Dear Dave,
Thanks for your rapid and informative reply. It's as you say :
query_heg_grid shows that my two observations have different minimum
wavelengths (the README confusingly mentions that it "will read the minimum
wavelength from a Chandra grating RMF" rather than the pha2 file mentioned
in the USAGE) :
$ query_heg_grid obs1_acisf_pha2.fits
0.500026
$ query_heg_grid obs2_acisf_pha2.fits
1.000000
while prism inspection of the N0002 CIAO2.1 HEG +/-1 RMF that I used shows
it also starts at 0.5 Angstroms. The RMF manifest suggested that the N0003
RMF grid would start at 1.0 Angstroms and so it has proved.
It's full steam ahead now, thanks.
Andy
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