Array (HERA), which consists of nine receivers arranged in a regular $3\times3$ grid with a spacing on the sky of 24. The beam size of each “pixel” at 230 GHz is 12 (FWHM). For a gaussian source of size 12 (FWHM) centered in the array, a negligible fraction ($<1\%$) of the signal in the central pixel is recorded in the outer pixels. The backend used was the VESPA autocorrelator, which was configured to provide $225 \times 1.63$ [kms$^{-1}$]{} channels for each of the nine receivers during the 2002 observing run, and $409 \times
1.63$ [kms$^{-1}$]{} channels during the 2003 observing run. A wide spectral coverage is essential because the CO lines of PN envelopes often have broad wings and the systemic radial velocities of many of the PNe have not previously been measured at optical wavelengths (see Section 5).

The telescope pointing was regularly monitored during the observations using reference sources, and is generally expected to be accurate within $\sim 2\arcsec$, although it is somewhat more uncertain during periods of high wind or rapidly changing weather conditions. The observations were made by switching in azimuth by 4 using the wobbler. The spectra were calibrated using the chopper wheel technique, and 