Commissioning FOURIER: the Magdalena Ridge Observatory Interferometer’s first-generation science beam combiner

Abstract

The Magdalena Ridge Observatory Interferometer (MROI) is a 10-telescope optical interferometer under construction in New Mexico, USA, as an international collaboration between the New Mexico Institute of Mining and Technology and the University of Cambridge. The first-generation science beam combiner for this observatory is known as the Free-space Optical multi-apertUre combineR for IntERferometry (FOURIER). It measures interferometric observables within a wavelength range of 1.1 to 2.4 microns using spectrally-dispersed fringes from up to 3 telescopes. In this paper, we report on the results of FOURIER’s ongoing commissioning after it was installed at the MROI. We discuss the performance of the instrument’s cryostat in terms of vacuum and temperature holding times. We also present the results of detector tests from within the cryostat, including FOURIER’s gain curve, photon transfer curves, and system noise. Finally, having achieved our first stellar fringes in July 2025, we present measurements of FOURIER’s wavelength calibration, dispersion axis brightness profile, and visibility based on preliminary on-sky tests.