Infrared telescopes are often required to work in a complex thermal environment. A long time of daytime heating will
cause the temperature of the telescope dome and the surrounding facilities different from the ambient air during the night.
Different levels of temperature controlling and the accuracy of forecasting will lead to the temperature departures
between the components in the system. Furthermore, the contaminated particles settled on the optical elements will
change the optical characteristics of the optical elements. All of these factors will degrade the stray light performances of
infrared telescopes. In this paper, taking Cassegrain as a typical example and using the optical analysis software, i.e.,
ASAP, the three-dimensional simulation models of the infrared telescope and the dome has been built up. On this basis,
the stray light performances and the variation of the systems have been simulated and analyzed for the different cases of
the different coating for the dome, the change of the temperature of primary and ambient, as well as the existence of the
mirror contamination. The effective emissivity has been introduced and the stray light performance of the systems has
been evaluated. The results indicate that the contaminated particles settled on the optical elements will degrade the
system performances significantly, whereas the influences of other factors are relatively small. Therefore, it is of great
importance to focus on the contaminated particles settled on the optical elements to adopt proper methods to improve the
stray light performances of infrared telescope systems.