|
|
Opacity increases strongly at the upper end of the EMIR 2mm band because of the atmospheric water line at 183.3GHz. Observations near the upper edge of the 2mm 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 20MHz 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 181.2GHz, 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.