Combining and plotting HETG spectra

From: Petri Savolainen <psavolainen_at_email.domain.hidden>
Date: Fri, 03 Jun 2011 14:40:17 -0400
Hi,

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

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

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

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

But when I plot again (same command), the error bars are tiny or 
nonexistent (and the chi squared quite large), and the residuals also 
seem better than they should be, as if they're from one of the 
individual spectra instead of the combined one; this, I think, is also 
supported by the fact that if I change the notice criteria to min_sum = 
1, the plotted residuals get messed up due to zero-count bins in the 
individual spectra.

So, what is this discrepancy due to, and can I eliminate it somehow?
Is the plotting function plot_data causing some of the issues?


I also tried plotting by the indices

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

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

Furthermore, I tried combine_datasets with weights based on the exposure 
times (which are very different for the two observations), but the 
resulting chi squared is even larger.

Regards,

Petri

-- 
Petri Savolainen, Predoctoral Fellow
Smithsonian Astrophysical Observatory
Harvard-Smithsonian Center for Astrophysics
60 Garden Street, MS 67
Cambridge, MA 02138, USA
Tel: +1-617-495-7956
Email: psavolainen_at_email.domain.hidden
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Received on Fri Jun 03 2011 - 14:40:30 EDT

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