Linear baselines

To take a linear baseline out of your spectrum use the command remove-linear-baseline. For example, I want to remove a linear baseline from the current spectrum. The following command will do it:

$>\!>$ r-l-b

If you are in interactive mode the screen will display the current spectrum with the first of the previously chosen boxes (if any) outlined in dot-dashed lines. If you are using Xwindows click first on the title bar at the top of the plot window - it will be labelled ``PGPLOT Window 1''; depending on how your system is set up clicking elsewhere in the window may give erroneous results. Then either

• tap the return key at this point to choose the range shown (which will then be displayed in solid lines), and display the second range. You have the option of taking this one too, in which case you are finished with this fit.
• or, alternatively, move the cursor or cross-hair to a somewhat flat part of the noise to the left of the spectral line and hit `l' (for `left') and then move a little to the right (still to the left of the spectral line) and hit `r' (for `right') and finally `a' (for `accept'). Now a box will appear on the screen corresponding to the range between the chosen l and r positions. Now go to the right side of the spectral line and do the same thing. When you hit `a' this time though, the baseline will have been subtracted and the end result will be in the current spectrum buffer.

To see the end result, all you need to do now is type `n'. Remember, to get out of the interactive mode, type `e' with the cursor anywhere inside the plot window. If you want to leave the interactive mode altogther use the command

$>\!>$ s-i n

In non-interactive mode the following exchange is typical. The result in this particular instance is shown in Figure . Remember, EOF is Control-d.

 >> r-l-b 
 Doing R-L-B for quadrant/sub-band #   1
   2 baseline regions currently defined
  Type intervals, one at a time, EOF to finish
 Current units are km/s  
 
 # [   46.00,   56.03] 30 50
 # [   80.00,   90.02] 80 100
 >>

Figure: The upper spectrum is the original after averaging several observations. Basically in this case there is an offset in the baselevel due to the continuum background of the source (G34.3). Below it is the effect of removing a linear baseline tied to clean (line-free) regions of the spectrum.