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
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.