Objective  The extra-wide field-of-view camera adopts an off-axis four-mirror optical system, and the second and forth mirror adopt an integrated free-form surface structure. A free-form surface is an asymmetric structure that is highly sensitive to thermal changes. Even with a uniform change in bulk temperature, the optical-mechanical structure still undergoes asymmetric geometric changes. In addition, the extra-wide field-of-view camera needs to meet the design requirements of a width of 860 km and a field of view of 65.6°, and the entrance mask adopts a wide special-shaped opening design. The sun shines directly on the inside of the hood for a duration of 5.5 minutes as it enters and exits the Earth's shadow. Both optical and mechanical errors are caused by heat cause line of sight drift (LOS) and wavefront distortion (WFE) in the camera. These issues seriously affect the optical transfer function of the system. Considering its structural characteristics and the thermal control challenges brought about by the large change of heat flow outside the light entrance, targeted thermal control measures must be taken for different parts of the camera to meet the thermal control index requirements of off-axis free-form surface cameras with special-shaped optical apertures in orbit.

  Methods  Six aspects of the camera are analyzed, including its on-orbit state, structural layout, task requirements, orbital environment, technical indicators, and heat sources. The thermal control design is implemented by using composite multi-layer heat insulation components for thermal isolation (Fig.7), graded thermal control for mirrors (Fig.8), and high thermal conductivity graphite film for thermal conduction (Fig.9). This design allows for precise control of the optical-mechanical structure and efficient heat dissipation of high heat consumption/heat flux electronic equipment. The temperature of the camera is simulated and analyzed under high and low temperature conditions using the finite element analysis software UG12.0/Space thermal. The effectiveness of the thermal control scheme is verified through thermal analysis, thermal test, and satellite on-orbit telemetry temperature data.

  Results and Discussions   The transient temperature changes of the camera mirror assembly and electronic components are presented. Under low temperature conditions, the primary mirror exhibits a temperature fluctuation ranging from 19.83 ℃ to 20.10 ℃, while the second and fourth mirrors experience a temperature fluctuation between 19.80 ℃ and 20.13 ℃. The temperature fluctuation of the third mirror falls within the range of 19.91 ℃ to 20.04 ℃ (Fig.13(a)). Similarly, under high temperature conditions, the temperature fluctuation of the primary mirror ranges from 19.88 ℃ to 20.10 ℃, while the second and fourth mirrors exhibit a fluctuation between 19.81 ℃ and 20.14 ℃. The temperature fluctuation of the third mirror ranges from 20.02 ℃ to 20.20 ℃ (Fig.13(b)). It is worth noting that the reflector assembly maintains a stable temperature, with fluctuations not exceeding ±0.2 ℃ under both high and low temperature conditions. During the non-camera period, the CMOS component maintains a temperature range of 19.5 ℃ to 19.8 ℃ (Fig.13(c)). However, during the imaging period, the temperature of the CMOS component varies between 19.6 ℃ and 23.6 ℃. The temperature fluctuation of the CMOS focal plane component does not exceed 4.5 ℃, and the imaging electrical box experiences a temperature fluctuation within 8 ℃ (Fig.13(d)). The overall temperature level of the camera, as determined by thermal analysis, is compared with the thermal test results (Tab.2). The table shows that the temperature of the primary mirror and the second and fourth mirrors remained constant during the test, while the temperature of the third mirror fluctuated by 0.23 ℃. The maximum deviation between the thermal analysis and thermal test results is within 5%. The comparison of the thermal analysis results with the thermal test results confirms the validity of the thermal analysis.

  Conclusions  The deviation between the on-orbit telemetry data and the thermal analysis and thermal test results is within ±0.5 ℃. This indicates that the thermal design of the camera is accurate and feasible, and the thermal analysis and test process are reasonable and reliable. The employed thermal control measures and design methods are suitable for the thermal design of extra-wide space optical camera.