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 \mu m ( 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 _ { 1.5 { THz } } = ( 95 \pm 10 ) \times \tau _ { 225 { GHz } } .
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 \sim 65 \pm 10 Jy ( 1 \sigma 1s ) 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 \sim 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 .