Re: Combining and plotting HETG spectra

    Petri> I'm trying to combine 8 Chandra HETG spectra (two closely spaced 
    Petri> observations, HEG & MEG + & - 1st orders) into two (HEG & MEG) spectra 
    Petri> in a statistically sensible way. What I was expecting to get is what I 
    Petri> see by plotting the spectra in these groups:

    Petri> plot_data({[1,2,5,6],[3,4,7,8]};...});

    Petri> Which plots two neat spectra with what I think are sensibly-sized error 
    Petri> bars. So I do the combination:

    Petri> variable h1_id, m1_id;
    Petri> match_dataset_grids(1,2,5,6);
    Petri> match_dataset_grids(3,4,7,8);

If I understand your indexing, h = [1,2,3,4] is one obs, with 1,2 being HEG and
3,4 being MEG.  The second set is h+4.

To match grids, you then want to do:

match_dataset_grids( [3, 1,2, 5,6] );

to put 1,2,5,6 onto the 1st MEG grid.  (1,2,5,6 alread match - IF they
are all HEG, and 3,4,7,8 already match, if they are all MEG).


    Petri> notice_values([1,2,5,6],1.2,9.2; min_val = 1, unit = "A");
    Petri> notice_values([3,4,7,8],1.2,9.2; min_val = 1, unit = "A");
    Petri> h1_id = combine_datasets(1,2,5,6);
    Petri> m1_id = combine_datasets(3,4,7,8);

    Petri> But when I plot again (same command), the error bars are tiny or 
    Petri> nonexistent (and the chi squared quite large), and the residuals also 

If you want to plot as combined, I think you need a negative index on
the data or the combined group ID:

plot_data( -m1_id );

or for all combined:

plot_data( -[1:8] );

(and for the residuals, set popt.res = 4).



    Petri> I also tried plotting by the indices

    Petri> plot_data({h1_id,m1_id};...});

This is incorrect syntax.  Those ID's would be interpreted as
histogram indices.  They are "handles" for the combined data.  E.g., 
if I do:

g = combine_datasets( [1,2,3,4] );

then g = 1.

print( combination_members( g ) );

would show me the members.


    Petri> But this produces an even weirder plot, with the HEG spectrum looking 
    Petri> like spectrum 1 and the MEG spectrum missing both error bars and residuals.


I'm not sure what you really want to do.  If you want to combine HEG
and MEG, then you need to match grids.  If you just want to combine
MEG,  and combine HEG, then you don't have to match grids.  E.g., for
the latter case:

heg_idx = [ 1,2, 5,6 ];
meg_idx = heg_idx + 2 ; 

gm = combine_datasets( meg_idx ) ; 
gh = combine_datasets( heg_idx ) ; 

plot_data( -gm, popt );
plot_data( -gh, popt) ;

(where popt is the structure that plot_data wants, and I set fields
appropriately).


combine_datasets sets a flag which means that when you do a fit, the
counts get summed, the model counts get summed, then the statistic
gets evaluated.  Extra work (plot_data, plot_counts, plot_unfold, from
Mike Nowak) are needed to do the visualization.

Hope this helps.


-- Dave

David Huenemoerder  617-253-4283 (o); -253-8084 (f); http://space.mit.edu/home/dph
MIT Kavli Institute for Astrophysics and Space Research
70 Vassar St., NE80-6065,
Cambridge, MA  02139
[Admin. Asst.: Elaine Tirrell, 617-253-7480, egt_at_email.domain.hidden
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