LIQUID-HELIUM COOLED SCANNING FAR-INFRARED FABRY-PEROT-INTERFEROMETERFOR ASTRONOMICAL OBSERVATIONS WITH A BALLOON-BORNE TELESCOPE

We have developed a new liquid-helium cooled far-infrared Fabry-Perot interferometer (SFP) with high spectral resolution which will be incor porated and flown with our balloon-borne far-infrared 60 cm phi telesc ope FIT-FIT. This telescope has been built and already flown with othe r radiometric equipment in collaboration with the Geneva Observatory. The new spectrometer is designed to measure astronomical emission line s between 140 and 220 mu m wavelength. Of special interest are the [NI I]-line at 205 mu m as well as the [CII]-line at 158 mu m. In order to achieve high sensitivity we had to minimize the thermal radiation fro m the interferometer and its optics. For this reason all optics, inclu ding the scanning Fabry-Perot interferometer SFP, are liquid-helium co oled. This paper describes special new features of the design, operati on, and test of its performance at liquid-helium temperature with opti cally pumped far-infrared gas lasers.