## 4 Thermal Analysis of Interstellar Laser Communication Payload
### Modeling of Inter-Satellite Laser Communication Payload  
The thermal control configuration of the inter-satellite laser communication payload is as follows. White paint is applied on the telescope barrel and baffle surfaces; black paint is applied on the inner surface of the optical tube; black anodizing is adopted for primary mirror skin, secondary mirror skin, tertiary mirror skin, quaternary mirror skin, primary mirror mount, secondary mirror mount, and tertiary mirror mount. Based on these thermal control design specifications, a 3D thermal simulation model for the inter-satellite laser communication payload has been established, as shown in Figure 11. For the satellite, continuous yaw control is performed to ensure that the Sun always remains within the XOZ plane of the satellite's body coordinate system. This means that the sunlight is always parallel to the \(\pm\)Y faces of the satellite, with the +X face continuously illuminated and the \(-\)X face not illuminated. The payload's angle tracking range is as follows: pitch from \(-70^{\circ}\) to \(70^{\circ}\) within a conical angle, and azimuth from \(0^{\circ}\) to \(360^{\circ}\).  
Figure 11: Thermal simulation model of the inter-satellite laser communication payload.  
Payload Thermal Analysis of Typical Operating Conditions for Inter-Satellite Laser Communication Payload  
The payload can experience high temperatures under the following conditions: (1) forward tracking at \(\beta=13.2^{\circ}\), (2) backward tracking at \(\beta=13.2^{\circ}\). According to the analysis in Section 4.1, when \(\beta=13.2^{\circ}\), the entire satellite is in a full-sunlight orbit. For both forward and backward tracking modes, the exposure conditions of the payload are symmetrical, resulting in the same average external heat flux over the orbit period and the same maximum instantaneous external heat flux, with only a phase difference. Therefore, we will focus on the backward tracking condition at \(\beta=13.2^{\circ}\) for this section's analysis.  
During the simulation, the satellite maintains a yaw attitude relative to the Sun, and the payload maintains an inter-satellite pointing attitude. The yaw and pitch angles are set according to the methods described in Section 2. Figure 12 shows the temperature variation curves for the primary mirror, secondary mirror, tertiary mirror, and quaternary mirror under the inter-satellite pointing attitude. Table 2 lists the maximum and minimum temperatures of each mirror, and Table 3 provides the calculation results as well as the maximum and minimum temperatures of the primary mirror tube measured in orbit.  
Under the inter-satellite pointing attitude, the calculated temperatures of each mirror range from 16.6 \({}^{\circ}\)C to 21.2 \({}^{\circ}\)C. Considering the in-orbit measurement results from the temperature sensors on the primary mirror barrel, four temperature sensors are arranged on the primary mirror barrel. The results show that the simulation results based on the inter-satellite pointing attitude are close to the in-orbit measurement values. The highest temperature under the inter-satellite pointing attitude is 20.9 \({}^{\circ}\)C, which differs from the in-orbit measured temperature by 2.3 \({}^{\circ}\)C. Therefore, using the external heat flux method for the inter-satellite pointing attitude proposed in this paper, the external heat flux conditions in orbit can be accurately analyzed, providing a theoretical basis for the thermal analysis of the payload under the pointing attitude.