  High fidelity iodine spectra provide the wavelength and instrument
calibration needed to extract precise radial velocities (RVs) from stellar
spectral observations taken through iodine cells. Such iodine spectra are
usually taken by a Fourier Transform Spectrometer (FTS). In this work, we
investigated the reason behind the discrepancy between two FTS spectra of the
iodine cell used for precise RV work with the High Resolution Spectrograph
(HRS) at the Hobby-Eberly Telescope. We concluded that the discrepancy between
the two HRS FTS spectra was due to temperature changes of the iodine cell. Our
work demonstrated that the ultra-high resolution spectra taken by the TS12 arm
of the Tull Spectrograph One at McDonald Observatory are of similar quality to
the FTS spectra and thus can be used to validate the FTS spectra. Using the
software IodineSpec5, which computes the iodine absorption lines at different
temperatures, we concluded that the HET/HRS cell was most likely not at its
nominal operating temperature of 70 degree Celsius during its FTS scan at NIST
or at the TS12 measurement. We found that extremely high resolution echelle
spectra (R>200,000) can validate and diagnose deficiencies in FTS spectra. We
also recommend best practices for temperature control and nightly calibration
of iodine cells.
