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A cryostat specifically designed for maintaining nonlinear optical crystals at temperatures between room temperature and -175°C is described. The sample is held in a vacuum environment, and its temperature is kept within 0.1°C of the desired value by applying regulated power to a sample holder which is connected to a liquid-nitrogen-cooled heat sink through a thermal resistance. The system is easily constructed and can be evacuated with a mechanical pump. It can operate longer than 6 h on a single filling of liquid nitrogen with the pump disconnected.