Functional Detection Star Simulator: Introduction

With the continuous development of deep space navigation technology, the application of space optical attitude sensors is becoming increasingly widespread. Among the commonly used space optical attitude sensors, star sensors (referred to as star sensors) have the highest accuracy, prompting people to conduct in-depth research on them. Receiving the light emitted by stars and quantifying it into stars is one of the key technologies of star sensors. Therefore, it is necessary to calibrate the optical signal receiving equipment of star sensors. In order to solve the problems of expensive and difficult maintenance in orbit calibration, star simulators have emerged as ground calibration and testing equipment for star sensors.

Star simulators can be divided into two types based on target simulation methods: static star simulators and dynamic star simulators. The dynamic star simulator is mainly composed of a backlight board and a liquid crystal light valve. It simulates star maps on the ground, generates real-time star maps that match the star sensor, and performs functional testing on star map recognition and star point extraction of the star sensor; The static star simulator mainly simulates the magnitude and position of stars, and tests the detection ability, spatial resolution, and accuracy performance of star sensors.

1. System working principle

The static star simulator mainly includes high-precision variable target sources and collimating optical systems. The high-precision variable target sources include driving circuit boards, LED light sources, star hole plates, etc. The equipment is installed in a sealed light chamber to prevent external interference. The working principle of the magnitude simulator is shown in Figure 1. The control signal sent by the computer is transmitted to a high-precision variable target source, and the driving circuit board controls the LED light source at a given position to emit light according to the command signal of the computer. The beam passes through the star hole plate to reach the collimating optical system, simulating the target light source as starlight from the “infinite” sky, reaching the exit pupil position of the collimating optical system and projecting it onto the entrance pupil position of the star sensor, forming a star map. LED light sources can achieve single point controllability and joint regulation, and the brightness of each star point can be set through a control unit.

Fig.1 Working principle diagram of star simulator

2. Optical system design

(1) Design parameters

The design of optical systems directly affects performance indicators such as position accuracy and simulation accuracy of simulated magnitude. To avoid wastage of luminous flux, the optical system should follow the principle of pupil connection, that is, the exit pupil of the magnitude simulator should coincide with the entrance pupil of the star sensor; To ensure the imaging quality of star maps and the simulation accuracy of magnitude, optical systems need to have high image quality. According to the system requirements, our research group has designed a collimating optical system with high imaging quality and an external exit pupil.

The diameter of the exit pupil of the star simulator is generally greater than or equal to the diameter of the entrance pupil of the star sensor. In order to match the two pupil sizes and not cause waste of luminous flux, the actual design value is taken to be equal to the diameter of the entrance pupil and the diameter of the exit pupil, which is 50mm, and the distance between the exit pupil is greater than 120mm

In summary, based on the ground calibration requirements of the star sensor, the design parameters of the high-precision static magnitude simulator optical system are determined as shown in Table 1.

Table 1 Design parameters of the optical system

(2) Structure selection

To meet the needs of weak light, a lens type structure is usually selected as the optical structure of the magnitude simulator. In addition, its optical structure usually adopts a double glued or three piece structure. To further improve imaging quality and correct axial image heterodynes, a 4-piece separation system is used. The optical path diagram of the optical system is shown in Figure 2

Fig.2 Optical path of the optical system

(3) Image quality evaluation

The aberration of the star simulator should not affect the usage requirements of the star sensor. To ensure the positional accuracy of the star map, the optical system of the star simulator requires that the distortion and magnification chromatic aberration should not be too large. The performance evaluation methods of the final optical system mainly include modulation transfer function (MTF) curve, point plot, field curve and distortion curve, wave aberration, etc.

1) MTF. The design of optical systems usually uses MTF curves to measure the collimation of the system, but it is not only necessary to pursue high-quality MTF curves, but also to ensure the dispersion of various fields of view.

Fig.3 MTF curve

2) Point chart. According to the relationship between the diffuse spot and the MTF curve, if the diffuse spot is small, the MTF value will be high, and the image quality of the system is relatively good. The point plot of the system is shown in Figure 4. The root mean square radius (RSM) values of the diffuse spot are all less than 0.005mm, and the deviation between the energy center and the position of the main beam is not significant, meeting the system requirements.

Fig.4 Spot diagram

3) Distortion. The size of distortion has no impact on the image quality of the system, but it directly affects the positional accuracy of the star points. Therefore, distortion is the most important aberration in system design, and distortion elimination is the key point in the system design process.

Fig.5 Curves of field curvature and destruction

4) Wave aberration. The wavefront of the system is shown in Figure 6. The mean square value of the system’s wavefront aberration is not greater than λ／ 34.4, meeting the requirement that the system wavefront aberration is less than λ／ 20, with good imaging quality.

Fig.6 Wave front diagram

3. Simulation of magnitude

To solve the problems of large volume, complex control, high cost, high heat generation, short lifespan, and poor stability of traditional halogen lamp light sources, this study selects LED lamps with small volume, long lifespan, and good spectral stability as the system’s light source. To meet the requirements of the full spectral range of starlight, the light source device should be in the full color visible light band range. The specific model selected is the XLampLED light source of CREE’s neutral white series, with a driving current of 1A, a related color temperature (CCT) of 3700-5000K, and a high luminous flux of up to 100lm. The spectral characteristics and current to relative luminous flux characteristic curves of XLampLED light source are shown in Figure 7 and Figure 8, respectively.

Fig.7 Spectrum characteristics of X Lamp LED source

Fig.8 Current and the relevant flux characteristics curves of X Lamp LED source

The essence of LED dimming control is to keep the applied voltage basically constant from turning off to on, and to ensure the simulation accuracy of star magnitude, the brightness contrast of the LED light source is first determined by controlling the change in current to make the LED change in brightness.

To solve the problem of color difference caused by the voltage drop of LED caused by simulated dimming, the LED illumination is adjusted linearly by a pulse width modulation (PWM) signal, while also ensuring stable color temperature of the light source. The dimming principle diagram of PWM is shown in Figure 9.

Fig.9 PWM dimming principle diagram

The control system of the light source mainly includes a computer control system, communication interface circuit, main controller, isolation circuit, send/receive matching device, driver, sampling circuit, and LED light source. The schematic structure diagram of the controller is shown in Figure 10.

Fig.10 Schematic diagram of controller

4. Star magnitude test

Before the star magnitude test, the control system is debugged. The experimental equipment uses ATmega16 microcontroller to generate 1.0kHz PWM. The relationship between the input PWM duty cycle and LED illumination is detected through an oscilloscope, and then the focal length of the star simulator is adjusted to achieve the best effect. The relationship curve between PWM duty cycle and LED illumination is shown in Figure 11.

Fig.11 Relationship between PWM duty cycle and LED illumination

Starting from the requirements of ground calibration equipment for star sensors, a high-precision star magnitude simulator with a spectral range covering the entire visible light has been designed. The system has a magnitude simulation range of 0-7, a control accuracy of 0.1 (repeated experimental points fall within the required range of ± 0.4%), a fine adjustment range of no less than half of the magnitude for each magnitude, and an illumination adjustment range of 1-640lx, The adjustment accuracy is better than 0.1lx.