  The Spectrometer/Telescope for Imaging X-rays (STIX) is a hard X-ray imaging
spectroscopy device to be mounted in the Solar Orbiter cluster with the aim of
providing images and spectra of solar flaring regions at different photon
energies in the range from a few keV to around 150 keV. The imaging modality of
this telescope is based on the Moire pattern concept and utilizes 30
sub-collimators, each one containing a pair of co-axial grids. This paper
applies Fourier analysis to provide the first rigorous description of the data
formation process in STIX. Specifically, we show that, under first harmonic
approximation, the integrated counts measured by STIX sub-collimators can be
interpreted as specific spatial Fourier components of the incoming photon flux,
named visibilities. Fourier analysis also allows the quantitative assessment of
the reliability of such interpretation. The description of STIX data in terms
of visibilities has a notable impact on the image reconstruction process, since
it fosters the application of Fourier-based imaging algorithms.
