Focal plane scanning near-field holographic surface shape measuring device and method for radio telescope antenna

Abstract

The invention provides a focal plane scanning near-field holographic surface shape measuring device and method for a radio telescope antenna, and the method comprises the steps: an emission source illuminates the center of a reflection surface at the curvature center of a to-be-measured parabolic antenna, and the distribution of a field reflected by a paraboloid is focused at a position, which is symmetrical to the emission source, about the normal of a central point of the antenna; amplitude and phase measurement is carried out on a focusing plane of an antenna by using a coherent detector, and then antenna surface shape error distribution can be obtained through wavefront inversion and error correction. The method has the following characteristics: 1) the surface shape error distribution of the whole aperture of the antenna can be obtained through small-range plane scanning in a focal plane field area; 2) a plane scanning motion mechanism is simple, the precision is easier to control, and the method is suitable for surface shape measurement of large-scale antennas; and 3) the problem that the required measurement distance is too long in large-scale antenna measurement is relieved.

Description

Technical Field

[0001] The present invention belongs to the technical field of telescopes, and in particular relates to a device and method for measuring the focal plane scanning near-field holographic surface shape of a radio telescope antenna. Background Art

[0002] Frontiers in the development of modern single-aperture radio telescopes include expanding the observation frequency band to higher-frequency submillimeter and terahertz bands, and achieving larger radio telescopes with larger apertures and greater light-collecting capabilities. Large radio telescopes typically utilize primary or secondary reflectors composed of spliced ​​reflective panels. Panel machining and splicing errors are the primary sources of overall surface errors in the reflector. Currently, three-dimensional coordinate measuring machines (CMMs) are commonly used to measure machining errors in large antenna panels. However, CMMs are contact-based, have a limited measurement range, and require a strict measurement environment. Radio holography offers high measurement accuracy and is generally used as an effective means for final fine-tuning of splicing errors in large reflector antennas. As the aperture of radio telescopes increases, the size of antenna panels also increases accordingly. As radio astronomical observations expand to higher-frequency submillimeter and terahertz bands, the surface accuracy requirements for reflector antennas are also increasing, reaching micrometer levels. Summary of the Invention

[0003] In order to overcome the above limitations of current measurement methods and utilize the high measurement accuracy of near-field holographic measurement technology, the present invention proposes a focal plane scanning near-field holographic surface measurement device and method for radio telescope antennas, providing a more efficient and flexible measurement method for the surface measurement of increasingly larger reflector antennas and panels.

[0004] To achieve the above object, the present invention adopts the following technical solutions:

[0005] In a first aspect, the present invention provides a focal plane scanning near-field holographic surface measurement device for a radio telescope antenna, comprising: a transmitting source, a coherent detector, a digital correlator and a processing unit; the transmitting source illuminates the center of a reflecting surface at the center of curvature of a parabolic antenna to be measured, and the focusing positions of the transmitting source and the field distribution reflected by the parabola are symmetrical about the normal line of the center point of the antenna to be measured; the coherent detector performs sampling measurement on the focal plane of the antenna to be measured; the digital correlator performs cross-correlation operation on the signal received by the coherent detector to obtain the complex distribution of the focal plane field; the processing unit performs wavefront inversion and error correction on the complex distribution of the focal plane field to obtain the surface error distribution and RMS of the antenna to be measured.

[0006] Optionally, the distance between the emission source and the antenna to be measured is twice the focal length; and the coherent detector performs sampling measurement on a focal plane at a distance twice the focal length from the antenna to be measured.

[0007] Optionally, the coherent detector includes a main receiving path and a reference receiving path; the main receiving path is located on a two-dimensional plane scanning frame and moves with the two-dimensional plane scanning frame, and is used to scan and sample the focal plane field distribution of the antenna to be tested; the reference receiving path is located on a stationary platform, and is used to receive a coherent signal of the same frequency as a reference for measuring the focal plane field phase distribution.

[0008] Optionally, the transmitting source includes an RF frequency synthesizer, a power splitter, a frequency multiplier, a power amplifier, an adjustable attenuator and a transmitting speaker; the RF signal generated by the RF frequency synthesizer is divided into two paths by the power splitter: one path passes through the frequency multiplier, the power amplifier and the adjustable attenuator in sequence, and then transmits an RF signal of a specific frequency and power to the antenna to be tested by the transmitting speaker; the other path is sent to the reference receiving path.

[0009] Optionally, the main receiving path includes a receiving speaker, mixer 1 and multiplier 3, and the reference receiving path includes a local oscillator frequency synthesizer, power divider 2, mixer 2, power amplifier 2 and multiplier 2; the receiving speaker receives the radio frequency signal reflected and focused by the antenna to be tested; the local oscillator signal emitted by the local oscillator frequency synthesizer is divided into two paths through power divider 2: one path is mixed with the signal received by the receiving speaker in mixer 1 after passing through multiplier 3, and the mixed signal is input to the digital correlator through a bandpass filter; the other path is mixed with the radio frequency signal sent from power divider 1 in mixer 2, and the mixed signal passes through power amplifier 2 and multiplier 2 in sequence, and then passes through a bandpass filter to be input to the digital correlator.

[0010] In a second aspect, the present invention provides a method for measuring the near-field holographic profile of a radio telescope antenna using a focal plane scanning method, which is performed using the measurement device described in the first aspect and includes the following steps:

[0011] Step 1: The main receiving path uses raster scanning to scan and sample the focal plane field distribution on the two-dimensional plane scanning frame. After reaching each scanning point, the main receiving path obtains the current position information of the two-dimensional plane scanning frame;

[0012] Step 2: The digital correlator performs real-time cross-correlation calculations between the main receiving path and the reference receiving path for each scanning point;

[0013] Step 3: The processing unit calculates the surface error distribution and RMS of the antenna under test based on the position information of the two-dimensional plane scanning frame and the real-time cross-correlation calculation results.

[0014] Optionally, step 1 is as follows: determine the center of the focal field sampling, and perform a center calibration at the center point of the main receiving path before starting the scan. After each row of sampling points is scanned, return to the center point to perform a center calibration, and finally return to the center point to perform a calibration after the scan is completed. The sampling points are scanned N*N times in total, and the center calibration is performed N+1 times in total. M data are saved at each scanning point and center calibration point, and finally a total of M*N*N scanning data and M*(N+1) center calibration data are saved.

[0015] Optionally, step 3 is as follows:

[0016] Step 3.1: Average the scan data of each scanning point to obtain the real and imaginary data of N*N sampling points, and further obtain the corresponding complex number;

[0017] Step 3.2: Average the center-calibrated data to obtain the real and imaginary data with N+1 sampling points, and further obtain the corresponding complex numbers;

[0018] Step 3.3: Use the interpolation method to obtain fitting curve data from the center calibration data obtained in step 3.2, and then divide the scan data obtained in step 3.1 by the fitting curve data to obtain the amplitude and phase distribution of the corrected focal plane field;

[0019] Step 3.4: Based on the position information of the two-dimensional plane scanning frame, perform wavefront inversion on the amplitude and phase distribution of the focal plane field obtained in step 3.3 to obtain the amplitude and phase distribution of the induced field generated by the illumination of the emission source on the surface of the antenna under test;

[0020] Step 3.5: Correct the effect of the phase distribution of the transmitting horn and the receiving horn on the phase distribution of the induced field;

[0021] Step 3.6: Correct the influence of the position and attitude offset of the transmitting horn and the focal plane on the phase distribution of the induced field;

[0022] Step 3.7: Based on the induced field phase distribution obtained in step 3.6, calculate the surface error distribution of the antenna under test and calculate its RMS.

[0023] Optionally, in step 3.6, the primary aberration on the parabola is fitted and corrected using Zernike polynomials.

[0024] Optionally, in step 3.7, the induced field phase distribution obtained in step 3.6 is multiplied by the cosine value of the angle between the incident light and the normal at each point on the antenna to be tested, divided by twice the wave number, to obtain the surface error distribution of the antenna to be tested.

[0025] The beneficial effects of the present invention are as follows: the proposed device and method for focal plane scanning near-field holographic surface measurement of radio telescope antennas can obtain the antenna's full-aperture surface error distribution by scanning a small area near the antenna focal point. Its simple scanning motion mechanism and easily controlled accuracy make it ideally suited for high-precision surface measurement of large-scale single-piece reflective panels, alleviating the problem of excessively long measurement distances required for large-scale antenna measurements. This invention will significantly expand the measurement capabilities of existing radio holographic technology and its application in terahertz astronomy, providing a more efficient and flexible measurement method for future large-aperture reflective antennas and panels. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 Schematic diagram of the near-field holographic surface measurement device for scanning the focal plane of a radio telescope antenna.

[0027] Figure 2 Schematic diagram of the geometric optics of near-field focusing of a parabolic antenna.

[0028] Figure 3 Schematic diagram of the composition of the emission source.

[0029] Figure 4 Schematic diagram of the composition of the coherent detector. DETAILED DESCRIPTION

[0030] The present invention will now be described in further detail with reference to the accompanying drawings.

[0031] The present invention proposes a method for measuring the focal plane scanning near-field holographic surface shape of a radio telescope antenna and constructs a set of measuring devices accordingly. Figure 1 As shown, the focal plane scanning near-field holographic measurement device and method mainly include three parts: formation of focal plane field distribution; sampling of focal plane field distribution; and data processing of focal plane field distribution.

[0032] 1. Formation of focal plane distribution

[0033] The reflector antenna of a radio telescope is a parabola. Illuminating the reflector antenna near the center of curvature of the parabola makes the field reflected by the parabola approximately focused near the center of curvature. In order to separate the focus position of the antenna from the illumination feed (i.e., the transmitting horn) in space so that the two do not affect each other, the illumination feed is defocused laterally, and the illumination direction of the feed is still at the center of the reflector antenna under test. At this time, the field distribution reflected by the parabola will be approximately focused near a position symmetrical to the illumination feed in the direction of the normal to the center of the parabola. Figure 2 The figure shows a geometric optical diagram of near-field focusing of a sector parabolic panel with a focal length of 2 meters. The illumination of the parabola is realized by the transmitting source, which mainly transmits the radio frequency signal of a specific frequency and power. Its components are as follows: Figure 3The parabolic antenna to be tested can be a segmented sector reflector panel or a complete circular aperture reflector antenna. Figure 1 The schematic diagram of the entire measurement system is described using a fan-shaped antenna panel as an example.

[0034] 2. Sampling of focal plane field distribution

[0035] A 2D scanning gantry is mounted near the focal area of ​​the reflector antenna under test. The normal to the gantry's 2D plane aligns with the normal to the focal plane of the reflector antenna under test. The coherent detector's main receiving path is mounted on the gantry and moves with it. The reference receiving path is located on a stationary surface behind the gantry. The gantry is controlled by computer programming, performing a raster scan within the 2D plane. The coherent detector samples the focal surface field distribution at corresponding points.

[0036] The main receiving path of the coherent detector receives the RF signal reflected and focused by the antenna of the reflector to be measured and down-converts, filters and amplifies the signal. The reference receiving path and the main receiving path share the same local oscillator signal source (i.e., local oscillator frequency synthesizer). A power divider is used to separate a path that is mixed with the signal coupled from the RF signal source and multiplied to the same frequency as the focal plane field receiving signal. The two signals are fed together to the digital correlator at the back end. The composition of the coherent detector is as follows: Figure 4 shown.

[0037] The digital correlator digitally samples the two signals from the coherent detector and performs real-time cross-correlation operations to obtain the amplitude and phase distribution of the detection signal. After the scanning frame reaches the specified sampling point, the correlator outputs the current amplitude and phase data to the file as the focal plane field sampling data of that point. The scanning frame then moves to the next target point for data collection and archiving. The scanning sampling and data storage of the focal plane are repeated in sequence for subsequent analysis and inversion of the surface error distribution of the antenna to be tested.

[0038] In this embodiment, the main receiving path uses a raster scanning method to complete focal plane field sampling on a two-dimensional plane scanning frame. The specific method is: first, the center of the focal plane field sampling is determined. If N*N points need to be sampled, the main receiving path is located at the center point for a center calibration before starting the scan. After each row of sampling points is scanned, the main receiving path returns to the center point for a center calibration, and finally returns to the center for a calibration after the scan is completed. The sampling points are scanned N*N times in total, and the number of center calibration points is N+1.

[0039] Real-time cross-correlation calculations are performed between the main receiving path and the reference receiving path at each raster scanning position. The real-time cross-correlation calculations are completed by a digital correlator, and M data (for example, M=5) are saved at each grid point. Therefore, a total of M*N*N focal field data points and M*(N+1) center calibration data points are saved after completing an N*N scan.

[0040] During the scanning process of the main receiving path, the two-dimensional scanning frame moves horizontally or vertically according to the instructions. After the main receiving path reaches the scanning point, the current position information of the scanning frame is obtained. Subsequently, it is sent to the processing unit together with the real-time cross-correlation calculation results of the coherent detector receiving signal to calculate the surface error distribution and RMS of the antenna under test.

[0041] 3. Data Processing of Focal Field Distribution

[0042] The specific method for the processing unit to calculate the telescope antenna surface error distribution includes the following steps:

[0043] 3.1. Scanning data averaging: The data of repeated sampling at each sampling point in the received information is averaged to obtain the real and imaginary data of N*N sampling points, and further obtain the corresponding complex number;

[0044] 3.2. Center calibration data averaging: The center calibration data is averaged to obtain the real and imaginary data with N+1 sampling points, and further obtain the corresponding complex numbers;

[0045] 3.3. Fit the center calibration data and use the fitting curve data to correct the scan data: Use the center calibration data obtained in step 3.2 to obtain the fitting curve data by interpolation, then divide the scan data processed in step 3.1 by the fitting curve data to obtain the corrected focal plane field complex distribution function;

[0046] 3.4. Perform wavefront inversion on the amplitude and phase distribution of the focal plane field obtained in step 3.3 to obtain the amplitude and phase distribution of the induced field generated by the parabolic antenna surface illuminated by the emission source;

[0047] 3.5. Correct the effect of the phase distribution of the transmitting horn and the receiving horn on the phase distribution of the induced field on the antenna surface: subtract the phase distribution of the transmitting source horn and the coherent detector horn from the induced field phase obtained in step 3.4;

[0048] 3.6. Correct the contribution of the position and attitude offset of the transmitting source and the focal plane to the phase distribution of the induced field on the antenna surface: Use Zernike polynomials to fit and correct the primary aberration on the parabola;

[0049] 3.7. The residual phase distribution obtained in step 3.6 is caused by the deviation of the antenna surface shape from the ideal parabola. Multiply the residual phase distribution by the cosine of the angle between the incident light and the normal at each point on the antenna divided by twice the wave number to obtain the surface error distribution of the antenna under test, and calculate its RMS.

[0050] The above are merely preferred embodiments of the present invention. The scope of protection of the present invention is not limited to the above embodiments. All technical solutions based on the principles of the present invention are within the scope of protection of the present invention. It should be noted that for those skilled in the art, various improvements and modifications that do not depart from the principles of the present invention should be considered within the scope of protection of the present invention.

Claims

1. A near-field holographic surface measurement device for focal plane scanning of a radio telescope antenna, characterized in that: include: An emission source, a coherent detector, a digital correlator and a processing unit; the emission source illuminates the center of the reflecting surface at the center of curvature of the parabolic antenna to be tested, and the focusing position of the emission source and the field distribution reflected by the parabola is symmetrical about the normal line of the center point of the antenna to be tested; The coherent detector performs sampling measurement on the focal plane of the antenna to be tested; the digital correlator performs cross-correlation operation on the signal received by the coherent detector to obtain the complex distribution of the focal plane field; the processing unit performs wavefront inversion and error correction on the complex distribution of the focal plane field to obtain the surface error distribution and RMS of the antenna to be tested.

2. The device for measuring the focal plane scanning near-field holographic surface shape of a radio telescope antenna according to claim 1, characterized in that: The distance between the emission source and the antenna to be measured is twice the focal length; the coherent detector performs sampling measurement on a focal plane at a distance twice the focal length from the antenna to be measured.

3. The device for measuring the focal plane scanning near-field holographic surface shape of a radio telescope antenna according to claim 1, characterized in that: The coherent detector includes a main receiving path and a reference receiving path; the main receiving path is located on a two-dimensional plane scanning frame and moves with the two-dimensional plane scanning frame, and is used to scan and sample the focal plane field distribution of the antenna to be tested; The reference receiving circuit is located on a stationary platform and is used to receive a coherent signal of the same frequency as a reference for focal field phase distribution measurement.

4. The device for measuring the focal plane scanning near-field holographic surface shape of a radio telescope antenna as claimed in claim 3, characterized in that: The transmitting source includes an RF frequency synthesizer, a power divider, a frequency multiplier, a power amplifier, an adjustable attenuator and a transmitting speaker; the RF signal generated by the RF frequency synthesizer is divided into two paths by the power divider: one path passes through the frequency multiplier, the power amplifier and the adjustable attenuator in sequence, and then the transmitting speaker transmits an RF signal of a specific frequency and power to the antenna to be tested; the other path is sent to a reference receiving path.

5. The device for measuring the focal plane scanning near-field holographic surface shape of a radio telescope antenna as claimed in claim 4, characterized in that: The main receiving path includes a receiving speaker, a mixer 1 and a frequency multiplier 3, and the reference receiving path includes a local oscillator frequency synthesizer, a power divider 2, a mixer 2, a power amplifier 2 and a frequency multiplier 2; the receiving speaker receives the radio frequency signal reflected and focused by the antenna to be tested; the local oscillator signal emitted by the local oscillator frequency synthesizer is divided into two paths through the power divider 2: one path is mixed with the signal received by the receiving speaker in the mixer 1 after passing through the frequency multiplier 3, and the mixed signal is input to the digital correlator through a bandpass filter; the other path is mixed with the radio frequency signal sent by the power divider 1 in the mixer 2, and the mixed signal is sequentially passed through the power amplifier 2 and the frequency multiplier 2, and then input to the digital correlator through a bandpass filter.

6. A method for measuring the near-field holographic surface shape of a radio telescope antenna by focal plane scanning, performed using a measuring device as claimed in any one of claims 3 to 5, characterized in that: The steps include: Step 1: The main receiving path uses a raster scanning method to scan and sample the focal plane field distribution on the two-dimensional plane scanning frame. After reaching each scanning point, the main receiving path obtains the current position information of the two-dimensional plane scanning frame; Step 2: The digital correlator performs real-time cross-correlation calculations between the main receiving path and the reference receiving path for each scanning point; Step 3: The processing unit calculates the surface error distribution and RMS of the antenna under test based on the position information of the two-dimensional plane scanning frame and the real-time cross-correlation calculation results.

7. The method for measuring the near-field holographic surface shape of a radio telescope antenna by focal plane scanning as claimed in claim 6, characterized in that: Step 1 is as follows: determine the center of the focal field sampling, and perform a center calibration at the center point of the main receiving path before starting the scan. After each row of sampling points is scanned, return to the center point for a center calibration until the scan is completed and finally return to the center point for a calibration. The sampling points are scanned N*N times in total, and the center calibration is N+1 times in total. M data are saved at each scanning point and center calibration point, and finally M*N*N scanning data and M*(N+1) center calibration data are saved.

8. The method for measuring the near-field holographic surface shape of a radio telescope antenna by focal plane scanning as claimed in claim 7, characterized in that: Step 3 is as follows: Step 3.1: Average the scan data of each scan point to obtain the real and imaginary data with N*N sampling points, and further obtain the corresponding complex number; Step 3.2: Take the average of the center-calibrated data to obtain the real and imaginary data with N+1 sampling points, and further obtain the corresponding complex numbers; Step 3.3: The center calibration data obtained in step 3.2 is interpolated to obtain fitting curve data, and the scan data obtained in step 3.1 is divided by the fitting curve data to obtain the amplitude and phase distribution of the focal field after correction; Step 3.4: According to the position information of the two-dimensional plane scanning frame, the amplitude and phase distribution of the focal plane field obtained in step 3.3 are subjected to wavefront inversion to obtain the amplitude and phase distribution of the induced field generated by the illumination of the emission source on the surface of the antenna to be tested; Step 3.5: Correct the influence of the phase distribution of the transmitting horn and the receiving horn on the phase distribution of the induced field; Step 3.6: Correct the influence of the position and attitude offset of the transmitting horn and the focal plane on the phase distribution of the sensing field; Step 3.7: Based on the induced field phase distribution obtained in step 3.6, calculate the surface error distribution of the antenna to be tested and calculate its RMS.

9. The method for measuring the focal plane scanning near-field holographic surface shape of a radio telescope antenna as claimed in claim 8, characterized in that: In step 3.6, the primary aberration on the parabola is fitted and corrected using Zernike polynomials.

10. The method for measuring the near-field holographic surface shape of a radio telescope antenna by focal plane scanning as claimed in claim 8, characterized in that: In step 3.7, the induced field phase distribution obtained in step 3.6 is multiplied by the cosine value of the angle between the incident light and the normal at each point on the antenna to be tested divided by twice the wave number to obtain the surface error distribution of the antenna to be tested.

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