  We present observations that were carried out with the Two HUndred Micron
PhotometER (THUMPER) mounted on the James Clerk Maxwell Telescope (JCMT) in
Hawaii, at a wavelength of 200 um (frequency 1.5 THz). The observations utilise
a small atmospheric window that opens up at this wavelength under very dry
conditions at high-altitude observing sites. The atmosphere was calibrated
using the sky-dipping method and a relation was established between the optical
depth, tau, at 1.5 THz and that at 225 GHz: tau_1.5THz = (95 +/-
10)*tau_225GHz. Mars and Jupiter were mapped from the ground at this wavelength
for the first time, and the system characteristics measured. A noise equivalent
flux density (NEFD) of ~65 +/- 10 Jy (1 sigma 1 second) was measured for the
THUMPER-JCMT combination, consistent with predictions based upon our laboratory
measurements. The main-beam resolution of 14 arcsec was confirmed and an
extended error-beam detected at roughly two-thirds of the magnitude of the main
beam. Measurements of the Sun allow us to estimate that the fraction of the
power in the main beam is ~15%, consistent with predictions based on modelling
the dish surface accuracy. It is therefore shown that the sky over Mauna Kea is
suitable for astronomy at this wavelength under the best conditions. However,
higher or drier sites should have a larger number of useable nights per year.
