Wavefront aberration, which caused by atmospheric turbulence, needs to be measured in the free space optical communication. The existing sensors of wavefront aberration measurement are mainly divided into two classes, wavefront sensors and image-based sensors. Wavefront sensors , such as Hartmann sensor and shearing interferometry, measure wavefront slope to calculate wavefront aberration. However, wavefront sensors always need most of the laser energy, which means it is hard to use wavefront sensors in free space optical communication in the daytime. Image-based sensors usually requires iteration, which means poor real-time and locally optimal solution. No existing method can measure wavefront aberrations in real time in free space optical communication in the daytime. In this article, a new method of measuring wavefront aberration with CNN is proposed, which can be used in free space optical communication in the daytime and have good real-time performance. We made some modifications in VGG to make it can be used to fitting the Zernike coefficients. The input to the network was the PSF of focal plane and defocus plane and the output was the initial estimate of the Zernike coefficients. 22000 pairs of images were collected in the experiment, which produced by liquid crystal and the wavefront was built by 64 Zernike coefficients when atmospheric coherent length(r0) is 5cm. 20000 pairs of images were used as training sets and the other were used as testing sets. The root-mean-square(RMS) wavefront errors of VGG is on average within 0.0487 waves and the time it needs is 11-12ms. We use RMS wavefront error less than 0.1 waves as the correct standard and the correct rate is 98.75% , while other RMS wavefront errors were properly close to 0.1 waves.
A satellite dedicated for quantum science experiments, has been developed and successfully launched from Jiuquan, China, on August 16, 2016. Two new optical ground stations (OGSs) were built to cooperate with the satellite to complete satellite-ground quantum experiments. OGS corrected its pointing direction by satellite trajectory error to coarse tracking system and uplink beacon sight, therefore fine tracking CCD and uplink beacon optical axis alignment accuracy was to ensure that beacon could cover the quantum satellite in all time when it passed the OGSs. Unfortunately, when we tested specifications of the OGSs, due to the coarse tracking optical system was commercial telescopes, the change of position of the target in the coarse CCD was up to 600μrad along with the change of elevation angle. In this paper, a method of reduce alignment error between beacon beam and fine tracking CCD is proposed. Firstly, OGS fitted the curve of target positions in coarse CCD along with the change of elevation angle. Secondly, OGS fitted the curve of hexapod secondary mirror positions along with the change of elevation angle. Thirdly, when tracking satellite, the fine tracking error unloaded on the real-time zero point position of coarse CCD which computed by the firstly calibration data. Simultaneously the positions of the hexapod secondary mirror were adjusted by the secondly calibration data. Finally the experiment result is proposed. Results show that the alignment error is less than 50μrad.
In this paper, a telescope control method to reject ground-based disturb is proposed to enhance the tracking precision of telescope systems. Telescope systems usually suffer some uncertainouter disturbances, some disturbance come from the torque disturb such as friction orwind loads, some of the others may come from the platform. For astronomical telescope, especially relative large volume telescope, disturb from ground istransferred to the telescope via the pier foundation. And the main mount of disturb is the resonance frequency of pier foundation. The frequency is about 10Hz.A complete vibration test was carried out on a quantum satellite-ground communicationground telescope. Some conclusion is achieved. And a control method based on data fusion to rejecting ground-based disturbance is proposed. The test showed that the method could reduce the track error from 1.5 angular second to 0.28 angular second.
In this paper, a method of acquisition between optical ground station and quantum communication satellite named Micius for establish optical links is proposed. The acquisition technological specification of the optical ground station system is analyzed. The acquisition strategy of optical ground station is designed. In order to point accurately to quantum satellite for optical ground station, system error modifying method is designed, using the telescope mount model to improve the absolute pointing precision. Finally the experiment result is proposed. Results show that the correction accuracy is better than 5μrad. The acquisition time is less than 5 seconds.
In this paper, a friction compensating method based on data fusion in telescope controller is proposed to enhance the tracking precision of telescope systems. Telescope systems usually suffer some uncertain disturbances, such as friction, wind loads and other unknown disturbances. Especially, when telescope change speed direction, the friction is the dominate track error. Thereby, to ensure the tracking precision, the friction compensating method which can attenuate the influence of friction is introduced. Besides, to improve the friction identify accuracy, a data fusion method which fuse accelerometer and encoder is adopted. Finally, a few comparative experimental results show that the proposed control method has excellent performance for reducing the tracking error of telescope system.