## 3 Experimental Setup  
### Instrument-Level Experiment  
A diagram of the instrument-level sub-experiment in the thermal vacuum condition is shown in Figure 8. The on-orbit operation environment of the PSAC was established  
Figure 7: Schematic diagram of the detector-level RTDC experimental method.  
in the vacuum chamber by the CS-1800 space environment simulation system. The PSAC was installed on the base plate of the vacuum chamber, and the fixed-point observation mode was used with a chopper to substitutability observe the integrating sphere and the dark reference. With the PSAC thermal control system, the temperature of the heat sink was maintained at approximately \(-15\)\({}^{\circ}\)C, and the temperature of the optical components was approximately 20 \({}^{\circ}\)C. The PST was set to \(-75\)\({}^{\circ}\)C\(<\) T \(<\)\(-35\)\({}^{\circ}\)C with the temperature controller of the detector. The stability of the integrating sphere radiance was greater than 2%/h, and its radiance was monitored synchronously during the experiment.  
To reduce the influence of background radiation and dark current on \(\mathrm{G_{ins}(T)}\) measurement, based on the signal acquisition of the PSAC adopting an Alternating Current (AC) coupling design, a chopper was used to modulate the light source and ensure normal data acquisition. The shape of the chopper was a symmetrical open-hole disc, and the disc was sprayed with an extinction black lacquer. The stepper motor drove the disc to rotate at a uniform speed, and the rotation speed matched the PSAC sampling time sequence. When the PSAC observed a dark reference, the DCR function was implemented to establish a zero-radiation reference.  
Limited by the PSAC installation state in the chamber, a turning light path was installed to ensure that the light source was incident to the PSAC, and a heating plate was mounted on the back of the planar mirror to reduce the distortion of the mirror, which would otherwise result in light path offsets at large temperature differences. In the experiment, the reflectance of the mirror and the vacuum chamber port window remained basically unchanged. Although their polarization could have caused a change in the polarization component, \(I\) in Equation (1) was relatively stable and had little influence on the \(\mathrm{G_{ins}(T)}\) measurement. To monitor the operation state of the PSAC in the experiments, temperature sensors were added to the PSAC body, the planar mirror, the vacuum chamber, and other key positions. Based on this thermal vacuum experimental condition, \(\mathrm{G_{ins}(T)}\) (T \(<\)\(-50\)\({}^{\circ}\)C) was measured, and an instrument-level bench stability experiment was also conducted.  
In the instrument-level sub-experiment in the ambient laboratory temperature, the PSAC also observed the integrating sphere through a chopper. With the operation of the active-passive detector temperature controller, the temperature of the heat sink was close to the ambient temperature, which limited the PST to higher than \(-35\)\({}^{\circ}\)C.  
Figure 8: Schematic diagram of the instrument-level thermal vacuum sub-experiment. The PSAC was installed on the base plate of the vacuum chamber, and the fixed-point observation mode was used with a chopper to substitutability observe the integrating sphere and the dark reference. A turning light path was installed to ensure that the light source was incident to the PSAC. Temperature monitoring was performed for the PSAC body, the planar mirror, the vacuum chamber, and other key positions.