Re: Fitting background-subtracted grating data in isis 1.4.9-4

FWIW, a sometimes convenient way to see the background is to set your
model norm(s) to 0.0, eval_counts, then plot_model_counts.  Since the
background is added to the model, you then see the scaled background.


-- Dave



>>>>> "John" == John Houck <houck_at_email.domain.hidden
    John> On Wed, Jan 30, 2008 at 14:51 +1100, Duncan Galloway wrote:
    >> 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 

    John> Isis does not subtract the background from the data.  Instead,
    John> the (scaled) background is added to the model.  The fit
    John> statistic should be the same, since
    John> (data - background) - model = data - (model + background)

    John> To plot the background-subtracted spectrum, one can retrieve
    John> the numerical values using

    John> get_data_counts, get_back, get_model_counts

    John> and generate a custom plot using hplot.  Mike Nowak has written
    John> custom plotting routines that do this.  This plotting
    John> functionality will eventually be incorporated into the isis
    John> distribution.

    >> (does the "0" mean it's not actually doing it?) 

    John> The define_back() function returns an integer status code to
    John> indicate whether or not the function succeeded.  A return value
    John> of zero indicates success (-1 indicates failure).

    >> 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

    John> Using combine_datasets to combine +/- order spectra has a
    John> number of advantages over creating a summed pha file. For
    John> example, combine_datasets allows each individual spectrum to be
    John> treated with the appropriate (separate) response files so
    John> there's no need to create summed response files. With
    John> combine_datasets it's also easier to examine the effect of
    John> leaving one or more spectra out of the sum.  For a more
    John> detailed discussion, see

    John> http://space.mit.edu/cxc/isis/combine_datasets.ps

    John> Thanks,
    John> -John
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-- 
Dave

David Huenemoerder  617-253-4283 (o); -253-8084 (f); http://space.mit.edu/home/dph
MIT Kavli Institute for Astrophysics and Space Research
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