2. Observations and data reduction
The data reported here were obtained during excellent observing conditions in 1994 Dec and 1995 Jan, mostly as part of an experiment in flexibly scheduled service observing at the JCMT. We used the single-channel SIS receiver, C2, with a broad-band digital autocorrelation spectrometer (DAS) as the backend. The beam (11 FWHM, or around 20 kpc at ) was nutated by 60 in azimuth, at a rate of 1 Hz, with the telescope position-switching by the same distance every 30 s to alternate the signal and reference beams. The maximum pointing offset during the observations was less than 4 . ranged from 1050 to 2300 K, and the sky transparency was excellent. A total of 105 min was spent on source, with a 175 per cent overhead for sky subtraction, position switching, calibration and pointing checks.
The maximum bandwidth of the DAS (920 MHz, or 600 km s-1 at 459.399807 GHz) is barely sufficient when searching for high-frequency lines, particularly when the target line is broad as was expected to be the case here. For this reason, four slightly overlapping spectra were obtained (with band centres at 458.74362, 459.39987, 460.05612 and 460.71237 GHz - see Table 1), giving a full velocity coverage of 1890 km s-1. These were reduced, using SPECX V6.7 (Padman 1993), baseline-subtracted (zero order baselines) and binned to give a velocity resolution of 40 km s-1. The overlap regions were averaged.
Table 1. Log of observations towards PC 1643+4631A.
Our conversion from atmosphere-corrected antenna temperatures, , to the scale assumes a forward spillover efficiency, , of 70 per cent, and is accurate to per cent. The conversion factor between the scale and flux density is Jy, which assumes an aperture efficiency of 42 per cent. The beam efficiency at 460 GHz, measured on Mars, was per cent, so , with an overall uncertainty of around 20 per cent.
© European Southern Observatory (ESO) 1998
Online publication: January 16, 1998