ABSTRACT
After the discovery of spectral emission of carbon monoxide (CO) at 115.3 GHz ( 7 = 1-0) ( J is the rotational state of a molecule and J = 1-0 represents a transition from the lowest excited state ( J = 1) to the ground state ( J = 0)) in 1970 (Wilson et al 1970) millimetre-wave radio astronomy experienced a rapid growth to become one of the most important branches of observational astronomy. The emission spectra of CO have been used extensively in the past to obtain valuable information on the physics and kinematics of interstellar clouds. Higher-J CO transitions, such as the ones at 230 GHz ( J = 2 - 1) and 345 GHz ( J = 3 – 2), also contain spectral and spatial information on the cosmic background, on very distant newly formed galaxies and on the early stages of star formation within molecular clouds in our galaxy. In addition to CO, many molecules have been found by spectroscopy of the interstellar medium using heterodyne receivers. From studies of the interstellar molecular transitions, our understanding of the nature of the interstellar medium has increased enormously. Since the signals received from those molecules are generally extremely weak, the quest for higher sensitivity has led to larger radio telescopes with a good surface accuracy (for reviews on radio telescopes see Baars et al 1994, Ukita and Tsuboi 1994) and the development of low-noise heterodyne receivers to fully exploit the telescopes’ potentiality (for reviews on receiver development see Blundell and Tong 1992, Carlstrom and Zmuidzinas 1996).
