Misleading chi-square

Hi John,

   I noticed that when I tried to simultaneously fit many data sets
extracted on a chip-by-chip basis, my chi-square dropped dramatically.
The problem turned out to be due to large wavelength regions where
there were no counts, and where the arf was identically zero (the
wavelengths fell off the chip).  I can always ignore such ranges but
that will become a logistical nightmare when juggling 15 or so data
sets.  I though about using get/put_data_counts and get/put_arf to
truncate the data and arf. However, the docs for put_data_counts seems
to preclude this.  Any other suggestions? Perhaps a mechanism can be
added to isis to handle this situation.  I suppose that I can use
combine_data to work around it.

Thanks,
--John
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