A global optical survey of the solar corona (ref 1) is presently accomplished by observations made with an Extreme ultraviolet Imaging Telescope (EIT), an experiment on-board of SOHO. The thermal issue was a main concern for this sensor. The thermal behaviors of the instrument and some subsystems have been extensively analyzed and tested; these can now be compared with the real behavior in space. These analyses and tests are reported. We outline the lessons learned from this experience for the thermal design of future scientific instruments and suggest improvements in the presently used methods as well as hand calculation methods which enable to easily model some specific problems and more directly extract the physical aspects of the problem.