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2. The Instrumentation of the 1.2m SMWT

 

2.1. The basic telescope system

 

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 tex2html_wrap_inline5389, which is better than tex2html_wrap_inline5391 at the wavelength of the tex2html_wrap_inline5227 line of tex2html_wrap_inline5325, tex2html_wrap_inline5397 (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 5tex2html_wrap5415 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 tex2html_wrap_inline5401, 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 8tex2html_wrap54178 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 tex2html_wrap_inline5241 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 tex2html_wrap_inline5325, or Bally et al. 1987, 1988 for tex2html_wrap_inline5327).

2.2. The system upgrade of the MPIfR in 1993

Before the tex2html_wrap_inline5241 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 tex2html_wrap_inline5241 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).

  figure616
Figure 1: The complete system of the 1.2m SMWT on Cerro Tololo (CTIO) for the tex2html_wrap_inline5241 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


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