Space gravitational wave detection telescope with coaxial entrance pupil and exit pupil and design method thereof

Abstract

The invention provides an exit pupil and entrance pupil coaxial space gravitational wave detection telescope and a design method thereof, which are used for solving the problems that the exit pupil and the entrance pupil of the existing off-axis four-mirror space gravitational wave detection telescope are not coaxial, so that the TTL coupling noise of the whole system is increased; and the signal-to-noise ratio of the space gravitational wave detection telescope is greatly reduced. An off-axis five-mirror structure is adopted, and the third mirror, the fourth mirror, the fifth mirror and the exit pupil are all located on the side, away from the secondary mirror, of the primary mirror. A primary mirror, a secondary mirror, a third mirror and a fourth mirror are reasonably arranged, laser reflected by the third mirror enters the fourth mirror and then is reflected by the fourth mirror to form parallel laser, and the formed parallel laser is emitted in the direction perpendicular to the central optical axis of an aperture diaphragm and then is emitted from an exit pupil after being reflected by a fifth mirror, so that the exit pupil and an entrance pupil of the space gravitational wave detection telescope are coaxial. TTL coupling noise of the space gravitational wave detection telescope is greatly reduced, and the high signal-to-noise ratio of the space gravitational wave detection telescope is ensured.

Description

Technical Field

[0001] The present invention relates to a space gravitational wave detection telescope, and in particular to a space gravitational wave detection telescope with coaxial entrance and exit pupils and a design method thereof. Background Art

[0002] Since the gravitational wave signals detected on the ground are not only easily affected by seismic wave noise, but also have the problem of limited detection frequency band, it is particularly important to develop space gravitational wave detection technology.

[0003] As a key component of space gravitational wave detection technology, the space gravitational wave detection telescope not only needs to transmit the watt-level laser energy emitted by the local base station to the satellite base station millions of kilometers away, but also needs to receive the picowatt-level weak laser signal sent from the satellite base station millions of kilometers away, so as to form a stable two-way laser detection link between the two base stations. Therefore, the space gravitational wave detection telescope usually requires an ultra-high signal-to-noise ratio. Existing space gravitational wave detection telescopes usually adopt an off-axis four-transmission structure, such as Figure 1 As shown, it includes an aperture stop 01, a primary mirror 02, a secondary mirror 03, a third mirror 04, a fourth mirror 05, and an exit pupil 06. The primary and secondary mirrors are aspherical reflectors (the primary mirror 02 is a parabolic reflector, and the secondary mirror 03 is a hyperbolic reflector), and the third mirror 04 and the fourth mirror 05 are both spherical reflectors. However, the existing off-axis four-transmission space gravitational wave detection telescope has a larger TTL (optical path tilt) coupling noise as the exit pupil 06 and the entrance pupil are not coaxial, which greatly reduces the signal-to-noise ratio of the space gravitational wave detection telescope. Summary of the invention

[0004] The purpose of the present invention is to solve the technical problem that the exit pupil and entrance pupil of the existing off-axis quadruple-transverse space gravitational wave detection telescope are not coaxial, resulting in an increase in the TTL coupling noise of the entire system, thereby greatly reducing the signal-to-noise ratio of the space gravitational wave detection telescope, and propose a space gravitational wave detection telescope with coaxial exit and entrance pupils and a design method thereof.

[0005] The technical concept of the present invention is as follows: when optimizing the existing off-axis four-transmission space gravitational wave detection telescope, if the off-axis four-transmission structure is continued to be used, more optimization variables need to be added to the optimization process. If the aspherical reflectors and spherical reflectors in the off-axis four-transmission structure are replaced with free-form surface reflectors, not only the processing and adjustment difficulty of the system is increased, but also the roughness level of the free-form surface is quite difficult. Therefore, based on the above considerations, the present invention proposes a space gravitational wave detection telescope with coaxial entrance and exit pupils based on an off-axis five-transmission structure.

[0006] In order to achieve the above purpose, the technical solution provided by the present invention is as follows: A space gravitational wave detection telescope with coaxial entrance and exit pupils, which is special in that it comprises an aperture stop, a primary mirror, a secondary mirror, a third mirror, a fourth mirror, a fifth mirror and an exit pupil, which are sequentially located on an external parallel laser incident light path; The primary mirror is a parabolic reflector, the secondary mirror is a hyperbolic reflector, the third and fourth mirrors are spherical reflectors, and the fifth mirror is a plane reflector; The primary mirror and the secondary mirror form a Cassegrain structure; The three mirrors, four mirrors, five mirrors and the exit pupil are all located on the side of the primary mirror away from the secondary mirror; The setting angles of the four mirrors must satisfy the following requirements: the laser reflected by the three mirrors is incident on the four mirrors and then reflected by the four mirrors to form parallel lasers, and the formed parallel lasers are emitted in a direction perpendicular to the central optical axis of the aperture stop; The five mirrors and the exit pupil are located on the central optical axis of the aperture stop; The primary mirror, secondary mirror, third mirror, fourth mirror and fifth mirror do not interfere with each other.

[0007] Furthermore, the aperture stop, primary mirror, secondary mirror, third mirror, fourth mirror, fifth mirror and exit pupil are arranged in the same vertical plane, which can ensure the miniaturization of the system.

[0008] Furthermore, the three mirrors are arranged to be deflected upwards, and at this time, the four mirrors are located above the three mirrors.

[0009] Furthermore, the three mirrors are arranged to be deflected downwards, and at this time, the four mirrors are located below the three mirrors.

[0010] At the same time, the present invention also provides a design method for the above-mentioned space gravitational wave detection telescope with coaxial entrance and exit pupils, which is special in that it includes the following steps: Step 1: Determine the distance between the primary mirror and the secondary mirror according to the entrance pupil diameter and geometric dimensions of the space gravitational wave telescope to be designed, and then build a Cassegrain structure with the primary and secondary mirrors coaxial; set the position of the primary real image plane of the space gravitational wave telescope to be designed, and optimize the variable parameters of the Cassegrain structure according to the off-axis amount of the aperture stop of the Cassegrain structure to ensure that the image plane of the Cassegrain structure is located at the primary real image plane and the imaging effect of the Cassegrain structure is close to the diffraction limit; Step 2, adding three mirrors to the laser light path reflected by the secondary mirror, so that the three mirrors are located behind the primary real image plane; adding four mirrors to the laser light path reflected by the three mirrors, so that the laser reflected by the four mirrors forms parallel lasers and is emitted in a direction perpendicular to the central optical axis of the aperture stop; Step 3, according to the incident wavelength, entrance pupil diameter, field angle, focal length and compression ratio of the space gravitational wave telescope to be designed, the variable parameters of the optical system after adding three mirrors and four mirrors are optimized with wave aberration as the evaluation index; Step 4, adding five mirrors to the parallel laser light path reflected by the four mirrors, and setting geometric constraints so that the exit pupil is located on the central optical axis of the aperture stop; optimizing the variable parameters of the optical system after adding the five mirrors using wave aberration as an evaluation index; Step 5, respectively performing wavefront aberration analysis, TTL coupling noise analysis, and backward stray light analysis on the optical system optimized in step 4; if the results of each analysis meet the corresponding requirements, the variable parameter optimization of the optical system is completed, and step 7 is executed; if any analysis result does not meet the corresponding requirements, the deflection angles of the three mirrors are adjusted, and the variable parameters of the optical system are re-optimized until the results of each analysis meet the corresponding requirements; Step 6, perform tolerance analysis on the optical system, and allocate mirror processing tolerances and installation tolerances for the primary mirror, secondary mirror, third mirror, fourth mirror, and fifth mirror. Then use the Monte Carlo method to perform simulation analysis to ensure that the mirror processing tolerances and installation tolerances of the primary mirror, secondary mirror, third mirror, fourth mirror, and fifth mirror meet the pass rate requirements, and complete the design of the space gravitational wave detection telescope with coaxial entrance and exit pupils.

[0011] Further, in step 5, the analysis result of the wave aberration analysis must satisfy: the wave aberration is better than λ / 80; The analysis results of the TTL coupling noise analysis must meet the following requirements: TTL coupling noise is less than 25 pm / μrad; The analysis result of the backward stray light analysis must meet the following requirements: the backward stray light energy is less than 10% of the external parallel laser energy. -10 .

[0012] Furthermore, in step 2, three mirrors are added to the laser light path reflected by the secondary mirror so that the three mirrors are located behind the primary real image plane. Specifically: Add three mirrors to the laser light path reflected by the secondary mirror, so that the three mirrors are located behind the primary real image plane and are deflected upward.

[0013] Furthermore, in step 2, three mirrors are added to the laser light path reflected by the secondary mirror so that the three mirrors are located behind the primary real image plane. Specifically: Add three mirrors to the laser light path reflected by the secondary mirror, so that the three mirrors are located behind the primary real image plane and are deflected downward.

[0014] Further, in step 1, the variable parameters of the Cassegrain structure include the radius of curvature of the primary mirror, the radius of curvature of the secondary mirror, and the quadratic surface coefficient of the secondary mirror; In step 3, the variable parameters of the optical system after adding three mirrors and four mirrors include the radius of curvature of the primary mirror, the radius of curvature of the secondary mirror, the radius of curvature of the three mirrors, the radius of curvature of the four mirrors, and the quadratic surface coefficient of the secondary mirror; In step 4, the variable parameters of the optical system after adding five mirrors include the curvature radius of the primary mirror, the curvature radius of the secondary mirror, the curvature radius of the third mirror, the curvature radius of the fourth mirror, the curvature radius of the fifth mirror and the quadratic surface coefficient of the secondary mirror.

[0015] Compared with the prior art, the present invention has the following beneficial effects: 1. The present invention adopts an off-axis five-reflector structure, wherein the primary mirror is a parabolic reflector, the secondary mirror is a hyperbolic reflector, the third mirror and the fourth mirror are spherical reflectors, and the fifth mirror is a plane reflector. The primary mirror, the secondary mirror, the third mirror and the fourth mirror are reasonably arranged so that the laser reflected by the third mirror is incident on the fourth mirror and then reflected by the fourth mirror to form a parallel laser, and the formed parallel laser is emitted in a direction perpendicular to the central optical axis of the aperture stop, and then emitted from the exit pupil after being reflected by the five mirrors, so that the exit pupil and the entrance pupil of the space gravitational wave detection telescope can be coaxial, the TTL coupling noise of the space gravitational wave detection telescope is greatly reduced, and the high signal-to-noise ratio of the space gravitational wave detection telescope is ensured; at the same time, because the laser has a large deflection angle when passing through the third mirror and the fourth mirror, the space gravitational wave detection telescope of the present invention has excellent backward stray light suppression capability.

[0016] 2. The present invention realizes a space gravitational wave detection telescope with coaxial entrance and exit pupils only through the reasonable layout of a parabolic reflector, a hyperbolic reflector, two spherical reflectors and a plane reflector. The overall optical structure is simple. Compared with the off-axis four-mirror structure based on free-form surface reflectors, the surface shape of each reflector in the present invention is simpler, which effectively reduces the difficulty of optical component processing and system-level assembly and adjustment.

[0017] 3. The present invention provides a design method for a space gravitational wave detection telescope with coaxial entrance and exit pupils. The method is based on a Cassegrain structure with coaxial primary and secondary mirrors, uses wave aberration as an evaluation index, and adopts the geometric constraint of coaxial entrance and exit pupils to optimize the optical system. The method has fewer optimization variables, is simple and practical, and can achieve good wave aberration. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 It is a schematic diagram of the structure of the existing off-axis quadruple-trans space gravitational wave detection telescope; Figure 1 The accompanying drawings are denoted as follows: 01-aperture stop; 02-primary mirror; 03-secondary mirror; 04-third mirror; 05-fourth mirror; 06-exit pupil.

[0019] Figure 2 This is a schematic structural diagram of Embodiment 1 of a space gravitational wave detection telescope with coaxial entrance and exit pupils according to the present invention; Figure 3 This is a schematic structural diagram of Embodiment 2 of a space gravitational wave detection telescope with coaxial entrance and exit pupils according to the present invention.

[0020] Figure 2-Figure 3 The reference numerals in the figures are as follows: 1- aperture stop; 2- primary mirror; 3- secondary mirror; 4- tertiary mirror; 5-quadruple mirror; 6-penta mirror; 7- exit pupil. DETAILED DESCRIPTION

[0021] In order to make the advantages and features of the present invention more clear, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0022] Embodiment 1 like Figure 2 As shown, a space gravitational wave detection telescope with coaxial entrance and exit pupils is an off-axis five-transverse structure, which includes an aperture stop 1, a primary mirror 2, a secondary mirror 3, a third mirror 4, a fourth mirror 5, a fifth mirror 6 and an exit pupil 7. In order to achieve system miniaturization, the present embodiment preferably arranges the aperture stop 1, the primary mirror 2, the secondary mirror 3, the third mirror 4, the fourth mirror 5, the fifth mirror 6 and the exit pupil 7 in the same vertical plane.

[0023] The primary mirror 2 is a parabolic reflector, the secondary mirror 3 is a hyperbolic reflector, the aperture stop 1 and the primary mirror 2 are located in sequence on the incident light path of the external parallel laser, and the secondary mirror 3 is located on the laser light path reflected by the primary mirror 2; wherein the primary mirror 2 and the secondary mirror 3 form a Cassegrain structure.

[0024] The three mirrors 4 , the four mirrors 5 , the five mirrors 6 and the exit pupil 7 are all located on the side of the primary mirror 2 away from the secondary mirror 3 , that is, the three mirrors 4 , the four mirrors 5 , the five mirrors 6 and the exit pupil 7 are all located on the non-reflective surface side of the primary mirror 2 .

[0025] The third mirror 4 and the fourth mirror 5 are both spherical reflectors. The third mirror 4 is located on the laser light path reflected by the secondary mirror 3, and the fourth mirror 5 is located on the laser light path reflected by the third mirror 4. In the present invention, the third mirror 4 can be deflected upward or downward. When the third mirror 4 is deflected upward, the fourth mirror 5 is arranged above the third mirror 4. When the third mirror 4 is deflected downward, the fourth mirror 5 is arranged below the third mirror 4. In this embodiment, since the aperture stop 1, the primary mirror 2, the secondary mirror 3, the third mirror 4, the fourth mirror 5, the fifth mirror 6 and the exit pupil 7 are arranged in the same vertical plane, and the third mirror 4 is deflected upward, the fourth mirror 5 is arranged above the third mirror 4, forming an "S"-shaped off-axis five-trans space gravitational wave detection telescope with coaxial entrance and exit pupils. At the same time, since the external parallel laser is non-parallel laser after being reflected by the primary mirror 2, the secondary mirror 3 and the third mirror 4, the setting angle of the fourth mirror 5 needs to satisfy: the laser reflected by the third mirror 4 is incident on the fourth mirror 5, and forms a small-diameter parallel laser after being reflected by the fourth mirror 5, that is, the large-diameter external parallel laser is converted into a small-diameter parallel laser through the primary mirror 2, the secondary mirror 3, the third mirror 4 and the fourth mirror 5 in sequence, and the formed parallel laser is emitted in a direction perpendicular to the central optical axis of the aperture stop 1, so that the third mirror 4 and the fourth mirror 5 form a laser collimation system. In addition, in order to prevent the mirror stray light of the primary mirror 2, the secondary mirror 3 and the third mirror 4 from affecting each other, the third mirror 4 needs to be set outside the primary mirror 2.

[0026] The five mirrors 6 are plane reflecting mirrors. The five mirrors 6 and the exit pupil 7 are sequentially located on the optical path of the parallel laser reflected by the four mirrors 5, and the five mirrors 6 and the exit pupil 7 are simultaneously arranged on the central optical axis of the aperture stop 1 to achieve coaxiality of the entrance and exit pupils.

[0027] The receiving optical path of the space gravitational wave detection telescope with coaxial entrance and exit pupils of the present invention is as follows: parallel laser light sent from a satellite base station a million kilometers away is incident on the surface of a primary mirror 2 through an aperture diaphragm 1, is reflected by the primary mirror 2 and then by a secondary mirror 3, and forms a real image surface between the secondary mirror 3 and a third mirror 4, and then is reflected by the third mirror 4, the fourth mirror 5 and the fifth mirror 6 in sequence, and finally emerges at the position of an exit pupil 7 as a parallel light beam.

[0028] The design parameters of the primary mirror 2, the secondary mirror 3, the third mirror 4, the fourth mirror 5 and the fifth mirror 6 in the optical system of this embodiment are shown in Table 1.

[0029] Table 1 Optical system design parameters

[0030] The present invention also provides a design method for the above-mentioned space gravitational wave detection telescope with coaxial entrance and exit pupils, which specifically comprises the following steps: Step 1: Build a coaxial Cassegrain structure of the primary and secondary mirrors, and optimize the variable parameters of the Cassegrain structure.

[0031] First, according to the entrance pupil diameter and geometric dimensions of the space gravitational wave telescope to be designed, the distance between the primary mirror 2 and the secondary mirror 3 is determined, and then the coaxial Cassegrain structure of the primary and secondary mirrors is built. Then, the position of the primary real image plane of the space gravitational wave telescope to be designed is set, and according to the off-axis amount of the aperture stop of the Cassegrain structure, the curvature radius of the primary mirror 2, the curvature radius of the secondary mirror 3, and the quadratic surface coefficient of the secondary mirror 3 are optimized to ensure that the image plane of the Cassegrain structure is located at the primary real image plane, and the imaging effect of the Cassegrain structure is close to the diffraction limit.

[0032] Step 2, add third mirror 4 and fourth mirror 5.

[0033] A third mirror 4 is added to the laser light path reflected by the secondary mirror 3, so that the third mirror 4 is located behind the primary real image plane and is deflected upward. A fourth mirror 5 is added to the laser light path reflected by the third mirror 4, so that parallel laser light is formed after reflection by the fourth mirror 5. The formed parallel laser light is emitted in a direction perpendicular to the central optical axis of the aperture stop 1. At this time, the third mirror 4 and the fourth mirror 5 form a stable laser collimation system.

[0034] Step 3, optimizing the variable parameters of the optical system after adding the three mirrors 4 and the four mirrors 5.

[0035] According to the incident wavelength, entrance pupil diameter, field angle, focal length and compression ratio of the space gravitational wave telescope to be designed, the curvature radius of the primary mirror 2, the curvature radius of the secondary mirror 3, the curvature radius of the third mirror 4, the curvature radius of the fourth mirror 5 and the quadratic surface coefficient of the secondary mirror 3 are optimized with wave aberration as the evaluation index. Among them, the optimization index is required to be better than λ / 80.

[0036] Step 4, adding five mirrors 6 and optimizing the variable parameters of the optical system after adding the five mirrors 6.

[0037] A fifth mirror 6 is added to the parallel laser light path reflected by the fourth mirror 5, and geometric constraints are set so that the exit pupil 7 is located on the central optical axis of the aperture stop 1. The curvature radius of the primary mirror 2, the curvature radius of the secondary mirror 3, the curvature radius of the third mirror 4, the curvature radius of the fourth mirror 5, the curvature radius of the fifth mirror 6 and the quadratic surface coefficient of the secondary mirror 3 are optimized with wave aberration as the evaluation index.

[0038] Step 5, performing wave aberration analysis, TTL coupling noise analysis and backward stray light analysis on the optical system optimized in step 4.

[0039] If the analysis results of wave aberration analysis, TTL coupling noise analysis and backward stray light analysis all meet the corresponding requirements, the variable parameter optimization of the optical system is completed. If any analysis result does not meet the corresponding requirements, the deflection angle of the third mirror 4 is adjusted, and the curvature radius of the primary mirror 2, the curvature radius of the secondary mirror 3, the curvature radius of the third mirror 4, the curvature radius of the fourth mirror 5, the curvature radius of the fifth mirror 6 and the quadratic surface coefficient of the secondary mirror 3 are re-optimized until all analysis results meet the corresponding requirements.

[0040] The analysis results of wavefront aberration analysis must meet the following requirements: wavefront aberration is better than λ / 80; the analysis results of TTL coupling noise analysis must meet the following requirements: TTL coupling noise is less than 25pm / μrad; the analysis results of backward stray light analysis must meet the following requirements: backward stray light energy is less than 10% of the external parallel laser energy. -10 .

[0041] Step 7, tolerance analysis of the optical system. Tolerance analysis usually includes machining tolerance and adjustment tolerance analysis. Therefore, it is necessary to reasonably allocate the mirror machining tolerance and adjustment tolerance of the primary mirror 2, secondary mirror 3, third mirror 4, fourth mirror 5 and fifth mirror 6 according to the existing optical processing capabilities. Then, the Monte Carlo method is used to simulate and analyze the mirror machining tolerance and adjustment tolerance of the primary mirror 2, secondary mirror 3, third mirror 4, fourth mirror 5 and fifth mirror 6 to ensure that the mirror machining tolerance and adjustment tolerance of the primary mirror 2, secondary mirror 3, third mirror 4, fourth mirror 5 and fifth mirror 6 meet the qualified rate requirements. At this point, the design of the space gravitational wave detection telescope with coaxial entrance and exit pupils is completed.

[0042] Embodiment 2 like Figure 3 As shown, the difference between this embodiment and the first embodiment lies only in the arrangement of the three mirrors 4 and the four mirrors 5 . In this embodiment, the three mirrors 4 are deflected downward, and the four mirrors 5 are arranged below the three mirrors 4 .

[0043] The design method of the space gravitational wave detection telescope with coaxial entrance and exit pupils in Example 2 is different from that in Example 1 only in that, in step 2, a third mirror 4 is added to the laser light path reflected by the secondary mirror 3, so that the third mirror 4 is located behind the primary real image plane and is deflected downward to form a "δ"-type off-axis five-transmission space gravitational wave detection telescope with coaxial entrance and exit pupils.

[0044] Taking the external parallel laser of 1 W as an example, the backward stray light suppression capability of the two embodiments of the present invention and the existing off-axis quadruple-transmission space gravitational wave detection telescope was analyzed under the same test conditions. The analysis results are shown in Table 2.

[0045] Table 2 Analysis results of backward stray light suppression capability of three optical structures

[0046] It can be seen that the backward stray light suppression capability of the space gravitational wave detection telescope with coaxial entrance and exit pupil based on the off-axis five-trans structure in the first embodiment of the present invention is better than the backward stray light suppression capability of the existing off-axis four-trans space gravitational wave detection telescope. Although the backward stray light suppression capability of the space gravitational wave detection telescope with coaxial entrance and exit pupil based on the off-axis five-trans structure in the second embodiment of the present invention is relatively low, it is the same as the first embodiment. The entrance and exit pupils of the system are coaxial, which greatly reduces the TTL coupling noise of the space gravitational wave detection telescope and ensures a high signal-to-noise ratio of the space gravitational wave detection telescope.

[0047] The above description is only used to illustrate the technical solution of the present invention rather than to limit it. For ordinary professional and technical personnel in the field, the specific technical solution recorded in the above embodiment can be modified, or some of the technical features therein can be replaced by equivalents, and these modifications or replacements do not make the essence of the corresponding technical solution deviate from the scope of the technical solution protected by the present invention.

Claims

1. A space gravitational wave detection telescope with coaxial entrance and exit pupils, characterized by: It includes an aperture stop (1), a primary mirror (2), a secondary mirror (3), a third mirror (4), a fourth mirror (5), a fifth mirror (6), and an exit pupil (7) which are sequentially located on the incident light path of an external parallel laser. The primary mirror (2) is a parabolic reflector, the secondary mirror (3) is a hyperbolic reflector, the third mirror (4) and the fourth mirror (5) are spherical reflectors, and the fifth mirror (6) is a plane reflector; The primary mirror (2) and the secondary mirror (3) form a Cassegrain structure; The three mirrors (4), the four mirrors (5), the five mirrors (6) and the exit pupil (7) are all located on a side of the primary mirror (2) away from the secondary mirror (3); The setting angles of the four mirrors (5) must satisfy the following conditions: the laser light reflected by the three mirrors (4) is incident on the four mirrors (5) and then reflected by the four mirrors (5) to form parallel laser light, and the formed parallel laser light is emitted in a direction perpendicular to the central optical axis of the aperture stop (1); The five mirrors (6) and the exit pupil (7) are located on the central optical axis of the aperture stop (1); The primary mirror (2), the secondary mirror (3), the third mirror (4), the fourth mirror (5) and the fifth mirror (6) do not interfere with each other.

2. The space gravitational wave detection telescope with coaxial entrance and exit pupils according to claim 1, characterized in that: The aperture stop (1), the primary mirror (2), the secondary mirror (3), the third mirror (4), the fourth mirror (5), the fifth mirror (6) and the exit pupil (7) are arranged in the same vertical plane.

3. A space gravitational wave detection telescope with coaxial entrance and exit pupils according to claim 1 or 2, characterized in that: The three mirrors (4) are arranged to be deflected upwards, and the four mirrors (5) are located above the three mirrors (4).

4. A space gravitational wave detection telescope with coaxial entrance and exit pupils according to claim 1 or 2, characterized in that: The three mirrors (4) are arranged to be deflected downwards, and the four mirrors (5) are located below the three mirrors (4).

5. A method for designing a space gravitational wave detection telescope with coaxial entrance and exit pupils as described in any one of claims 1 to 4, characterized in that: The following steps are involved: Step 1, according to the entrance pupil diameter and geometric dimensions of the space gravitational wave telescope to be designed, determine the distance between the primary mirror (2) and the secondary mirror (3), and then build a Cassegrain structure in which the primary and secondary mirrors are coaxial; set the position of the primary real image plane of the space gravitational wave telescope to be designed, and optimize the variable parameters of the Cassegrain structure according to the off-axis amount of the aperture stop of the Cassegrain structure to ensure that the image plane of the Cassegrain structure is located at the primary real image plane and the imaging effect of the Cassegrain structure is close to the diffraction limit; Step 2, adding a third mirror (4) to the laser light path reflected by the secondary mirror (3), so that the third mirror (4) is located behind the primary real image plane; adding a fourth mirror (5) to the laser light path reflected by the third mirror (4), so that the laser light reflected by the fourth mirror (5) forms parallel laser light and is emitted in a direction perpendicular to the central optical axis of the aperture stop (1); Step 3, according to the incident wavelength λ, entrance pupil diameter, field angle, focal length and compression ratio of the space gravitational wave telescope to be designed, the variable parameters of the optical system after adding three mirrors (4) and four mirrors (5) are optimized with wave aberration as an evaluation index; Step 4, adding five mirrors (6) to the parallel laser light path reflected by the four mirrors (5), and setting geometric constraints so that the exit pupil (7) is located on the central optical axis of the aperture stop (1); optimizing the variable parameters of the optical system after adding the five mirrors (6) using wave aberration as an evaluation index; Step 5, performing wave aberration analysis, TTL coupling noise analysis and backward stray light analysis on the optical system optimized in step 4; If all analysis results meet the corresponding requirements, the variable parameter optimization of the optical system is completed and step 6 is executed; if any analysis result does not meet the corresponding requirements, the deflection angle of the three mirrors (4) is adjusted and the variable parameters of the optical system are optimized again until all analysis results meet the corresponding requirements; Step 6, perform tolerance analysis on the optical system, allocate mirror processing tolerances and installation tolerances for the primary mirror (2), secondary mirror (3), third mirror (4), fourth mirror (5) and fifth mirror (6); then use the Monte Carlo method to perform simulation analysis to ensure that the mirror processing tolerances and installation tolerances for the primary mirror (2), secondary mirror (3), third mirror (4), fourth mirror (5) and fifth mirror (6) meet the pass rate requirements, and complete the design of the space gravitational wave detection telescope with coaxial entrance and exit pupils.

6. The design method of a space gravitational wave detection telescope with coaxial entrance and exit pupils according to claim 5, characterized in that: In step 5, the analysis result of the wave aberration analysis must satisfy: the wave aberration is better than λ / 80; The analysis results of the TTL coupling noise analysis must meet the following requirements: TTL coupling noise is less than 25 pm / μrad; The analysis result of the backward stray light analysis must meet the following requirements: the backward stray light energy is less than 10% of the external parallel laser energy. -10 .

7. The design method of a space gravitational wave detection telescope with coaxial entrance and exit pupils according to claim 6, characterized in that: In step 2, a third mirror (4) is added to the laser light path reflected by the secondary mirror (3), so that the third mirror (4) is located behind the primary real image plane. Specifically: A third mirror (4) is added to the laser light path reflected by the secondary mirror (3), so that the third mirror (4) is located behind the primary real image plane and is deflected upward.

8. The design method of a space gravitational wave detection telescope with coaxial entrance and exit pupils according to claim 6, characterized in that: In step 2, a third mirror (4) is added to the laser light path reflected by the secondary mirror (3), so that the third mirror (4) is located behind the primary real image plane. Specifically: A third mirror (4) is added to the laser light path reflected by the secondary mirror (3), so that the third mirror (4) is located behind the primary real image plane and is deflected downward.

9. The method for designing a space gravitational wave detection telescope with coaxial entrance and exit pupils according to any one of claims 5 to 8, characterized in that: In step 1, the variable parameters of the Cassegrain structure include the radius of curvature of the primary mirror (2), the radius of curvature of the secondary mirror (3), and the quadratic surface coefficient of the secondary mirror (3); In step 3, the variable parameters of the optical system after adding the three mirrors (4) and the four mirrors (5) include the radius of curvature of the primary mirror (2), the radius of curvature of the secondary mirror (3), the radius of curvature of the three mirrors (4), the radius of curvature of the four mirrors (5) and the quadratic surface coefficient of the secondary mirror (3); In step 4, the variable parameters of the optical system after adding the five mirrors (6) include the radius of curvature of the primary mirror (2), the radius of curvature of the secondary mirror (3), the radius of curvature of the third mirror (4), the radius of curvature of the fourth mirror (5), the radius of curvature of the fifth mirror (6) and the quadratic surface coefficient of the secondary mirror (3).

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