spectrum has a good signal-to noise ratio of 120:1. The FTS was operated under vacuum conditions.

The new VIS-FT setup allowed for a two-times improvement in the width of measured lines (0.08 [cm$^{-1}$]{}FWHM) as well as a better signal-to-noise ratio relative to the old grating spectrograph. Previously our CO spectra were contaminated with a dense and intense coverage of Th frequency-calibration lines. This is no longer necessary given the intrinsically linear frequency scale of the Fourier-transform spectrometer and much cleaner spectra can be obtained. A detailed description of the improved setup will be the subject of a future publication.

In Fig. \[fig:FTBXspec\] a spectrum of the [B$^1\Sigma^+-$A$^1\Pi$]{}$(0,0)$ band of $^{13}$C$^{16}$O is presented. Also present are lines from the forbidden [B$^1\Sigma^+-$e$^3\Sigma^-$]{}$(1,0)$ and [B$^1\Sigma^+-$d$^3\!\Delta$]{}$(0,4)$ bands, which borrow intensity from [B$^1\Sigma^+-$A$^1\Pi$]{}$(0,0)$ transition. Line positions were measured by fitting Voigt lineshape functions to the experimental lines in a Levenberg-Marquardt procedure [@Levenberg1944; @Marquardt1963] included in the OPUS$^\textrm{TM}$ software package [@OPUS]. An absolute calibration of the wavenumber scale accurate to within 0.003[cm$^{-1}$]{}was made with reference to the first harmonic of a frequency-stabilized He-Ne laser. This laser monitors the FTS optical path difference and is stable over 1h of operation within a fractional variation of $\pm2\times10^{-9}$.

The 