A high accuracy FTS for laser frequency combs, lamps and other sources

We present results from a custom Fourier Transform Spectrograph (FTS) optimized for characterizing broadband, high repetition-rate laser frequency combs (``astro-combs'') as well as other sources such as calibration lamps and solar spectra. The FTS is a 2.4 m maximum optical path difference (OPD) Michelson interferometer with a resolving power at 500 nm of $R\sim8$ million for a single transverse mode and long coherence length source (e.g., a laser frequency comb) and $R\sim1$ million for a source with multiple transverse modes and short coherence length (e.g., Th:Ar lamp). In our FTS, the reference laser co-propogates with the light being characterized, canceling finite aperture frequency shifts and is locked to an atomic clock via a laser frequency comb providing intrinsic accuracy of the FTS at the part per trillion level. Due to residual systematic effects, we realize an accuracy of $<2$ MHz on multiple transverse mode sources. We will present results from our ongoing use of the FTS to characterize various sources.