We describe the design and performance of a cryogenic Fourier transform spectrometer (Cryo-FTS) operating at a
temperature of approximately 15 K. The instrument is based on a porch-swing scanning mirror design with active
alignment stabilization using a fiber-optic coupled diode laser and voice-coil actuator mechanism. It has a KBr
beamsplitter and has been integrated into an infrared radiometer containing a calibrated Si:As blocked impurity band
(BIB) detector. Due to its low operating temperature, the spectrometer exhibits very small thermal background signal
and low drift. Data from tests of basic spectrometer function, such as modulation efficiency, scan jitter, spectral range,
and spectral resolution are presented. We also present results from measurements of faint point-like sources in a low
background environment, including background, signal offset and gain, and spectral noise equivalent power, and discuss
the possible use of the instrument for spectral characterization of ground-based infrared astronomy calibration sources.
The Cryo-FTS is presently limited to wavelengths below 25 micrometers but can be in principle extended to longer
wavelengths with changes in beamsplitter and detector.