  Double-Fourier interferometry is the most viable path to sub-arcsecond
spatial resolution for future astronomical instruments that will observe the
universe at far-infrared wavelengths. The double transform spatio-spectral
interferometry couples pupil plane beam combination with detector arrays to
enable imaging spectroscopy of wide fields, that will be key to accomplishing
top-level science goals. The wide field of view and the necessity for these
instruments to fly above the opaque atmosphere create unique characteristics
and requirements compared to instruments on ground-based telescopes. In this
paper, we discuss some characteristics of single-baseline spatio-spectral
interferometers. We investigate the impact of intensity and optical path
difference noise on the interferogram and the spectral signal-to-noise ratio.
We apply our findings to the special case of the Balloon Experimental Twin
Telescope for Infrared Interferometry (BETTII), a balloon payload that will be
a first application of this technique at far-infrared wavelengths on a flying
platform.
