Making very long spectra

 There is a much less trivial application than the above which occurs when one has spectra which occupy both subsystems of the DAS. Consider a CO 4-3 observation with receiver C2 in the special wideband mode; in this case there are 16 subbands, using both subsystems, and extending over nearly 1.8 GHz. Each subsystem consists of eight subsections. Loading this spectrum into SPECX shows that two positions in the stack are used, one for each subsystem:

 >> r-g-d 46
 GSD version  5.3
          (x,y) offset = ( 159.9,   0.5) arcsec
          rotation angles: x2y =  -90.0 deg.; v2y =    0.0 deg.
          (r,d) offset = ( 159.9,   0.5) arcsec
 
 
 Stack posn    Scan no    Title
     X           46      0046.000_B  SATURN   JCMT 
     Y           46      0046.000_A  SATURN   JCMT 
 >>

The following sequence of commands will reduce this to a single spectrum (dialogue has been omitted):

>> r-g-d 46
>> das-merge\#\n\
>> xy
>> das-merge\#\n\
>> xy
>> concat
>> merge-quad\n\

Here one first performs the das-merge on each of the subsystems separately, then concatenates the two spectra to produce a single version which has overlapping channels around the centre, and finally merges both spectra together to form the final result. Normally the comamnd merge-quadrants is used only as part of das-merge, but it can be used separately, as in this instance.

Doing this to the next spectrum, averaging the result, and so on for all the spectra in a set one can form the final average (see Figure ). It would make sense to construct a procedure to do this (see later).