Example T_sky − T_hot spectra from simultaneous observations in the L1 (red) and L2 (green) band, showing two ozone lines (the broad features toward the middle of the L1 and L2 passband) and two CO lines (the narrow peaks on the left). Note that the two CO lines appear at the same velocity on a v_LSR scale, because the receiver was tuned to two CO lines for the astronomical observations. The two black curves are independent fits of T_sky − T_hot for the two bands. The red and green lines between the measured spectra are the result of a common fit, which simultaneously fits a single value of pwv to both bands. In a common fit, a consistent value of pwv cannot be derived for both bands.