## 2 Measuring System  
### Environmental Control Box  
The high-precision biaxial inclinometer's absolute accuracy is significantly affected by the level of its installation, as well as the ambient temperature and humidity during measurement. For this experiment, we utilized the LEICA Nivel-210, a commercially available two-axis precision inclination sensor with a resolution of 0.001 mrad. The main technical parameters of the sensor are listed in Table 1. To maintain optimal measurement accuracy, a flatness tolerance within 0.01 mm and a platform inclination of \(\pm\)1.51 mrad are required. The sensor's operational temperature range is \(-\)20 \({}^{\circ}\)C to 50 \({}^{\circ}\)C, with a zero-point error of 0.005 mrad induced by every 1 \({}^{\circ}\)C of temperature drift. The seasonal temperature range at Nanshan Observatory, located in Urumqi, China, spans from \(-\)40 \({}^{\circ}\)C to 60 \({}^{\circ}\)C. The significant disparity in temperature can have a notable impact on the precision of inclinometer measurements and may even result in irreparable harm to its internal optical components. Therefore, it is imperative to implement stringent measures for the environmental protection of the inclinometer. An ECB was designed and constructed to ensure optimal functioning and precise performance of the inclinometer. The components of the ECB are shown in Figure 4a, which includes a base (1), a seal ring groove (2), three sets of leveling screws (3), temperature and humidity sensors (4), data acquisition and  
Figure 3: The 25 m aperture NSRT. temperature control PCB (5), a relay switch (6), four sets of heating modules (7), an inner protective cover (8), and an outer protective cover (9). The ECB is depicted in Figure 4b post-manufacturing and assembly. The shell of the ECB has extremely high self-weight and stiffness to avoid interference with sensor measurements. The inclinometer's level is achieved by adjusting the three sets of leveling screws, ensuring that it falls within the expected range for accurate measurements, and keeping stability. A temperature and humidity sensor, along with a microcontroller unit (MCU), is integrated into a printed circuit board (PCB) within the enclosure. The outer protective cover is equipped with two waterproof interfaces, one of which provides a 12 V/DC linear DC power supply for control and communication, especially for high-precision sensor power supply, while the other supplies 220 V/AC power to four heating modules located on the side of the inner protective cover. The inner protective cover not only protects the inclinometer but also plays a role in evenly conducting heat and insulation. As depicted in Figure 4c, the interior of the outer protective cover and base plate of the inclinometer has been coated with a layer of white aerogel blanket, which serves as another effective means of passive temperature control. The communication protocol and wiring path of the measurement system are illustrated in Figure 5. A personal computer is connected to the MCU via a TCP/IP to RS422 bus converter, which then connects to the inclinometer through an MCU-encoded RS422 to RS232 converter, facilitating data transmission between the computer and inclinometer. The temperature and humidity inside the box, measured by the sensor on the PCB, are transmitted to the MCU in real time. Once the MCU acquires temperature data from the sensor, it will compare it with the pre-set temperature threshold. The MCU also includes a logic program to control the activation and deactivation of relays for heating or stop relays as a means of active temperature control. In this experiment, the heating temperature threshold was established at 30 \({}^{\circ}\)C. When the temperature falls below 30 \({}^{\circ}\)C, the MCU maintains relay closure to activate heating modules. Conversely, the relay is deactivated, and heating