A method for measuring seeing in layered atmosphere based on dual small telescopes

Abstract

The present invention discloses a method for measuring the seeing of a layered atmosphere based on dual small telescopes, and the steps include: two small telescopes simultaneously point to the same guide star region and keep the optical path conjugate; use a direct imaging sensor to shoot the observation images of the two small telescopes; use the aperture diameter, aperture spacing and each guide star offset of the two telescopes as initial input to obtain the overall seeing; use the variance of the above parameters as initial input to obtain the seeing of each atmospheric height layer, and calculate the ground layer seeing, and then inversely calculate the atmospheric turbulence profile at each height. The two small telescopes used in the present invention have small apertures, small volumes, and light weights, and are easy to carry; a rigid tripod is used to limit the two telescopes, which can effectively avoid the influence of relative vibration errors and guide errors; no wavefront sensor is needed, and many defects such as relatively low resolution, limited dynamic range, and high difficulty in accurate calibration of the Shack-Hartmann wavefront sensor can be effectively eliminated.

Description

Technical Field

[0001] The present invention belongs to the field of atmospheric optics technology, and specifically relates to a method for measuring the seeing of a layered atmosphere based on dual small telescopes. The present invention is the result of the China Space Station Project "Survey Telescope Exoplanet Imaging Coronagraph Module", the National Natural Science Foundation of China Astronomy Joint Key Support Project "Research on Key Technologies for High-Contrast Imaging of Exoplanets for Large-Aperture Spliced ​​Mirror Telescopes" (U2031210), the National Major Scientific Research Instrument Development Project "Development of High-Contrast Imaging Detection Instruments for Exoplanets Around Faint Stars" (11827804), and the National Natural Science Foundation of China General Project "Research on Portable Multi-Aperture Daytime Atmospheric Turbulence Profile Measurement Technology Based on Differential Image Motion" (11873068). Background Art

[0002] The measurement of atmospheric turbulence profiles is crucial for evaluating astronomical observatory sites. The study of the function of altitude is becoming more and more important for the performance estimation and optimization of future adaptive optics and multi-layer conjugate adaptive optics. The atmospheric seeing r0 is an important parameter for evaluating the turbulent characteristics of the atmosphere. By measuring the seeing values ​​of each turbulent layer, the atmospheric refractive index structure constant at that altitude can be calculated.

[0003] As the vertical distribution of atmospheric turbulence becomes increasingly important for existing adaptive optics and multi-layer conjugate adaptive optics, many methods have emerged. The most popular methods for measuring atmospheric seeing include: (1) differential image motion seeing monitoring method, which is based on two sub-apertures located in the same telescope to observe the same star and is used to measure the overall seeing. However, it does not have the resolution to obtain the layered atmospheric turbulence profile. (2) Daytime differential image motion seeing monitoring similar method, which is used on the new 1.6-meter solar telescope of the Big Bear Lake Observatory in the United States. However, the new solar telescope used in this method is a large-aperture solar telescope with an aperture of 1.6 meters, which is unrealistic for new sites without large solar telescope equipment. (3) Advanced multi-subaperture daytime seeing monitoring method, although this technology can use two telescopes, it uses the algorithm similar to the daytime differential image motion seeing monitoring method, and introduces a jitter elimination optimization algorithm to eliminate ground vibration, making it impossible to measure the atmospheric seeing in the ground layer. None of them can meet the requirements of simultaneously obtaining the layered atmosphere (including the ground turbulent layer) and the overall seeing parameter value. Summary of the invention

[0004] In order to overcome at least the following defects: (1) the inconvenience of using a large telescope; (2) the inability to obtain the overall seeing; and (3) the inability to obtain the seeing parameters of each layered atmospheric turbulent layer including the ground layer, the present invention provides a method for simultaneously obtaining the overall seeing and the seeing parameters of each layered atmospheric turbulent layer with a more portable device. The method of the present invention proposes a method for measuring the seeing of the layered atmosphere based on two small telescopes. By using granulation guide stars at different angular intervals from the sun as initial input, the seeing of the upper atmospheric layers other than the ground layer is obtained, and a single guide star is combined as the initial input to obtain the overall seeing. The seeing parameters of the ground layer are obtained through the operational relationship between the overall seeing and the layered seeing.

[0005] To achieve the above object, the present invention provides the following technical solutions:

[0006] A method for measuring the seeing of a layered atmosphere based on dual small telescopes, the method comprising the following steps:

[0007] Step 1: using two small telescopes, the two small telescopes are simultaneously pointed at the same guide star area, and the optical paths are kept conjugated;

[0008] Step 2: Use a direct imaging sensor to capture observation images of the two small telescopes, and record the aperture diameters, aperture spacing, and guide star offsets of the two telescopes;

[0009] Step 3: Using the aperture diameters, aperture spacing and guide star offsets of the two telescopes as initial inputs, obtain the overall seeing Fried parameters;

[0010] Step 4: Using the telescope aperture diameter, aperture spacing, and the variance of each guide star offset combination as initial input, obtain the Fried parameters of each atmospheric height layer through matrix singular value decomposition and nonlinear least squares method, calculate the ground layer seeing, and then inversely calculate the atmospheric turbulence profile at each height.

[0011] Furthermore, the two small telescopes are fixed on the same rigid tripod.

[0012] Furthermore, step 2 also includes recording the initial height of the atmospheric seeing stratification node determined by the guide star combination.

[0013] Furthermore, a linearly arranged guide star array is used to recover the atmospheric turbulence profile.

[0014] Furthermore, guide star combinations with different angular spacings and different numbers are selected, and the expected vertical resolution and maximum measurement height are obtained by changing the guide star combination.

[0015] Further, in step 4, the ground layer seeing is:

[0016]

[0017] Where: r0 represents the Fried parameter of atmospheric seeing; r0(h0) represents the value of ground seeing; r0(h n ) represents the atmospheric seeing at each conjugate altitude.

[0018] Further, in step 4, the overall seeing is:

[0019]

[0020] Where: λ represents the measurement wavelength; φ is the zenith angle; is the atmospheric refractive index structure constant.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] (1) The present invention uses two small telescopes and a linearly arranged guide star array to restore the atmospheric turbulence profile, and can select any guide star combination with different angular spacing and any number. By changing the guide star combination, the method of the present invention can obtain good vertical resolution and satisfactory maximum measurement altitude.

[0023] (2) In the method of the present invention, the two telescopes are fixed on the same rigid tripod. This device restricts the two telescopes to point to the same guide area at the same time and maintains the conjugation of the optical path, which can effectively avoid the influence of relative vibration error and guidance error in the traditional method. The method of the present invention can omit the additional step of measuring the overall seeing.

[0024] (3) The method of the present invention does not require the use of a Shack-Hartmann wavefront sensor, which can effectively eliminate its disadvantages such as relatively low resolution, limited dynamic range, and difficulty in accurate calibration.

[0025] (4) The two small telescopes have small apertures, small volumes, and light weight, making the instrument of the present invention more convenient to carry. Considering that potential observatory sites do not have large telescope equipment, and even existing large telescopes are difficult to apply for a large amount of observation time, the method of the present invention is portable, economical, and effective.

[0026] (5) The present invention does not require the addition of new hardware equipment, does not change the existing optical path structure, can fully utilize the existing telescope equipment, and has strong practicality. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is a schematic diagram of the dual small telescope system and the optical path;

[0028] Figure 2 This is a schematic diagram of the conjugate elevation of the optical path of the triangulation method;

[0029] Figure 3 This is a flow chart of a method for measuring the seeing of a layered atmosphere using two small telescopes according to the present invention;

[0030] Markings in the figure: 1. Light source wavefront; 2. Small telescope; 3. Catadioptric optical structure; 4. Rigid scaffolding; 5. Imaging sensor; 6. Image processing and restoration system. DETAILED DESCRIPTION

[0031] The present invention will be further described in detail below in conjunction with the accompanying drawings.

[0032] like Figure 3 As shown, a method for measuring the seeing of a layered atmosphere based on two small telescopes in this embodiment includes the following steps:

[0033] Step 1: The method is based on Figure 1 The optical system shown in the figure includes a light source wavefront 1, a small telescope 2, a catadioptric optical structure 3, a rigid scaffold 4, an imaging sensor 5, and an image processing and recovery system 6. The two small telescopes 2 are fixed on the same rigid tripod 4, and the rigid tripod 4 restricts the two small telescopes 2 to point to the same guide star area at the same time and keep the optical path conjugate. Figure 2 This is a schematic diagram of the conjugate elevation of the optical path using the triangulation method.

[0034] Step 2: Record the observation images of the two small telescopes 2 taken by the imaging sensor 5, record the aperture diameters and aperture spacing of the two telescopes, record the offsets of each guide star, and record the initial height of the atmospheric seeing stratification node determined by the guide star combination.

[0035] Step 3: Using the telescope aperture diameter, aperture spacing, and guide star offsets as initial input, obtain the overall seeing Fried parameter, denoted by r0.

[0036] When the two telescopes are pointed at the same guide star at the same time, the method is converted to the traditional differential image motion monitoring method, which can measure the overall atmospheric seeing. When the two telescopes are pointed at different guide star combinations, the method can measure the Fried parameters of atmospheric seeing at high altitudes except the ground layer. The corresponding height value is expressed as r0(h n )express.

[0037] Step 4: Using the telescope aperture diameter, aperture spacing, and the variance of each guide star offset combination as initial input, obtain the Fried parameters of each atmospheric height layer through matrix singular value decomposition and nonlinear least squares method, calculate the ground layer seeing, and then inversely calculate the atmospheric turbulence profile at each altitude.

[0038] For the N-layer atmospheric turbulence, the ground-layer seeing value is expressed as r0(h0), and the upper-layer seeing value is expressed as r0(h1) to r0(hN-1 ), the relationship between the overall seeing and the seeing of each layer is expressed as:

[0039]

[0040] Once the overall seeing value and the high-level seeing value are obtained, the ground-level seeing value can be obtained by subtracting them. It can be expressed as:

[0041]

[0042] Atmospheric refractive index structure constant The relationship with atmospheric seeing is:

[0043]

[0044] Get the Fried parameters r0(h n ) can be used to obtain the atmospheric refractive index structure constant at each altitude, and then the atmospheric turbulence profile can be obtained.

[0045] Where φ is the zenith angle. The overall seeing can also be calculated from the above formula

[0046]

[0047] In this embodiment, two small telescopes with an aperture diameter of 10 cm are specifically used, which are fixed on the same rigid tripod with a distance of 40 cm between them. Four solar granulation guide stars and four thin layers of atmosphere with altitudes of 0 km, 3 km, 8 km, and 12 km are generated by YAO, a professional optical software suitable for adaptive optical system simulation. The intensity ratios of the simulated four layers of atmosphere are 0.4, 0.3, 0.2, and 0.1 respectively. The number of image frames collected by the imaging sensor is 200 frames.

[0048] According to the method of steps (2)-(4), it can be calculated that the four-layer atmospheric turbulence is accurately inverted with a small error. It is proved that the method of the present invention can effectively and accurately distinguish the atmospheric stratified seeing. The comparison of the input and output results and the error are shown in Table 1.

[0049] Table 1 Comparison of input and input results of this method

[0050]

[0051] The method of the present invention can obtain good vertical resolution and a satisfactory maximum measurement height. The example of the present invention demonstrates the process of restoring four layers of atmosphere, and the effective restoration accuracy is better than 4%. It is also possible to increase the number of arbitrary guide stars to obtain higher vertical resolution, not just limited to the four layers of atmospheric turbulence in the example. The example proves that the method proposed by the present invention for measuring the seeing of layered atmosphere based on dual small telescopes is effective and accurate, and can replace the layered seeing measurement that could only be achieved by large telescopes in the past. It can simultaneously obtain the ground layer, high layer and overall seeing, and does not need to use the traditional Shack-Hartmann sensor, thus avoiding the calculation errors caused by the defects of the sensor itself.

[0052] In summary, the present invention provides a method for measuring the seeing of a layered atmosphere based on dual small telescopes. A combination of two small telescopes with a certain spacing is used to measure the atmospheric seeing and the atmospheric turbulence profile. By selecting small areas in the solar granular structure with different angular spacings as guide stars, the turbulence intensity at the corresponding height can be measured according to the triangulation principle. In the specific calculation, the variance of different guide star offsets is first calculated, and then the Fried parameters at different heights are obtained through a recovery algorithm, and finally the turbulence profile is obtained. This method is different from the traditional method of measuring seeing with a single telescope. The two small telescopes have small apertures, small volumes, and light weights, making the instrument of the present invention more convenient to carry. In addition, the method of the present invention does not require the use of a Shack-Hartmann wavefront sensor, which can effectively eliminate its shortcomings such as relatively low resolution, limited dynamic range, and difficulty in accurate calibration, and has strong practicality.

[0053] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modification, equivalent replacement and improvement made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for measuring the seeing of layered atmosphere based on dual small telescopes, characterized in that: The method comprises the following steps: using two small telescopes, the two small telescopes simultaneously point to the same guide star region and keep the optical path conjugate; using a direct imaging sensor to capture observation images of the two small telescopes, and recording the aperture diameters, aperture spacings and guide star offsets of the two telescopes; using the aperture diameters, aperture spacings and guide star offsets of the two telescopes as initial inputs, obtaining the overall seeing Fried parameters; using the aperture diameters, aperture spacings and variances of the guide star offset combinations of the telescopes as initial inputs, obtaining the Fried parameters of each atmospheric height layer through matrix singular value decomposition and nonlinear least squares method, and calculating the ground layer seeing, and then inversely calculating the atmospheric turbulence profiles at each height; The ground layer seeing is: Where: r0 represents the Fried parameter of atmospheric seeing; r0(h0) represents the value of ground seeing; r0(h n ) represents the atmospheric seeing at each conjugate altitude; The overall seeing is: Where: λ represents the measurement wavelength; φ is the zenith angle; is the atmospheric refractive index structure constant.

2. The method for measuring the seeing of layered atmosphere based on dual small telescopes according to claim 1, characterized in that: The two small telescopes are fixed on the same rigid tripod.

3. The method for measuring the seeing of layered atmosphere based on dual small telescopes according to claim 1, characterized in that: Step 2 also includes recording the initial height of the atmospheric seeing layer node determined by the guide star combination.

4. The method for measuring the seeing of layered atmosphere based on dual small telescopes according to claim 1, characterized in that: Recovering atmospheric turbulence profiles using a linearly arranged guide star array.

5. The method for measuring the seeing of layered atmosphere based on dual small telescopes according to claim 1, characterized in that: Guide star combinations with different angular spacings and different numbers are selected, and the expected vertical resolution and maximum measurement height are obtained by changing the guide star combination.

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