Balloon-borne far-infrared Fabry-Perot spectrometer for astrophysical observations

The balloon-borne cryogenic far-infrared spectrometer (FIS) described in this paper is a liquid-helium cooled direct-detection spectrometer which covers the wavelength range from 140 mu m to 220 mu m. The spectral resolution R is about 2000. FIS is based on two scanning Fabry-Perot interferometers (SFPs) whose reflector gaps can be varied continuously over more than 200 mu m by the magnetic force of superconducting coils. FIS has been developed to measure far-infrared emission lines from star-forming regions. The most prominent among these are the 157 mu m [CII] line and the 205 mu m [NII] line. In order to perform the calibration of FIS during the flight preparation at the balloon-launch facility of CNES in Aire sur 1'Adour (France), we have devised a new method of calibration of FIS which is performed exclusively with the help of an evacuable absorption cell. In-flight tests in the stratosphere have confirmed that all components of the instrument work. This test permitted us to estimate the system noise equivalent power (NEPsys). It is in the expected range of about 10(-14) W/H-1/2 at the wavelength, of 157 pm and of about 2 x 10(-14) W/Hz(1/2) at 205 mu m wavelength. (C) 1998 Elsevier Science B.V. All rights reserved.