Introduction
The 1.2 meter Millimeter-Wave Telescope at the CfA has been studying the distribution and properties of dense, star-forming molecular clouds in our Galaxy and its nearest neighbors for over three decades. Built by Prof. Patrick Thaddeus and colleagues in 1974, the telescope was operated from a Columbia University rooftop overlooking the Manhattan skyline for over 10 years before being moved to the CfA in 1986. A twin instrument was constructed at Columbia and shipped to Cerro Tololo, Chile in 1982. Together, these two instruments have obtained what is by far the most extensive, uniform, and widely-used Galactic survey of interstellar carbon monoxide (CO), the best general tracer of the largely invisible molecular hydrogen that constitutes most of the mass in molecular clouds. A total of 24 PhD dissertations have so far been written based on observations or instrumental work with these telescopes, and many more undergraduate students at Columbia and Harvard have participated in the observations either in course laboratories (currently Astro 191 and Astro 16) or as observing assistants (currently James Grundy, Joseph Megnia, Meng Li, and Yizi Shang). 

Major Scientific Achievements
Designed and constructed in-house by our research group just a few years after the first detection of CO in space (Wilson, Jefferts, & Penzias 1970), the 1.2 meter telescope has played an important or dominant role in all of the important general findings on molecular clouds (MCs) listed below. Many of these are now considered conventional wisdom but some were originally very controversial (e.g., the very existence of giant molecular clouds, their ages, and their confinement to spiral arms).

• CO is the best general-purpose tracer of MC mass (Thaddeus 1977)
• Galactic CO emission peaks in a broad "molecular ring" at R~4 kpc (Cohen & Thaddeus 1977)
• MCs are excellent tracers of galactic spiral structure (Cohen et al. 1980, Dame 1983)
• MCs are relatively short-lived galactic objects (Dame et al. 1980, Cohen et al. 1980)
• The MC mass spectrum is steep, with most of the mass in the largest clouds (Dame & Thaddeus 1982, Dame 1983)
• MC are mainly confined to a thin Gaussian layer ~100 pc wide (Cohen & Thaddeus 1977), but a faint layer ~3 times as wide also exists (Dame & Thaddeus 1994)
• Giant molecular complexes containing more than a million solar masses are not kinematic artifacts--as some had argued--but are well-defined objects that can be readily located throughout the Galaxy (Dame et al. 1986)
• Intercomparision of CO, HI, and diffuse gamma ray emissions provides perhaps the best large-scale calibration of CO as a molecular mass tracer (Lebrun et al. 1983; the term "X factor" coined in this paper)
• Roughly half of the interstellar gas within the solar circle is molecular (Bronfman et al. 1988)
• MCs are dark nebulae both in the optical (Dame & Thaddeus 1985) and the near infrared (Dame et al. 1989, Kent, Dame, & Fazio 1991)
  Note: Our earliest publications on each subject are cited; often more complete studies were published later.

Because molecular clouds are so large, contain such a large fraction of the interstellar gas and dust, and are the source of so many conspicuous young objects, we and our CO surveys have long played an important role in interpreting the Galactic emission in every major wavelength band.

Current Research
Over the past few years the 1.2 meter telescope has been mainly dedicated to surveying all of the northern sky at |b| < 50° lying outside the sampling boundary of the composite CO survey of Dame et al. (2001). As of June 2007 this survey has obtained 253,280 spectra covering ~16,000 sq-deg with 1/4° sampling. A current sky map is given here. Because NASA’s next-generation gamma ray telescope GLAST will be able to detect small molecular clouds of the sort being discovered by the survey (Torres, Dame, & Digel 2005), the GLAST project funded telescope operations this season.

We have also recently carried out pilot C¹⁸O observations in several nearby molecular clouds, including the Taurus and Orion B clouds. The most basic and important finding of the pilot observations is that the distribution of C¹⁸O emission is everywhere very different from that of CO. Such striking differences generally do not exist between CO and ¹³CO maps. The implication is that C¹⁸O observations with the 1.2 m are both practical and worthwhile, providing information complementary to that of the CO line on the denser regions of the clouds.

In late 2006 our spectroscopy laboratory succeeded in measuring precise rotational line frequencies for several negative molecular ion (anions), and soon thereafter was able to identify one of them, C6H–, in the dense interstellar cloud TMC–1—the first detection of a molecular anion in interstellar space (McCarthy et al. 2006). Widely thought to be too fragile to exist in the severe environment of space, the surprisingly large abundance measured for C6H– suggests that many other anions may be detectable both in the laboratory and in space. The 1.2 meter telescope recently carried out a sensitive search in a variety of Galactic sources for the negative molecular ion CN-, and other such searches are planned.