A calibration method for multi field star sensors based on a three-axis turntable is proposed. This method utilizes the three rotational degrees of freedom of the turntable to calibrate and collect data for any axial field of view without the need for reinstallation. By modeling and optimizing the integration of measurement models, structural parameter models, and external parameter models, a laboratory calibration model is obtained. Use Levenberg Marquardt nonlinear least squares method to solve the measurement model parameters for each field of view and the structural model parameters between each field of view. This method does not require the determination of structural model parameters through field observation data, greatly saving the workload of calibration data collection and avoiding parameter estimation errors caused by the influence of the atmosphere on the stellar vector. The effectiveness of this method was verified through simulation experiments on a digital three field of view star sensor and actual experiments on a dual field of view star sensor prototype. Compared with traditional methods based on outfield observations, the average inter satellite angular distance error in the field of view has been reduced by 20.32%, and the average inter satellite angular distance error in the field of view has been reduced by 59.34%
The calibration type star simulator has a simple structure and is easy to install and adjust. The conventional calibration type star simulator mainly consists of a control power supply, a high uniformity light source, a bandpass filter, a laser direct writing microporous star point plate, a high-precision collimation optical system, etc. The structural composition is shown in Figure 1.
Figure 1 Schematic diagram of the structural composition of the standardized star simulator
The working principle of a calibration type star simulator is as follows: a transparent microporous star point reticle plate with star point position distribution made using laser direct writing technology is placed on the focal plane of a high-precision collimating optical system. The light source illuminates the star point reticle plate and has high uniformity in the star map display area. The star spectrum is corrected through a neutral bandpass filter to form a simulated star point that meets spectral requirements. Due to the placement of the star point reticle on the focal plane of the collimating optical system, the light passing through the star point micropores is emitted in parallel through the collimating optical system. The star sensor receives the parallel light and converges on its image plane to form a complete fixed celestial simulated star map, thus achieving the simulation of stars at infinity by the simulator and the observation of simulated star light from infinity by the sensor. To ensure the safe operation of the simulator, control the power supply to supply power to the light source, and adjust the brightness of the light source within a certain range to change the simulated star level.
The collimating optical system is a key factor in achieving the ground testing ability of star sensors in a calibrated star simulator. It can accurately simulate the position of star points in a fixed sky area at infinity, and its performance directly affects the ground calibration accuracy of the simulator for star sensors.
(1) Design principles and parameters
Due to the need for a calibrated star simulator to have the ability to simulate large spatial size star maps, the designed collimating optical system is essentially a large field of view collimator; Considering that the docking method between the star sensor and the simulator in the testing experiment is pupil connection, it is required that the designed collimating optical system has an external pupil and must have high-quality imaging characteristics such as small distortion and apochromatic aberration. According to the ground calibration requirements of high-precision star sensors, the design parameters of the calibration type star simulator collimation optical system are shown in Table 1.
Table 1 Design indicators of optical systems
(2) Design results
According to the design requirements of the collimating optical system of the star simulator, eliminating distortion is one of the most important tasks in system design. It was found that changing the basic structural parameters of the lens cannot effectively correct distortion and cannot achieve the requirement of relative distortion less than 0.1%. Therefore, optimization operations were set to control relative distortion. Finally, according to the technical specifications, the structure of the completed star simulator is shown in Figure 2.
Figure 2 Structure diagram of the designed star simulator
As shown in Figure 3, the relative distortion curve of the planned collimating optical system is shown. The maximum relative distortion error in the full field of view is not more than 0.08%, and the maximum relative distortion error in the center wavelength is not more than 0.06%.
Figure 3 Field Curve and Destruction Curve of Collimated Optical System
MTF is the most comprehensive criterion among all optical system performance criteria, and the planned MTF curve of the collimated optical system is shown in Figure 4.
Figure 4 MTF diagram of collimating optical system
The main beam points of the planned collimating optical system are shown in Figure 5.
Figure 5: Main ray points of the collimating optical system
The deviation between the energy center of the collimating optical system and the main beam can be calculated through the point plot, and then the theoretical deviation of the star simulation angle generated by the optical system can be calculated. The calculation results are shown in Table 2.
Table 2 Theoretical deviation of star point simulation angle
The accuracy of the key parameters of the calibrated star simulator itself is the key to determining its accuracy in calibrating the star sensor.
This article designs a high-precision calibration type star simulator for the specific requirements of ground calibration of star sensors on the performance of testing equipment. Proposed the overall design scheme of the simulator, including system composition and working principle; A simulator collimation optical system with external pupil was designed, which has the advantages of large field of view, wide spectrum, and small distortion; Elaborated on the testing methods for key parameters of the simulator, built an experimental platform, and conducted actual testing on typical star maps. The results indicate that the measured results of the core parameters meet the requirements of technical indicators. Therefore, the designed calibrated star simulator can serve as an important equipment for high-precision ground performance testing of star sensors.
A calibration method for multi field star sensors is proposed in the article. This method uses a high-precision three-axis turntable as an angle generating device, and its three rotational degrees of freedom can collect data from any axial field of view without changing the installation method of the star sensor. The installation error of the star sensor remains unchanged during the data collection process of each field of view, allowing for the separation of external parameters of the structural model from the installation error in the model and accurate calculation. At the same time, keeping the external parameters constant during the repeated data collection process in multiple fields of view reduces the occurrence of parameter coupling, and also makes the estimation of measurement model parameters more accurate. This method can be completed in the laboratory, saving the workload of traditional methods for field observation and avoiding the impact of atmospheric misting and turbulence on the calibration results.
Send us a message,we will answer your email shortly!
TY-Space Technology (Beijing) Ltd. is professional focusing on advanced attitude optical sensors, especially Star Trackers for space industry.
