FOURIER is the first-generation science beam combiner for the MROI. It is a three-way, J, H and K band image plane combiner. The FOURIER design emphasises low visibility losses and high optical throughput and is designed around a low-noise SAPHIRA detector. Based on laboratory measurements of its throughput and visibility losses, FOURIER is expected to reach limiting magnitudes of 12.3, 13.2 and 11.7 in the J, H and K bands, respectively, within 5 minutes of incoherent integration assuming 0.7′′ seeing and a detector read noise of 0.3 electrons. As FOURIER observes as red as the K band, the detector and most of its optics are placed within a liquid nitrogen cryostat. We present the design of FOURIER’s cryostat, as well as laboratory tests of the instrument’s cryogenic performance. We also report room temperature characterisation of the optics. Finally, we discuss the path forward from the current status of the instrument to first fringes in 2023.
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