(ON) and two emission-free reference positions (OFFs) selected by the telescope control program to straddle the ON in elevation. The fraction of the time spent on each OFF was adjusted so that the time-weighted average system temperature at the OFFs was equal to that at the ON, resulting in baselines that were flat, and residual offsets that were typically less than 1 K. This offset was generally removed by simply fitting a straight line to the emission-free ends of the spectrum. Away from the plane in those regions where only one or two relatively narrow carbon monoxide lines are found, frequency-switching by 10–20 MHz at a rate of 1 Hz was often used instead of position switching. Since spectral lines remain within the range of the spectrometer in both phases of the switching cycle, data could be obtained twice as fast as with position switching, although higher order polynomials, typically 4th or 5th order, were required to remove the residual baseline. A telluric emission line from carbon monoxide in the mesosphere, variable in both intensity and LSR velocity, is detected in frequency-switched spectra; because the LSR velocity of the line could be predicted exactly, blending with galactic emission could be avoided by appropriate scheduling of the observations. In a few cases of large surveys (e.g., Taurus and Orion) a model of the telluric line was fit daily to spectra free of galactic emission and used to remove the line from all spectra. References External links CfA 1.2 m Millimeter-Wave Telescope (CfA_mini) on the internet The Milky Way in Molecular Clouds survey of the galaxy Radio telescopes