Aperture in the optics of a Fourier-transform spectrometer, typically between the source and the interferometer, designed to be the limiting aperture when no other optical element such as the detector serves this purpose.
Source:
PAC, 2021, 93, 647. 'Glossary of methods and terms used in analytical spectroscopy (IUPAC Recommendations 2019)' on page 752 (https://doi.org/10.1515/pac-2019-0203)