  Solar observations at sub-THz frequencies detected a new flare spectral
component peaking in the THz range, simultaneously with the well known
microwaves component, bringing challenging constraints for interpretation.
Higher THz frequencies observations are needed to understand the nature of the
mechanisms occurring in flares. A THz photometer system was developed to
observe outside the terrestrial atmosphere on stratospheric balloons or
satellites, or at exceptionally transparent ground stations. The telescope was
designed to observe the whole solar disk detecting small relative changes in
input temperature caused by flares at localized positions. A Golay cell
detector is preceded by low-pass filters to suppress visible and near IR
radiation, a band-pass filter, and a chopper. A prototype was assembled to
demonstrate the new concept and the system performance. It can detect
temperature variations smaller than 1 K for data sampled at a rate of
10/second, smoothed for intervals larger than 4 seconds. For a 76 mm aperture,
this corresponds to small solar burst intensities at THz frequencies. A system
with 3 and 7 THz photometers is being built for solar flare observations on
board of stratospheric balloon missions.
