This is the final version of the article. Available from Society of Photo Optical Instrumentation Engineers (SPIE) via the DOI in this record.MIRC-X is an upgrade of the six-telescope infrared beam combiner at the CHARA telescope array, the world's largest baseline interferometer in the optical/infrared, located at the Mount Wilson Observatory in Los Angeles. The upgraded instrument features an ultra-low noise and fast frame rate infrared camera (SAPHIRA detector) based on e-APD technology. We report the MIRC-X sensitivity upgrade work and first light results in detail focusing on the detector characteristics and software architecture.MIRC-X is funded, in parts, by a Starting Grant from the European Research Council (ERC; grant agreement No. 639889, PI: Kraus) and builds on earlier investments from the University of Michigan and the National Science Foundation (NSF, PI: Monnier). This research has made use of the Jean-Marie Mariotti Center OIFits Explorer service (http://www.jmmc.fr/oifitsexplorer)
