Aiming at completing functional tests on high precision star sensor of stars′positional accuracy and star magnitude,an optical system of liquid crystal on silicon(LCOS)-based and high precision dynamic star simulator is designed.The general designing program is presented after the analysis of the collimating and the illuminating optical system.In order to improve the simulating precision,a optical splicing method of LCOS is put forward,designing process of a collimating optical system with large field of view,large relative aperture and long exit pupil is given in detail as well as the aberration diagrams.To satisfy the illuminating conditions ordered by LCOS and achieve simulation of-1 to 7 magnitude stars, an illuminating optical system is emulated.Experimental test indicates that the angular distance accuracy is less than12″,and-1 magnitude to 7 magnitude stars can be simulated, which meets the calibration requirements of high precision star sensor.
When a space vehicle is operating in space, it uses space light navigation sensors to capture and extract its flight attitude. Star sensors are the most accurate space light navigation sensors, providing real-time carrier position information by identifying stars in the sky. With the rapid development of high-precision star sensors, the requirements for ground detection devices are also increasing, such as increasing the refresh speed of star maps to the millisecond level; The accuracy of inter satellite angular distance has been improved from tens of seconds to tens of seconds; The simulation range of magnitude has been expanded to 10 magnitude stars.
A dynamic star simulator is a device that tests the functionality of star sensors, enabling real-time simulation of star sensors observing the sky. The existing dynamic star simulators mostly use thin film transistor liquid crystal display screens (TFT-LCD) and digital micro mirror devices (DMD) components to display star maps. Among them, TFT-LCD has a contrast ratio of 600:1 and a light energy utilization rate of less than 10%, which cannot meet the simulation requirements of multiple stars. The simulation range of star magnitude is mostly between 2 and 6 stars; The size of a single micro mirror in DMD is 14 μ M × fourteen μ m. The gap between adjacent micro mirrors is 1 μ m. It is difficult to achieve high-precision simulation of star point positions, and the relevant data shows that the accuracy of star to star angular distance can only reach 40 ″. Compared with TFT-LCD and DMD, reflective silicon based liquid crystal (LCOS) has a light energy utilization rate of up to 55%, and can achieve a single pixel size of 8 μ M × eight μ m. Pixel gap is only 0.538 μ m. As a star map display device, it has obvious advantages. Therefore, in order to meet the standards of inter star angular distance and magnitude for a high-precision star sensor, LCOS is used as a star map display device to develop a large field of view, large relative aperture, and high-precision dynamic simulator. This article proposes a technical solution for optical splicing of two pieces of LCOS based on the design concept of improving device simulation accuracy; In response to the design difficulties of correcting the aberrations of the asymmetric system with an external exit pupil and matching the LCOS lighting, the focus is on optimizing the optical system of the equipment and conducting simulation analysis.
(1) Main technical indicators of the simulator
According to the ground testing requirements of high-precision star sensors, the main technical parameters of the designed dynamic star simulator are shown in Table 1.
(2) The Working Principle of Simulator and LCOS Splicing
The designed dynamic simulator mainly consists of a computer control system, lighting optical system, spatial light modulation system, collimation optical system, etc. Its composition is shown in Figure 1. When the simulator is working, the computer control system inputs display commands to the spatial light modulation system, and the modulated light is projected parallel light through the collimating optical system to simulate the infinite starry sky.
LCOS is a key component of spatial light modulation systems. According to the requirements of the simulator’s field of view and accuracy indicators, the number of pixels in LCOS cannot be less than 1872 × In 1872, considering foreign export restrictions and technological limitations, the multiple LCOS devices currently available in the market were unable to meet the actual design requirements. To solve the problem of insufficient resolution of single chip display devices, a splicing scheme using two pieces of LCOS and a dielectric film polarizing prism (PBS) adjacent working surface for short edge pixel overlap is adopted, which increases the number of display pixels after splicing to 1920 × 1920. Figure 2 shows the splicing principle of LCOS. PBS can provide a conjugate surface with equal optical path, so that two LCOs share the same optical engine architecture, simplifying the optical engine structure and reducing the volume of the optical modulation system; Considering the working principle that LCOS only modulates S-line polarized light, a single chip is used λ/ The 2-wave plate causes a phase delay of π in the transmitted P-line polarized light path, forming an S-line polarized light to illuminate the LCOS facing the light source, which can meet the requirements of the device’s contrast light polarization. In order to meet the requirements of accurate simulation of star point magnitude in the star map and ensure equal brightness of the image planes of the two LCOS splices, the computer control system divides the display grayscale of the two LCOS to eliminate the impact of the transmitted light path passing through the wave plate twice on energy.
The optical system is one of the core components of the dynamic star simulator, including two parts: collimation and illumination system. The former requires accurate simulation of the inter star angular distance by the equipment, while the latter needs to meet the lighting conditions of the display components and achieve accurate simulation of the magnitude of the stars.
(1) Design of collimating optical system
According to the principle of star simulator projection light, LCOS is placed on the focal plane of the collimating optical system. The spatial light modulation system receives a computer control system to generate star map display commands that can be observed by the star sensor at the current time from the star catalog data. The light modulated by LCOS is projected parallel light through the collimating optical system to simulate real-time changes in the position of stars in the infinite sky. Therefore, the performance of the collimating optical system of the dynamic star simulator directly affects the accuracy index of the star simulator. When designing, four main objectives are considered: 1) To ensure that the simulated star points have high imaging position accuracy for the star sensor, the distortion of the system must be strictly corrected; 2) The use of modulation transfer function (MTF) to evaluate the overall design of the system requires that the point array graphics of the off axis field of view and the on axis field of view should be as consistent as possible, and the geometric radii should not differ significantly; 3) The star sensor detects the energy center of the simulated star point, hoping that the symmetry of the scattered spots in each field of view of the system is good, and the deviation between the energy center and the main ray is minimized as much as possible; 4) To facilitate star sensor extraction of star points, it should be ensured that 80% of the system’s energy is concentrated within the LCOS single pixel range.
The designed collimation system has a relatively large aperture, which increases the spherical aberration of the edge aperture and causes significant collimation error. It is necessary to use a reasonable combination of positive and negative lenses to correct the spherical aberration; The system aperture is external, and the large exit pupil distance increases the incident height of the main beam, causing a significant increase in vertical aberration. Asymmetric structures can be introduced to balance these aberrations; The required spectral range for the indicator is 500-800nm. The positional and multiplicative chromatic aberration caused by the different refractive indices of various colors of light can cause errors in the position of simulated star points. The former causes the image planes of different colors of light to not coincide, leading to the displacement of the geometric center and energy center of the optical system. The latter seriously damages the clarity of the axis extraterrestrial points. Therefore, it is necessary to combine the power distribution and select appropriate materials to minimize the system chromatic aberration.
The planned system has a focal length f ‘=56.02mm, as shown in Figure 3. All lenses adopt a compact traditional surface structure. The system adopts a+-+power distribution form, with the first two positive lenses quickly deflecting the light towards the optical axis; The intermediate negative power group has an asymmetric structural feature, which is used to correct coma and distortion, and to cancel out the accumulated positive and negative aberrations of the previous group; H-FK61 is selected as the material for the rear group of positive lenses, with a refractive index of 1.4970 and an Abbe number of 81.5947, which can correct chromatic aberration to a certain extent. At the same time, a crescent shaped lens is introduced into the positive power group to correct field curvature and balance residual aberration.
(2) Design of Lighting Optical System
In the design of lighting systems, special attention is paid to their matching with LCOS components and the acquisition of uniform light spots on LCOS. From the perspective of the working principle of PBS, PBS only produces polarization splitting with a fixed extinction ratio for incident beams that meet the Brewster condition. According to the selected PBS, the beam convergence angle of the lighting optical system cannot exceed 6 °. Considering the characteristics of LCOS itself, the larger the beam incidence angle, the more significant the decrease in image contrast. Therefore, in terms of usage requirements, the lighting system is required to have collimation; The uniformity of illumination received by LCOS directly affects the simulator’s precise control of magnitude. Based on the brightness calculation of magnitude, the uniformity of illumination in the lighting system should not be less than 90%. In terms of technical functions, the uniformity of emitted light from the lighting system is required. Therefore, the specific problem to be solved in the design of lighting systems is how to obtain a collimated and uniform lighting spot. The selected lighting system structure is shown in Figure 5,
The light emitted by the light source is shaped by a parabolic reflector and projected parallel to the front surface of the first compound eye lens. The lens array divides the light source image into several parts, and the front main surface of the second compound eye lens coincides with the back focal surface of the first compound eye lens. The first compound eye lens is segmented into an image at infinity, and the additional mirror is designed in a dual separation form, which collimates the through-hole of the second compound eye lens and images it on the irradiation surface, To some extent, it reduces the color difference and thus reduces the impact of wide spectral dispersion.
The main basis for selecting light sources is the simulated magnitude range and star map size. It is known that the brightness difference between each adjacent magnitude is 2.512 times. To achieve simulation of stars ranging from -1 to 7, the contrast M between the lighting system and the spatial light control system should meet 1585
Based on the design of the collimation optical system and illumination optical system, a simulation model for the optical part of the dynamic star simulator is established, as shown in Figure 7. The simulation model uses Lighttools software to track the illumination distribution obtained by ray tracing. The results show that the irradiance of the light emitted by the star simulator is consistent and uniform within the range full of pupils, reaching 95%, which is beneficial for the simulator to control the magnitude and meet the equipment’s usage requirements.
For the designed optical system, the simulated inter star angular distance error and magnitude accuracy are key technical indicators that determine the calibration level of the dynamic star simulator for star sensors. The designed optical system is connected to a computer control system and a spatial light modulator for testing.
Design the optical system of a large field of view and high-precision dynamic star simulator based on LCOS splicing. The collimation optical system meets the design requirements of wide spectrum, small distortion, and flat image field. The illumination optical system meets the requirements of high uniformity and high collimation in terms of output light. Compared with existing related equipment, it has advantages such as high simulation accuracy of star position, good uniformity, and a large magnitude range. Establish a dynamic star simulator correction method. After simulation analysis and practical testing, the designed optical system can meet the testing requirements of high-precision star sensors in dynamic star simulation. The inter star focal length error of the simulated star points is better than 12 ″, and the magnitude is adjustable from -1 to 7, meeting the testing requirements of high-precision star sensors.
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