An on-orbit data processing method for a space-borne soft X-ray single photon imaging camera

Abstract

The application discloses an on-orbit data processing method of a spaceborne soft X-ray single-photon imaging camera, and belongs to the technical field of X-ray pulsar detection imaging. The method performs dark field background correction on an original image of soft X-ray single photons received by a spaceborne area array detector; then, threshold value method is used to perform line-by-line detection on the image, when a detection pixel DN value exceeds a lower threshold value and does not exceed an upper threshold value, the pixel is detected, and DN values of a peripheral n*n region and the pixel coordinate value are extracted as an effective single-photon event; after full image detection, imaging auxiliary data and all single-photon event data in a dynamic memory are integrated to form an event frame image. Finally, the original image and a plurality of event frames are transmitted to realize effective detection efficiency under minimum bandwidth.

Description

Technical Field

[0001] This invention relates to an on-orbit data processing method for a spaceborne soft X-ray single-photon imaging camera, belonging to the field of X-ray pulsar detection and imaging technology. Background Technology

[0002] Pulsars are the remnants of the evolution, collapse, and supernova explosions of massive stars. They emit signals in various electromagnetic frequency bands, including radio, infrared, visible, ultraviolet, X-rays, and gamma rays. Pulsars that emit signals in the X-ray band are typically called X-ray pulsars. X-rays are high-energy photons, concentrating the majority of the pulsar's radiation energy, making them easy to detect and process with miniaturized equipment. However, they are difficult to penetrate Earth's dense atmosphere, therefore they can only be observed in outer space. X-ray pulsar detection is mainly used for astrophysical research, pulsar navigation research, and spacetime time standard technology research.

[0003] Since the launch of the ARGOS satellite in 1999, major economies around the world have successively carried out in-orbit X-ray pulsar detection activities. The latest satellite, eROSITA, jointly developed by Germany and Russia and launched in 2019, has an observation energy range of 0.5keV to 10keV, and its core sensor is a pn-CCD. my country launched Nanjing University-1 in July 2018, carrying a lobster-eye X-ray camera for in-orbit X-ray detection.

[0004] Crab pulsar is currently the pulsar with the highest known radiative flux, requiring the highest temporal resolution. The X-ray photon flux density of pulsar PSR B0531+21 detected in near-Earth orbit is λ. p =1.54×10 4 ph / m 2 / s, its background radiation intensity is about ten times the X-ray photon energy flux density of pulsar radiation, λ n =1.54×10 5 ph / m 2 / s.

[0005] The detection area (optical aperture) is 0.1m. 2 The optical system, with a detection efficiency of 10%, receives 154 pulsar photons and 1540 background photons. The photons arriving at the image plane are received by n detectors, whose distribution conforms to a discrete uniform distribution. Each detector receives approximately [number missing] photons per second. Taking a 1K×1K pixel detector as an example, when operating at a frame rate of 1fps, due to focusing, photons are focused into the central 2K pixels. Each pixel receives an average of 0.85 photons per frame, hence the term X-ray single-photon detection. A single X-ray photon bombards an electron cloud in a silicon-based detector, which is collected and amplified to generate an imaging signal. One electron is generated for every 3.65 eV of energy. Therefore, the number of photogenerated electrons generated at 0.2–10 keV (6.2–0.124 nm) ranges from 54 to 2739 e-. This means that X-ray photons of different wavelengths appear as different DN values ​​in the image. The stronger the energy, the larger the corresponding DN value (brighter the image). However, due to the dark current noise of the silicon-based detector, the linear relationship between the incident photon energy and the image DN value does not pass through zero; there is a positive intercept. This means that a DN value will be generated even when there is no X-ray photon input. For CCD / CMOS detectors, the positive intercept is different for each pixel. In addition, X-ray detection systems typically employ Wolter-type or lobster-eye-type optical systems, such as... Figure 1 Appendix Figure 2 As shown, due to the focusing characteristics of the optical system, the X-ray photon incident detector is not perpendicular to the incident point. The absorption depth of X-ray photons in the silicon-based detector is about 10 to 120 μm, and the photogenerated electrons diffuse to adjacent pixels. Therefore, there is an error in the measurement results of the optical system. Summary of the Invention

[0006] The technical problem solved by this invention is to overcome the shortcomings of the prior art and propose an on-orbit data processing method for a spaceborne soft X-ray single-photon imaging camera, which can effectively extract the correct X-ray single-photon events.

[0007] The technical solution of this invention is:

[0008] A method for on-orbit data processing from a spaceborne soft X-ray single-photon imaging camera includes:

[0009] The spaceborne array detector receives the raw image and imaging auxiliary data of soft X-ray single photons, and performs dark-field background correction on the raw image;

[0010] A threshold method is used to perform line-by-line retrieval of the image. When the DN value of a detected pixel exceeds the lower threshold but does not exceed the upper threshold, the pixel is detected and the DN value of the surrounding n×n region and the coordinate value of the pixel are extracted and stored in the dynamic memory as a valid single-photon event.

[0011] After the full image search is completed, the imaging auxiliary data and the data in the dynamic memory are integrated to form an event frame image.

[0012] The original images at fixed intervals are combined with multiple event frame images and output sequentially to the whole satellite antenna system.

[0013] Furthermore, dark-field background correction is performed on the original image using the following formula:

[0014] Y i,j =a i,j ×X i,j +b i,j +c

[0015] Among them, Y i,j X represents the image values ​​in the i-th row and j-th column after correction; i,j To correct the image values ​​in the i-th row and j-th column; a i,j b is the first-order correction coefficient for the dark field background correction of the pixel in the i-th row and j-th column; i,j is the constant term correction coefficient for dark field background correction of the i-th row and j-th column pixel; c is the full-image offset correction coefficient for dark field background correction.

[0016] Furthermore, the correction factor a i,j b i,j The initial values ​​of c are obtained from ground calibration, stored in the onboard non-volatile memory, and updated in orbit.

[0017] Furthermore, a thresholding method is used to perform a line-by-line search of the image to check whether the DN value of the detected pixels exceeds the lower threshold and does not exceed the upper threshold. The upper and lower thresholds are determined by the target spectral range. The specific method is as follows:

[0018] The lower threshold is determined by the minimum detectable photon plus background noise, specifically:

[0019]

[0020] Among them, D_TH min EG is the lower limit threshold for judgment. min D_dark represents the minimum detectable X-ray photon energy, and D_dark represents the detector's dark noise.

[0021] The upper limit threshold is determined by the maximum detectable photon count plus background noise, and the specific formula is as follows:

[0022]

[0023] Where D_TH max EG is the upper limit threshold for the determination. min The maximum detectable X-ray photon energy.

[0024] Furthermore, when the detected pixel DN value exceeds the lower threshold but does not exceed the upper threshold, the pixel is detected and the DN value of the surrounding n×n region and the pixel coordinate value are extracted and placed into the dynamic memory. The n×n region is determined by the number of pixels that the X-ray single photon travels through in the detector at its maximum travel distance.

[0025] Furthermore, the method for selecting values ​​for the n×n region is as follows:

[0026]

[0027] Where ceil() is the floor function; S Xray θ represents the maximum travel distance of an X-ray single photon within the detector. max S is the incident angle of a single photon; Pixel This refers to the detector pixel size.

[0028] Further, the DN values ​​of the 3×3 region surrounding the pixel are extracted, where the pixel is the center pixel; among the region DN values, the DN values ​​in the first row are the DN values ​​of the first pixel, the second pixel, and the third pixel, respectively; the DN values ​​in the second row are the DN values ​​of the fourth pixel, the center pixel, and the sixth pixel; and the DN values ​​in the third row are the DN values ​​of the seventh pixel, the eighth pixel, and the ninth pixel.

[0029] When the center pixel is located in the first row of the entire image, the DN values ​​of the first, second, and third pixels are all 0;

[0030] When the center pixel is located in the last row of the entire image, the DN values ​​of the seventh, eighth, and ninth pixels are all 0;

[0031] When the center pixel is located in the first column of the entire image, the DN values ​​of the first, fourth, and seventh pixels are all 0;

[0032] When the center pixel is located in the last column of the entire image, the DN values ​​of the third, sixth, and ninth pixels are all set to 0.

[0033] Furthermore, the imaging auxiliary data is integrated with the data in the dynamic memory. The imaging auxiliary data includes satellite attitude and orbit control information at the imaging time, imaging time information, optical system pointing, and exposure time. The data integration specifically involves stitching the imaging auxiliary data and all valid single-photon events together, and adding image frame headers and frame tails.

[0034] Furthermore, a single original image frame is transmitted every 3 to 5 minutes. The individual original images are combined with multiple event frames and output sequentially to the satellite antenna system. The frames are stored in the onboard non-volatile memory at intervals and updated in orbit.

[0035] Furthermore, the original images at fixed intervals are combined with multiple event frames, and data is transmitted using a mode of superimposing one original image with 100 event frames, and then output to the whole satellite antenna system.

[0036] The advantages of this invention compared to the prior art are:

[0037] (1) Compared with the method of downloading all raw frame images, the present invention can effectively reduce the amount of data on orbit. In the face of the low transmission rate of the space-ground data transmission channel, it can realize a large amount of effective data transmission, increase the on-orbit observation time, and improve the efficiency of scientific mission completion.

[0038] (2) The present invention adopts a fixed-interval original frame image downlink method, which can also retain the existing methods to effectively identify detector blind elements, camera dark current characteristics and other features, and ensure the effective acquisition of scientific data.

[0039] (3) The present invention is simple to implement, has low hardware consumption, can be implemented in a 2 million gate FPGA and a small capacity dynamic memory, and has been verified to be effective and reliable through long-term on-orbit verification. Attached Figure Description

[0040] Various other advantages and benefits will become apparent to those skilled in the art upon reading the following detailed description of preferred embodiments. The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Furthermore, the same reference numerals denote the same parts throughout the drawings. In the drawings:

[0041] Figure 1 This is a schematic diagram of an X-ray detection system using a Wolter-type optical system;

[0042] Figure 2 A schematic diagram of an X-ray detection system employing a lobster-eye-shaped optical system;

[0043] Figure 3 The image shown is after dark background correction according to an embodiment of the present invention;

[0044] Figure 4 This is a flowchart of the on-orbit data processing method of a spaceborne soft X-ray single-photon imaging camera according to an embodiment of the present invention;

[0045] Figure 5 This is a schematic diagram illustrating the method for selecting the surrounding n×n region in an embodiment of the present invention. Detailed Implementation

[0046] Exemplary embodiments of the present disclosure will now be described in more detail with reference to the accompanying drawings. While exemplary embodiments of the present disclosure are shown in the drawings, it should be understood that the present disclosure may be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art.

[0047] This invention proposes an on-orbit data processing method for a spaceborne soft X-ray single-photon imaging camera. Targeting the photoelectric characteristics of X-ray single photons in CCD / CMOS detectors, an event-oriented information extraction approach is adopted. A 3×3 region is retrieved and determined as a single-photon event based on a threshold, and event characteristics are analyzed to effectively eliminate pseudo-events such as cosmic high-energy rays and solar heavy particles. All single-photon events in the image are stored and downloaded in a timely manner. The event extraction process includes: dark-field background correction, event retrieval, event frame storage, and mixed transmission of full-image frames and event frames. Notably, the dark-field background correction parameters and event retrieval thresholds are both subject to on-orbit correction based on the detector's cumulative radiation effect.

[0048] The method specifically includes the following steps:

[0049] S1: Dark-field background correction is performed on the input soft X-ray single-photon image. The dark-field background correction adopts a linear correction method, which can achieve minimum on-orbit resource deployment. The correction formula is:

[0050] Y i,j =a i,j ×X i,j +b i,j +c

[0051] Among them, Y i,j X represents the image values ​​in the i-th row and j-th column after correction; i,j To correct the image values ​​in the i-th row and j-th column; a i,j b is the first-order correction coefficient for the dark field background correction of the pixel in the i-th row and j-th column; i,j is the constant term correction coefficient for dark field background correction of the i-th row and j-th column pixel; c is the full-image offset correction coefficient for dark field background correction;

[0052] Correction factor a i,j b i,j The initial values ​​of c are obtained from ground calibration, stored in the onboard non-volatile memory, and can be updated on orbit.

[0053] S2: Using the threshold method for... Figure 3 The image shown is searched line by line, with the threshold divided into a lower limit D. min and upper limit D max Less than the lower threshold D min The pixels are classified as noise, exceeding the upper threshold D. max Pixels classified as high-energy particle radiation pixels or bad pixels are not considered valid soft X-ray single-photon events. This applies when the detected pixel value D ≥ D. min And D≤D max When a pixel is detected, the values ​​of the surrounding n×n region, including the coordinates of the center point, are extracted and stored in the dynamic memory as a valid single-photon event.

[0054] The lower and upper thresholds are determined by the target spectral range, and the specific method is as follows:

[0055] The lower threshold is determined by the minimum detectable photon plus background noise:

[0056]

[0057] Where D_TH min EG is the lower limit threshold for judgment. min The minimum detectable X-ray photon energy is determined by the mission or optical system design and is typically in the range of 0.1 keV to 0.5 keV. 3.65 eV is the bandgap of silicon-based semiconductors, and D_dark is the dark noise of the detector, which is obtained by quantization of the dark pixels of the detector.

[0058] The upper threshold is determined by the minimum detectable photon plus background noise:

[0059]

[0060] Where D_TH max EG is the upper limit threshold for the determination. min The maximum detectable X-ray photon energy is determined by the mission or optical system design and is typically in the range of 5keV to 10keV. 3.65eV is the bandgap of silicon-based semiconductors, and D_dark is the dark noise of the detector, which is obtained by quantization of the dark pixels of the detector.

[0061] The n×n region is determined by the number of pixels traversed by the maximum path of an X-ray single photon within the detector, such as... Figure 5 As shown, n is chosen as follows:

[0062]

[0063] Where ceil() is the floor function; S Xray This represents the maximum path length of an X-ray single photon within the detector; for example, a 10 keV X-ray single photon has a maximum path length of 120 μm in a silicon-based detector. max The angle of incidence for a single photon is usually determined by the optical system design; S Pixel This refers to the detector pixel size.

[0064] In this embodiment, the numerical values ​​of a 3×3 region, including the coordinates of the center point, are extracted and stored in a dynamic memory as a valid single-photon event. The stored information should include: center cell row value, center cell column value, center cell DN value, 1-pixel DN value, 2-pixel DN value, 3-pixel DN value, 4-pixel DN value, 6-pixel DN value, 7-pixel DN value, 8-pixel DN value, and 9-pixel DN value.

[0065] S3: After the full image retrieval is completed, the satellite attitude and orbit control information, imaging time information, optical system pointing, exposure time, and other information are integrated with the data in the dynamic memory. This data is called an event frame image. During storage, false soft X-ray single-photon events caused by bad pixels need to be removed. The coordinates of bad pixels are stored in the onboard non-volatile memory. The coordinates of bad pixels are obtained from long-term observation events and then fed to the onboard processing unit. Based on the analysis of on-orbit X-ray pulsars and cosmic background radiation dose, the data storage space of the dynamic memory should meet the information storage requirements of at least 128 valid single-photon events.

[0066] S4: Combine single-frame raw images at fixed intervals with multiple event frames, stitching together auxiliary data and all valid single-photon events from a single image, and adding a frame header and trailer. Auxiliary data should include the capture time of this frame, overall satellite attitude and orbit control data, full-image offset correction coefficient c, and lower limit threshold D. min Upper limit threshold D max In addition to other camera operating status parameters, and based on the needs of on-orbit use, a mixed transmission method of raw images and event images is required. A raw image frame is transmitted every 3 to 5 minutes, and the interval time is stored in the on-board non-volatile memory, which can be updated on orbit.

[0067] This invention employs a data transmission mode of one original image frame + several event frames (default 100 frames), where the original image consists of the original image captured by the detector and imaging auxiliary data. The event frame processing method is as follows: dark-field background correction is performed on the original image of the soft X-ray single photon received by the spaceborne array detector; then, a threshold method is used to perform line-by-line retrieval of the image. When the detected pixel DN value exceeds the lower threshold but does not exceed the upper threshold, the pixel is detected, and the DN value of the surrounding n×n region and the pixel coordinate value are extracted and placed in dynamic memory as a valid single-photon event; after the full image retrieval is completed, the satellite attitude and orbit control information, imaging time information, optical system pointing, exposure time, and other information are integrated with all single-photon event data in the dynamic memory to form an event frame image; finally, single-frame original images at fixed intervals are combined with multiple event frame images and output sequentially to the entire satellite antenna system. The on-orbit data processing method proposed in this invention has been verified on the Nanjing University-1 satellite.

[0068] The embodiments described above are merely preferred embodiments of the present invention. Ordinary variations and substitutions made by those skilled in the art within the scope of the technical solution of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for on-orbit data processing of a spaceborne soft X-ray single-photon imaging camera, characterized in that, include: The spaceborne array detector receives the raw image and imaging aid data of soft X-ray single photons, and performs dark-field background correction on the raw image; A threshold method is used to perform line-by-line image retrieval. When the DN value of a detected pixel exceeds the lower threshold but does not exceed the upper threshold, the pixel is detected and its surrounding pixels are extracted. n × n The region DN value and the pixel coordinate value are stored in the dynamic memory as a valid single-photon event; After the full image search is completed, the imaging auxiliary data and the data in the dynamic memory are integrated to form an event frame image. The original images at fixed intervals are combined with multiple event frame images and output sequentially to the whole satellite antenna system; Dark-field background correction is performed on the original image using the following formula: in, For the corrected first i line, number j The image values ​​in the column; For the first correction i line, number j The image values ​​in the column; For the first i line, number j The first-order correction coefficient for dark-field background correction of a column of pixels; For the first i line, number j The constant term correction coefficients for dark field background correction of the column pixels; Full-image offset correction coefficients for dark-field background correction; correction coefficients , , The initial values ​​are obtained from ground calibration, stored in the onboard non-volatile memory, and updated in orbit. A thresholding method is used to perform a line-by-line search of the image to check whether the DN value of the detected pixels exceeds the lower threshold and does not exceed the upper threshold. The upper and lower thresholds are determined by the target spectral range. The specific method is as follows: The lower threshold is determined by the minimum detectable photon plus background noise, specifically: in, The lower threshold for judgment, The minimum detectable X-ray photon energy. This is the detector's dark noise; The upper limit threshold is determined by the maximum detectable photon count plus background noise, and the specific formula is as follows: in The upper limit threshold for judgment, For the maximum detectable X-ray photon energy; When the DN value of a detected pixel exceeds the lower threshold but does not exceed the upper threshold, the pixel is detected and its surrounding area is extracted. n × n The region DN value and the pixel coordinate value are stored in dynamic memory. n × n The region is determined by the number of pixels traversed by the X-ray single photon during its maximum travel within the detector; n × n The method for determining the region value is as follows: Where ceil() is the floor function; This represents the maximum travel distance of an X-ray single photon within the detector. The incident angle of a single photon; This refers to the detector pixel size.

2. The on-orbit data processing method for a spaceborne soft X-ray single-photon imaging camera according to claim 1, characterized in that, Extracting the periphery of pixels n × n The region DN value, where the cell is the center cell. n When the value is 3, the DN values ​​in the first row of the region are the DN values ​​of the first pixel, the second pixel, and the third pixel, respectively; the DN values ​​in the second row are the DN values ​​of the fourth pixel, the center pixel, and the sixth pixel; and the DN values ​​in the third row are the DN values ​​of the seventh pixel, the eighth pixel, and the ninth pixel. When the center pixel is located in the first row of the entire image, the DN values ​​of the first, second, and third pixels are all 0; When the center pixel is located in the last row of the entire image, the DN values ​​of the seventh, eighth, and ninth pixels are all 0; When the center pixel is located in the first column of the entire image, the DN values ​​of the first, fourth, and seventh pixels are all 0; When the center pixel is located in the last column of the entire image, the DN values ​​of the third, sixth, and ninth pixels are all set to 0.

3. The on-orbit data processing method for a spaceborne soft X-ray single-photon imaging camera according to claim 1, characterized in that, The imaging aid data is integrated with the data in the dynamic memory. The imaging aid data includes satellite attitude and orbit control information at the imaging time, imaging time information, optical system pointing, and exposure time. The data integration is specifically: the imaging aid data and all effective single-photon events are stitched together, and image frame headers and tails are added.

4. The on-orbit data processing method for a spaceborne soft X-ray single-photon imaging camera according to claim 1, characterized in that, A single original image frame is transmitted every 3 to 5 minutes. The individual original images are combined with multiple event frames and output sequentially to the satellite antenna system. The frames are stored in the onboard non-volatile memory at intervals and updated in orbit.

5. The on-orbit data processing method for a spaceborne soft X-ray single-photon imaging camera according to claim 1, characterized in that, The original images at fixed intervals are combined with multiple event frames. Data is transmitted using a mode that superimposes one original image with 100 event frames and outputs it to the whole satellite antenna system.

Images