Testing of eddy current edge displacement sensor on LAMOST

The edge displacement sensor, serving as a core component in large-aperture astronomical telescopes, plays a crucial role in maintaining the co-focusing of mirrors by measuring tiny displacements between sub-mirrors. Therefore, the performance of the sensor is of paramount importance to ensure that the telescope can obtain clear and accurate images. The challenging on-site environmental conditions place high demands on the sensor's performance. The key characteristics of the edge displacement sensor include nanometer resolution, low nonlinearity, minimal temperature drift, and temporal stability. Simultaneously achieving all these performance criteria poses significant challenges. We have developed an ultra-stable eddy current edge displacement sensor with a resolution better than 1nm (RMS) within a range of 250μm. During on-site testing at the Guoshoujing Telescope (the Large Sky Area Multi-Object Fiber Spectroscopic Telescope LAMOST) telescope, the sensor exhibited a temperature drift within ±2nm/°C after temperature compensation and achieved a drift of less than 25nm over 5 weeks. The environmental temperature varied within the range of - 17~+14°C, and humidity fluctuated between 20% and 80% RH during the testing. Additionally, a co-focusing test was conducted on a small system composed of three selected sub-mirrors. Six sets of sensors were installed on this small system, enabling real-time measurement of sensor outputs for attitude correction. The test results indicate that, by relying on sensor feedback, the telescope has the potential to achieve high-quality co-focusing over extended periods and significant temperature ranges, thereby enhancing the operational efficiency and observation quality of the telescope.