High-precision background & controllable about the star simulator map

Star sensors, as a space attitude optical sensor with high measurement accuracy, have been increasingly widely used in the aerospace field. With the rapid development of China’s aerospace technology, more requirements have been put forward for the measurement objects and accuracy of star sensors. Not only does the star sensor need to be able to test the position of a single star, but it also needs to have the ability to identify the background light of the star.

In order to meet the testing requirements of star sensors, the star simulator, as its ground calibration and testing equipment, should have star map simulation function and background light simulation function. Traditional star simulators generally only focus on the accuracy of star map simulation, and to some extent achieve the requirements of large field of view, high accuracy, high dynamics, miniaturization, and lightweight use. However, in terms of star map background simulation, especially in static star map simulators, due to technical limitations, almost only opaque thin films are used as the background to simulate black sky without simulating variable background light. This leads to the star sensor not being tested for star map recognition in the ground calibration stage under the condition of simultaneous background light and star points.

Using a high-precision static target standard source as the core device of the star map simulator, combined with a brightness controllable lighting system, to achieve simultaneous simulation of star points and background, and designing a collimating optical system to make the simulated star map emit parallel light with the background, generating a star map at the exit pupil of the star sensor, achieving a controllable background and high-precision star map simulation. Use it as a testing benchmark to conduct ground functional testing and indicator verification on the star sensor.

1. Composition and working principle of the simulator

The high-precision background controllable star map simulator can provide star sensors with star map images with a single star position simulation accuracy better than 10 “. At the same time, the star map background can be adjusted 26 times according to usage requirements. On the basis of the original star map simulator, establish the overall structure diagram of a high-precision background controllable star map simulator as shown in Figure 1.

The light source component emits a wide spectrum of light, which passes through a filter to form a light that meets the spectral range requirements, illuminating the static target standard source located at the focal plane position of the collimating optical system. The static target standard source uses the method of spot etching based on the aberration of the optical system and the machining and installation errors of the mechanical system, and achieves high-precision star marking based on the position value of a single star. By collimating the optical system, the simulated star map is projected out in the form of parallel light. On the other hand, the background light source component is located at the focal plane of the projection optical system, and after passing through the projection optical system, it projects background light of the same size as the static target standard source. After passing through the beam combiner, the static target is overlaid with the background light to achieve high-precision simulation of the controllable background star map.

Fig.1 Schematic layout of high precision variable star chart background star map simulator

2. Simulator design

In order to ensure that the high-precision background controllable star map simulator can not only simulate the distance of stars, but also achieve 26 times adjustment of the background light of the star map, and has a single star position simulation accuracy better than 10 “, a high imaging quality collimation optical system is designed using Zemax software. The high-precision theodolite is used to measure the positions of each point, establish a measurement model, and obtain the simulated positions of the star points through repeated calculations. Optimize the design controller to control the brightness of the background light source to achieve 26 times adjustment.

(1) Optical System Design

The designed collimating optical system has a focal length of 624 mm, a field of view of 3.1 °, a wavelength range of 500~800 nm, an exit pupil aperture of 85 mm, an exit pupil distance of 25 mm, a total length of 700 mm, and an image height of 32 mm. The optical system path diagram is shown in Figure 2.

Fig.2 Layout of optical system

Figure 3 shows the distortion curve of the optical system, with a central wavelength of 650 nm. For simulators, distortion has the greatest impact on the accuracy of single star position simulation, so the system optimization design distortion is better than 0.002%, which is conducive to improving the accuracy of the simulator; Figure 4 shows the magnification color difference curve, which shows that the position deviation caused by different wavelengths is less than 1.1 μ M; Figure 5 shows the point plot of the system, where the circles at each field of view represent the size of the Airy spot. The point plots of all fields of view on the image plane are basically located within the Airy spot, indicating optical properties close to the limit of diffraction theory; Figure 6 shows the MTF of the optical system. From the curve, it can be seen that the system has good imaging and can meet the design requirements of high-precision star map simulators.

Fig.3 Destruction curve

Fig.4 Lateral chromism

Fig.5 Spot diagram

Fig.6 MTF curves

(2) Design of static target standard source

The position error of a single star, as the main technical indicator of a high-precision background controllable star map simulator, depends on the design accuracy of the static target standard source. The key component of the static target standard source is the star point reticle. The star point dividing board is engraved with multiple circular micropores, and the position of each micropore is calculated based on the azimuth and elevation angles of the simulated star point itself.

(3) Background brightness simulation design

Design the circuit diagram of the background light source and achieve 26 times brightness adjustment through current control. Figure 7 shows the design diagram of the light source circuit board.

3. Star map simulation accuracy and star map background testing

There are two key indicators for high-precision background controllable star map simulator: firstly, the accuracy of star map simulation, which is measured by the position error value of a single star; The second is the adjustment range of the star map background, measured by the lowest and highest brightness.