Abstract: A contaminant control system for reducing or substantially preventing the ingress of contaminants into a positioning system to prevent degradation in precision and accuracy of the positioning system. The contaminant control system directs a gas flow through the interior of the positioning system and creates an internal pressure within the positioning system that is greater than the ambient air pressure. The contaminant control system may include a temperature conditioner to heat and/or cool the gas flowing through the positioning system. Temperature conditioning may provide additional positioning system precision and accuracy by mitigating thermal effects. The contaminant control system includes a controller to control both the gas flow and gas temperature.