# Qualitative comparison of raw spectra

Figure: Two examples of unprocessed VESPA spectra calibrated with MIRA and MRTCAL (black and red in the lower panels). The upper panels show the difference spectrum. Both polarizations and all consistent scans (same setup and observing mode) have been averaged. The three central channels of each spectrum have been masked as spikes are expected in the center of the three adjacent 40 MHz units for VESPA due to technical reasons (Paubert 2002).

Figure: Comparison of unprocessed (except for blanking of spikes and a spurious feature in E3) FTS200 spectra calibrated with MIRA and MRTCAL. The spectra are overplotted in black and red in the lower panels. The difference spectrum is displayed in the upper panels. Both polarizations and all scans (4 scans for setup E1 POINT2MM and E3 L338890, 6 for E1 L170250 and 10 for E230 L224250) have been averaged for each setup and observing mode, indicated in the upper right of each plot. All visible narrow features are spectral lines.

Figure compares two MIRA and MRTCAL-calibrated lines covered at high resolution with VESPA. Figure compares MIRA and MRTCAL-calibrated FTS spectra for all four frequency setups. In all cases, default calibration settings were used. Both polarizations and all consistent scans were averaged. Spikes and a spurious signal at intermediate frequencies 5.62 and 11.240.01 GHz in the E330 setup, which stems from interference with a network switch in the backend room, have been blanked. No futher processing was applied. In particular, no baselines were subtracted.

The line shape observed at high spectral resolution show variations of the order of at most a few percents (cf. Fig. ). Systematic differences are discernable on spectral lines in Fig. , with either MIRA- or MRTCAL-calibrated spectra showing higher intensities depending on the frequency. For instance, the differences are exacerbated for setup E150/L170250 that is situated close to the upper edge of the atmospheric window of the 2mm band (it shows a notable increased in the baseline noise at the high frequency end). These differences will be quantified and explained in the next sections