Fitting background-subtracted grating data in isis 1.4.9-4

Hi,
   I am trying to fit combined +/- order spectra once the background  
has been subtracted off, and just can't work out how to do this in  
isis. I've used add_grating_orders to produce the single pha (type I)  
file from the pha2 file, and then tg_bkg to produce a (single)  
background file from the output (this gives errors, but appears to  
work). Then I load in my source file and assign the background:

isis> define_back (1,"sum_MEG_1_bkg.pha");
0
isis>

But this does not appear to actually subtract the background (does  
the "0" mean it's not actually doing it?) Am I going about this  
completely the wrong way? Is there a simpler way, e.g. one that just  
uses the background columns in the pha2 file? any suggestions much  
appreciated!
Cheers
Duncan



----------------------------------------------------------------------
  Duncan Galloway, Monash Fellow             phone: +61 (0)3 9902-0393
  School of Physics &                          fax: +61 (0)3 9905-3637
    School of Mathematical Sciences         mobile: +61 (0)414 139-763
  Monash University, VIC 3800, Australia  chat: duncangalloway_at_email.domain.hidden  Duncan.Galloway_at_email.domain.hidden


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