Results of thermal mode mathematical evaluation and thermal vacuum tests of the star sensor developed for the “Yamal” project were compared. “TERM” software package was used for mathematical evaluation. The comparison of its results with information, obtained during thermal vacuum tests of sensor in shadow shows the following: 1. Calculated temperature of unit elements differs from the temperature measured in experiment with the mean square deviation 1.5°C. This deviation does not exceed the mean square error of the model. 2. The possibility of objective contamination does exist. 3. The star image enlarging does exist due the lens temperature changes. The recommendations are proposed to avoid these processes.