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The paper is concerned with multilayer and fine thermal control of an optical reference cavity for space applications, under development at National Metrology Institute, Turin, Italy. The cavity, made by ultra low expansion glass (Corning ULEreg), must be kept close to the zero-expansion temperature (close to 25degC) of the glass. The target can only be met by active control, while leaving the cavity free of sensor and actuators. This is obtained by two concepts: thermal bath and reference thermal sink, the latter allowing zero-expansion temperature to be reached by heaters in a wide environment range. Preliminary experiments of the two concepts leading to cavity design are detailed.