, the position of the small plane mirror is continuously changed to achieve co-phase interference of the light spot on the imaging camera, and the state of the interference fringes between each sub-telescope on the optical path detection camera is recorded at this time. Optical path closed-loop control is then activated, and the computational control unit calculates the translation amount of the delay line small plane mirror in the optical path, driving the small plane mirror to translate and correct the optical path difference, maintaining the interference fringes on the optical path detection camera in the recorded state.

[0119] 4) According to the trajectory equation of the entrance pupil mechanism, adjust the entrance pupil of the combining telescope to the designated position and move it according to the position-time relationship in the trajectory equation. According to the formula for the angle of the K-mirror at the current time, rotate the K-mirror to the corresponding angle and maintain the rotation so that the direction of the north celestial pole behind the K-mirror always coincides with the north direction.

[0120] 5) At t is The moment begins to be exposed, until t ie The exposure ends at a certain point, and a set of baseline interferometric images are acquired.

[0121] 6) Repeat steps 2)-5) until all baseline interferometric images have been acquired.

[0122] 7) Register and accumulate all the images to complete image reconstruction, and then perform image deconvolution to obtain a clear, high-resolution image.

[0123] Compared with the prior art, the main innovations of this invention include:

[0124] 1. This invention uses non-coaxial telescopes distributed on the ground.

[0125] 2. This invention uses a direct imaging mode, rather than a Michelson indirect imaging mode.

[0126] 3. A combined telescope is used to keep the field of view fixed, a K-lens is used to correct the field of view direction in real time, and the entrance pupil mechanism of the combined telescope is used to change the exit pupil distribution in real time. Both of these methods together ensure the pupil mapping relationship in the direct imaging mode.

[0127] Preferably, the present application also has the following alternative options:

[0128] 1. In the example of this invention, the sub-apertures are on the same horizontal plane. In reality, the sub-apertures may not be on the same horizontal plane. Only the delay line length, UV coverage, and entrance pupil equation of the combining telescope will be slightly different.

[0129] 2. In the example of this invention, the sub-aperture is fixed relative to the ground. In reality, the sub-aperture can be fixed on the guide rail and its position can be adjusted for different targets or at different times when observing the same target to improve UV coverage.

[0130] 3. In this invention, maintaining the pupil mapping relationship can also be achieved by keeping the entrance pupil mechanism of the combining telescope and the sub-telescope array similar in the horizontal coordinate system, and by having the combining telescope and the sub-telescope track synchronously (maintaining the same direction). Alternatively, the combining telescope can be fixed in place, and equivalent devices such as heliostats or astrostats can be used in front of the combining telescope.

[0131] 4. In addition to using K-lens, Dowell prisms, Beacon prisms, etc. can also be used to control the rotation of the field of view.

[0132] 5. The x-axis and y-axis in the text are chosen only to conform to common observation habits. Other directions can also be chosen, but the rotation angle of the K mirror and the length of the delay line will need to be changed to meet the requirements.

[0133] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A novel ground-based distributed optical synthetic aperture direct imaging method, characterized in that, The method includes the following steps: Step 1: Based on the right ascension and declination of the target, determine the observation time period and the position of each sub-telescope, and move the sub-telescopes to their respective positions; if the telescopes are fixed on the ground, only the observation time needs to be determined; the observation time periods are denoted as t1, t2, ..., tt n The start and end times of each observation period are denoted as t. is and t ie , i = 1 to n; Step 2: at t is Beforehand, the azimuth and elevation angles of each sub-telescope are simultaneously adjusted to point at the observation target and track it; four light spots are observed on both the imaging camera and the image stabilization camera; the light spots on the imaging camera are sequentially translated to the same designated position, and the position of each light spot on the image stabilization camera is recorded; closed-loop control is activated to keep the positions of the four light spots on the image stabilization camera at the recorded positions; Step 3: Based on the condition of equal optical path length, adjust the delay line length of each sub-telescope in real time to compensate for the optical path difference, so that the optical path detection camera can detect the interference fringes between the sub-telescopes. Within the adjustment range of the small plane mirror, continuously change the position of the small plane mirror to achieve co-phase interference of the light spot on the imaging camera, and record the state of the interference fringes between each sub-telescope on the optical path detection camera at this time; activate the optical path closed-loop control, and the calculation and control unit calculates the translation amount of the delay line small plane mirror in the optical path, drives the small plane mirror to translate and correct the optical path difference, and maintains the interference fringes on the optical path detection camera in the recorded state; Step 4: According to the trajectory equation of the entrance pupil mechanism, adjust the entrance pupil of the combining telescope to the designated position and move it according to the relationship between position and time in the trajectory equation; according to the formula of the K-mirror angle at the current time, rotate the K-mirror to the corresponding angle and maintain the rotation so that the north celestial pole direction behind the K-mirror always coincides with the north direction. Step 5: At t is The moment begins to be exposed, until t ie The exposure ends at a certain point, and a set of baseline interferometric images is acquired; Step 6: Repeat steps 2-5 until all baseline interferometric images have been acquired; Step 7: Register and accumulate all the images to complete image reconstruction, and then perform image deconvolution to obtain a clear, high-resolution image.

2. An apparatus for implementing the novel ground-based distributed optical synthetic aperture direct imaging method as described in claim 1, characterized in that, The device includes several sub-telescopes, a K-mirror, a delay line, an entrance pupil mechanism, and a beam combining telescope; wherein... Each of the sub-telescopes is distributed at a designated position on the same horizontal plane, fixed on the ground or in a track. Each sub-telescope uses an independent horizontal gantry. During the Earth's rotation, each sub-telescope can point to the same target and conduct long-term tracking observations of celestial bodies. The parallel beam of light from the celestial body passes sequentially through the primary mirror, secondary mirror, and folding mirror group of the sub-telescope. After passing through the folding mirror group, the beam direction is vertically downward and the position is stationary relative to the gantry. Each sub-telescope has the same beam compression ratio. Each sub-telescope is equipped with a K-mirror behind its folding mirror group. Each K-mirror rotates independently around its own optical axis to change the direction of the target imaging field of view. The rotation axis of the K-mirror coincides with the center of the beam to ensure that the beam does not translate during rotation. The rotation angle of the K-mirror ensures in real time that during the imaging process, before the beam enters the combining telescope for imaging, the U direction in the UVW coordinate system of the target field of view is rotated to always be aligned with the east direction in the horizontal coordinate system, and the V direction in the UVW coordinate system of the target field of view is rotated to always be aligned with the north direction in the horizontal coordinate system. The beam emitted from the K-mirror is reflected by a plane mirror fixed to the sub-telescope, turning it into a horizontal propagation. The beam then passes through the delay line, which consists of a plane mirror fixed to the ground at a 45° angle to the incident light, a parabolic mirror, and a small plane mirror. The small plane mirror is fixed on a piezoelectric displacement stage and is used to translate back and forth in front of the focal plane of the parabolic mirror for high-frequency optical path adjustment. The parabolic mirror and the small plane mirror are fixed as a whole on a motion stage to change the optical path over a wide range, ensuring that the optical path of the beams passing through different sub-telescopes to the focal plane of the combining telescope is equal. The light rays emitted from the delay line enter the entrance pupil mechanism of the beam combining telescope. Each entrance pupil mechanism consists of two plane mirrors. The normal of the first plane mirror is horizontal and forms a 45° angle with the incident light. The reflected light is perpendicular to the incident light and propagates along the horizontal plane. The second plane mirror converts the horizontal incident light into vertically downward propagating reflected light, which then enters the beam combining telescope. The first plane mirror performs a one-dimensional translation along the direction of its incident light. In addition to translating synchronously with the first plane mirror in this direction, the second plane mirror also performs a horizontal translation in a direction perpendicular to it. Therefore, the projection of the beam emitted from the entrance pupil mechanism of the beam combining telescope onto the horizontal plane undergoes a two-dimensional movement within the horizontal plane. The translation of the entrance pupil mechanism ensures that during the imaging process, the projection of the beam emitted from the entrance pupil mechanism onto the horizontal plane is similar to the projection of the sub-telescope array onto the UV plane, with the similarity ratio being the reciprocal of the beam compression ratio of the sub-telescope. The light beam enters the beam combining telescope through the entrance pupil mechanism, and the beam combining telescope remains stationary. Several beam splitters are set on the focal plane, and optical path detection cameras, pointing detection cameras, pupil detection cameras, and imaging cameras are placed on the focal planes corresponding to each beam splitter.

Images