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All three switching modes available for spectral line observations were observed with the same EMIR 1mm band setup in order to check the consistency of the line intensities between the different switching mode. To our knowledge, this is the first time this kind of study is done at the 30m-Telescope.
Figures 11 compares the integrated line intensities for seven lines in position, wobbler, and frequency switching mode. In all cases, the individual scans were corrected for the gain-elevation curve as the source is barely resolved. Moreover, both polarizations were averaged. Integrated line intensities from the frequency-switched spectra may suffer from a higher uncertainty due to strong baseline ripples that are associated with this switching mode. This is especially true for the weaker lines.
The integrated line intensities in position-switching are consistent
between both calibration software, with MRTCAL intensities reaching
991% (mean over the seven lines and its 1
standard deviation)
of the MIRA intensities.
For wobbler-switching, the integrated line intensities amounts to
931% of the intensities in position-switching for both MIRA- and
MRTCAL-calibrated spectra. This reduction of line intensity is expected
because of the reduced anntena gain when tilting the subreflector. For a
tilt of 90
, the antenna main beam gain at 230GHz is reduced to
0.93 (see report by Peñalver 2016), in agreement with the observed value.
For frequency-switching, the integrated intensities reach 941% of the
position-switching intensities for the MRTCAL-, and 89
1% for
MIRA-calibrated spectra. MRTCAL delivers integrated intensities that
better match the position switching ones, probably because the frequency
calibration and folding of the two phases is done in a different way than
in the usual combination of MIRA and CLASS. Indeed, Winkel et
al. (2012) indicate that the usual approach underestimates line intensities
when the line strengths is not negligible compared to the system temperature.
The remaining difference of 61% between frequency-switching and
position-switching when the data is calibrated by MRTCAL could be
related to a lost of receiver efficiency when the frequency throw is
large. In our case, it was set to the maximum value of
23.4MHz
because IRC+10216 lines are relatively broad. This effect will be further
explored in new observations.