Caveats

The defaults for das-merge work well for most purposes; that is, taking the default number of channels for removal, then refusing the vertical adjustment between subbands. Long integrations suggest that it is correct not to take the vertical subband adjustment. The intrinsic flatness of the DAS response is excellent, and applying the subband adjustment actually can introduce low-level offsets between subbands, which, particularly for wide weak lines, could result in spurious detections. Hence one should use the default command, equivalent to

$>\!>$ das-merge$\backslash$#$\backslash$n$\backslash$

for almost all cases.

One particular case which can readily introduce baselevel offsets if one applies the vertical subband adjustment is when one has a bright line in the central overlap region. One such example is shown in Figure . In such case one should most definitely use the default `n' option.

Figure: The same CO(3-2) spectrum of W3(OH), reduced using das-merge with subband offsets (upper spectrum) and without (lower spectrum). The presence of the strong line in the overlap region introduces an artificial baseline step in the former instance. One should use das-merge$\backslash$#$\backslash$n$\backslash$ here for sure.

Possibly the only time the `n' option may introduce artificial effects is when observing a planet. Even then the effect is likely to be small. See Figure .

Figure: The upper spectrum is an observation of Saturn using the 8-subband 760-MHz mode with B3i; the result is normalized to an observed value of 90 K T$^*_a$ (i.e. the actual value is about 45 K), and subband offsets have been applied in this case when using das-merge . The lower curve shows the difference introduced when not using the subband offsets, as compared with the upper spectrum. Using das-merge$\backslash$#$\backslash$n$\backslash$ here introduces additional small steps corresponding to a maximum of about 1% of the total signal.