The Principles

If one were to plot a spectrum before using das-merge one would obtain something like the spectrum in Figure .

Figure: A spectrum consisting of 4 subbands (500 MHz total bandwidth) taken with a very short integration with receiver A2, plotted without applying das-merge. The edge effects of the overlapping subbands appear as sharp spikes; one spike, at 70 km/s is however due to interference within the bandpass.

For illustration purposes, if we stagger the subbands vertically using the set-quadrant-display and offset commands in tandem, then the overlap between subbands becomes clear, as in Fig. .

Figure: The spectrum before applying das-merge. Quadrants are offset from each other for clarity.

das-merge in fact combines two commands. First it strips off the spiky ends of each subband, using the drop-channels command. The best number of channels dropped from each end of each subband depends on the bandwidth to some extent, but is typically 15-30. The default is set to half the subband overlap. Using the same staggered subband display for clarity the end result looks like that in Fig. .

Figure: The spectrum after applying das-merge. Quadrants offset from one another for clarity.

Note that the subbands still have significant overlap in velocity space. This overlap is used to facilitate the second part of the das-merge action, which averages signals in the overlap region. This can be done separately using the merge-quadrants command. The end result in this instance is shown in Figure .

Figure: The spectrum in Figure  after application of das-merge. The subbands are successful merged into a continuous spectrum. Only the interference spike remains.

Note that if one has previously modified the effective rest frequency using the s-l-r-f command (see Section ) then one must reset the rest frequency to accept the default header values associated with the next spectrum before das-merge can be used. That is, one must issue the command

$>\!>$ s-l-r-f 0.0

first. The consequence of not doing this is that the first subband of the new spectrum appears to das-merge to not have the correct frequency, and the error message

-- SPECX#033 -W- Can't MERGE - quadrants do not overlap --

will result. This is your clue to the above common problem, if you are given to mess with the frequencies.