The 1.2m SMWT was built in the period from 1980 to 1983 as the
southern twin of the 1.2m NMWT, then at Columbia University, New York
City, U.S.A. The SMWT started operation at CTIO at the beginning of 1983
(Cohen 1983). The antenna is a fast Cassegrain reflector with
a 1.2 meter parabolic primary and a 18.8 cm hyperbolic secondary
(Grabelsky et al. 1987). The effective f/D of the telescope
is 3.79, and the surface accuracy of the primary is 
, which is
better than 
at the wavelength of the 
line of
, 
(Cohen 1983).
The primary is made from a single aluminium casting, and the complete
telescope is housed in an astrodome whose slit is completely covered by a
thin screen of Griffolyn, a polyolefin fabric almost totally transparent at
2.6 mm. The mount of the telescope can move by 5
in less than 1 s, and
hence for position switching one can use reference locations degrees
away from the source position in switching cycles of only 30 s. This
allows rapid position switching (Bronfman et al. 1988).
The telescope has a superheterodyne receiver, which is tunable from
109 GHz to 120 GHz. Its first stage is cooled to 77 K by liquid
nitrogen and consists of a resonant ring LO diplexer with a signal loss
of 0.2 dB, a double-sideband Schottky barrier diode mixer with a noise
temperature of 110 K and a conversion loss of 5.2 dB, an impedance
matching transformer, and a GaAs field-effect-transistor (FET)
amplifier operating at the first intermediate frequency (IF) of
1390 MHz. The amplifier has a noise temperature of 15 K and a gain of
, constant to 1 dB over a bandwidth of 150 MHz. The second
stage, at ambient temperature, consists of standard commercial
components which further amplify the IF signal and convert it to the
second IF of 150 MHz (Bronfman et al. 1988, 1989).
The full width at half-maximum (FWHM) of the main beam of the telescope
was measured to be 8
8 at 115.3 GHz, with sidelobes more than
18 dB below the main beam (Bronfman et al. 1988,
1989).
The spectrometer permanently installed at the telescope is a 256
channel filterbank with a resolution of 100 kHz (Palmer
1984). Thus, the total bandwidth of this backend is 25.6 MHz. This
corresponds to only 69.9 at 109.8 GHz, the 
frequency. This small bandwidth is insufficient for Galactic center
observations since the CO emission is known from previous surveys to cover
the velocity range from -250 to +300 (see e.g.\
Bania 1977, 1980, 1986 for , or
Bally et al. 1987, 1988 for 
).
Before the survey could be started, the
telescope control software and hardware had to be improved, a broadband
backend installed, and data reduction facilities established.
First, a computer system based on an Apple Macintosh was installed at the SMWT. This allowed to run the control software of the NMWT after adjusting it to the Cerro Tololo site.
The Max-Planck-Institut für Radioastronomie
[4] (MPIfR) in
Bonn, Germany, made available a broadband (total bandwidth = 795 MHz)
acousto-optical spectrometer, hereafter AOS, (see Linhart
1994 for a detailed description of the AOS). The optical system of
the AOS laser is designed to illuminate 1499 pixels of the charge coupled
device (CCD). Each pixel has a frequency separation of 0.536695 MHz. The
frequency resolution of the dispersion of the laser due to the
acoustical signal is 0.78 MHz.
For the use at the 1.2m SMWT, the AOS had to be modified in a number
of ways to be compatible with the existing system (see Linhart
1994 for details). One requirement of the system affected the
observations significantly: Because the telescope control software was
designed to work with 256 channel backends, the AOS had to be adjusted to
read out only 512 of the 1499 channels; furthermore, 2 AOS channels were
combined to produce 1 channel. Thus, the AOS became a 256 channel
backend with a frequency separation of 1.07339 MHz per channel. This
corresponds to 2.93 at the frequency and
a total bandwidth of 274.8 MHz (= 750 ). This nearly completely
utilized the total receiver bandwidth of 300 MHz. It covered the total
velocity range of CO emission in the Galactic center region and, in
addition, allowed for enough range in velocity to accurately determine
baselines. The resolution was adequate for accurate measurements of the
wide lines common in the Galactic center region.
On-site data reduction, essential to ensure homogenous, high quality data, was made possible by the installation of a separate workstation, and the implementation of the Grenoble Image and Line Data Analysis System (GILDAS) software package. A dedicated program package, called CTOL, for data transfer via serial link and translation to the format used by the Continuum and Line Analysis Single-dish Software (CLASS) which is part of GILDAS was developed (see Dahmen 1995 for details).
The complete system including all upgrades is illustrated in Fig. 1 (click here).
Figure 1: The complete system of the 1.2m SMWT on Cerro Tololo (CTIO)
for the Galactic Center Survey. The receiver
signal was split into the branch for the filterbank and the branch for
the AOS. The Macintosh controls the telescope drive, the frequency
synthesizer, the filterbank, and the AOS. It is connected to the Sun
workstation for data reduction via the serial link