  The origin and nature of ultra-high energy cosmic rays (UHECRs) are hot
topics in the astroparticle physics community. The Fluorescence detector Array
of Single-pixel Telescopes (FAST) is a design for a next-generation
ground-based UHECR observatory, addressing the requirements for a large-area,
low-cost detector suitable for measuring the properties of the highest energy
cosmic rays with an unprecedented aperture. We have developed a full-scale
prototype consisting of four 200 mm photomultiplier tubes at the focus of a
segmented mirror of 1.6 m in diameter. Over the last three years, we have
installed three prototypes at the Telescope Array Experiment in Utah, USA.
These telescopes have been steadily taking data since installation. We report
on preliminary results of the full-scale FAST prototypes, including
measurements of UHECRs, and distant ultra-violet lasers used to study the
atmospheric transparency. Furthermore, we discuss the installation of an
additional identical FAST prototype at the Pierre Auger Observatory in
Argentina. Possible benefits to the Telescope Array Experiment and the Pierre
Auger Observatory include a comparison of the transparency of the atmosphere
above both experiments, a study of the systematic uncertainty associated with
their existing fluorescence detectors, and a cross-calibration of their energy
and Xmax scales.
