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Astron. Astrophys. 326, 59-68 (1997)

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2. Observations

All observations were carried out with the IRAM 30-m antenna. The measurements were made with SIS receivers tuned to a single sideband with image sideband rejections of typically 7 dB. An error in the determination of the sideband rejection of [FORMULA] 1 dB would result in an additional calibration uncertainty of [FORMULA] 5%. The temperature scale used throughout this paper is main-beam brightness temperature ([FORMULA]). System temperatures were between 540 K and 580 K [FORMULA] in the 3 mm wavelength range, between 600 K and 900 [FORMULA] at 2 mm wavelength and between 1200 K and 1800 K [FORMULA] at 1.3 mm. All transitions were observed with either a 512 [FORMULA] 1 MHz filterbank or a 864 channel acousto-optical spectrometer (AOS) with a total bandwidth of 500 MHz.

The observed transitions, including their designations, frequencies and energies above ground (Mangum & Wootten 1993, Anderson et al. 1990), and the beamwidth of the telescope, are summarized in Table 1.


Table 1. Summary of the observed transitions. For [FORMULA], o and p denote ortho and para transitions.

For broad lines encountered in external galaxies, it is not only the system noise that determines the accuracy of line intensies and ratios, but also the flatness of the spectrometer bandbass, which is affected by the stability of the weather and the electronic components. We used a wobbling secondary mirror, switching with a frequency of 0.5 Hz between the source and two reference positions placed symmetrically at 4 [FORMULA] offset in azimuth. The backends are of high stability. Thus, the baselines we obtained are very flat. Therefore, only baselines of order zero or one were subtracted from the spectra, and the data were smoothed to a resolution of [FORMULA] 10 [FORMULA] @

From continuum cross scans on nearby sources, made every [FORMULA] 3 hours, we estimate the pointing accuracy to be of order 5 [FORMULA] or better. To check the calibration of the telescope, we usually observed line emission from strong Galactic sources like W3(OH) in each period to achieve internal consistency. We expect the calibration to be correct on a level of [FORMULA] %.

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© European Southern Observatory (ESO) 1997

Online publication: April 20, 1998