The Submillimeter Wave Astronomy Satellite - The Instrument

V. Tolls, G. J. Melnick, N. Erickson, P. Goldsmith, M. Harwit, R. Schieder, R. Snell, J. Stauffer

Proceedings of "The Physics and Chemistry of Interstellar Molecular Clouds", held in Zermatt, Switzerland, , 21-24 September 1993, G. Winnewisser and G. C. Pelz (Eds.), Springer 1995


The Submillimeter Wave Astronomy Satellite (SWAS) is part of NASA's Small Explorer Program (SMEX). During its planned 3-year lifetime, SWAS will survey dense molecular clouds within our galaxy in five astrophysically important transitions of H2O, H218O, O2, CI and 13CO. The launch for SWAS is scheduled in 1995.

In this paper I will present the current status of the SWAS instrument; the scientific objectives of the mission are described in a separate paper .
The SWAS instrument consists of three main subsystems: (1) the telescope, (2) the receivers, and (3) the spectrometer. Additional instrument subsystems include the star tracker, which is used in conjunction with the Attitude control system to position the satellite, the radiators for the passive cooling of the receiver system and a continuum back-end for calibration.

The telescope is a highly efficient off-axis Cassegrain antenna with a 54 x 68 cm diameter primary mirror and a small chopping secondary mirror (2 Hz or 1/4 Hz chopping rate with an amplitude of 10 arc minutes along a single axis), both of which are made of aluminum. The use of a Gaussian edge taper of 11 dB results in an overall aperture efficiency of 81% and a main beam efficiency of 90% for this system.

The receiver system consist of two independent receivers with second harmonic Schottky diode mixers pumped by frequency-tripled InP Gunn oscillators. The oscillator frequencies are 81.5 GHz for the lower frequency receiver, which is used to observe O2, and CI at 487 GHz and 492 GHz respectively, and 92.3 GHz for the higher frequency receiver, which is used to observe H2O, 13CO, or H218O at 557 GHz, 551 GHz, and 548 GHz respectively. Both oscillators are phase locked to a single reference oscillator at 5.114 GHz. The receivers are operating in orthogonal linear polarizations.

The first IF stages consist of HEMT amplifiers, which are passively cooled to between 130 K and 150 K together with the input optics and the mixers resulting in an expected receiver noise temperature of about 1500 K (DSB). In the IF system both receiver IF's are down converted and diplexed to the spectrometer input band of 1.4 GHz to 2.8 GHz.

The SWAS spectrometer is a single acousto-optical spectrometer (AOS) with 1400 1-MHz channels. This yields a velocity channel spacing of approximately 0.6 km/s and a total bandwidth of 840 km/s, or about 200 km/s per line. The SWAS-AOS incorporates redundant laser diodes, a Bragg cell for the spectroscopic analysis of the IF and a CCD line sensor readout. SWAS will carry two broadband (700 MHz) continuum detectors, one for each receiver, which will be used for observing point-like sources, such as Jupiter, for pointing and calibration.

The receivers are being built by Millitech Corporation; the AOS is being provided by the University of Cologne; and the optics, cooling radiators, star tracker, and instrument structure as well as the thermal design, systems integration, and testing are the responsibility of Ball Aerospace Systems Group. The spacecraft bus and its subsystems are being built by the NASA Goddard Space Flight Center.