Quantitative impact of the MRTCAL frequency dependent calibration

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Quantitative impact of the MRTCAL frequency dependent calibration

Figure 8: Relative difference of integrated line intensities in MIRA and MRTCAL-calibrated spectra above 179.4 GHz in setup E150/L170250, at the edge of the 2mm atmospheric window. Top panel: with default 20 MHz calibration bandwidth in MRTCAL, which traces better the strong increase of atmosphere opacity at the high frequency end. Bottom panel: with MRTCAL calibration bandwidth equal to the natural hardware unit size, i.e. 1.35 GHz for FTS200. Horizontal blue solid and dashed lines indicate the mean and 1 standard deviation of all data points.

Figure 9: System temperature derived by MIRA (dashed black horizontal lines, one per 1.35 GHz hardware unit of the FTS200) and MRTCAL (dashed blue line) for the upper outer subband of setup E150/L170250. The respective spectra are shown with solid lines.

Figure 10: Ratio of integrated line intensities in MIRA and MRTCAL-calibrated spectra (black) at the upper frequency end of setup E150/L170250, compared with the ratios of system temperature (red) at the line frequencies.

Opacity increases strongly at the upper end of the EMIR 2 mm band because of the atmospheric water line at 183.3 GHz. Observations near the upper edge of the 2 mm atmospheric band will thus exacerbate the change in the calibration strategy that should better take into account the frequency dependency.

That the differences in line intensities between MRTCAL and MIRA close to the atmospheric window are a direct consequence of the different calibration bandwidths, is shown in Fig. 8. The top panel shows the relative difference of integrated line intensities for MRTCAL calibration with default settings (i.e., in 20 MHz steps), while the bottom panel shows it for a MRTCAL calibration in steps of the natural hardware units, the MIRA calibration bandwidth. In the second case, the relative differences are reduced from up to 100% to 11%. Residual differences in the bottom panel are possibly due to the remaining difference in the correction of the intermediate frequency bandpass.

The spectra and system temperature derived by both calibration softwares at the edge of the atmospheric window are shown in Fig. 9. In the rightmost third of this figure, the system temperature ( ) derived by MRTCAL better traces the strong increase of atmospheric contribution than the single mean value used by MIRA. In agreement with this, line peaks in the MIRA-calibrated spectrum are underestimated compared to those from MRTCAL in the regions where the MIRA value of is below that of MRTCAL (i.e., above 181.2 GHz, see also figure 7), and vice versa. Moreover, the baseline noise at the high frequency end is increased in the MRTCAL-calibrated spectrum due to the proportionality to , providing a more realistic magnitude in this region of low atmospheric transmission.

Figure 10 shows a numerical comparison of the ratio of the integrated line intensities in MIRA- and MRTCAL-calibrated spectra with the ratio of the respective system temperature applied at the line frequencies. The ratio of system temperature indeed shows the same trends as the ratio of line integrated intensities. The relative error between the two ratios is of the order of 20%. This could be due either to uncertainties in line intensities or to the scheme used to correct for the bandpass shape that is slightly different between MIRA and MRTCAL. Additional work is required to explore this discrepancy.