Hi,
This is about the creation of a spectrally-weighted instrument map in an
energy band. I was trying to understand the weights produced by a member of
our group for various chips, using Sherpa. The weights are drastically
different from one chip to another, which does not make sense to me. Then I
was reading the online thread (http://asc.harvard.edu/ciao/threads/spectral_weights/), which is complicated and seems to problematic. However, I do
understand the help on the routine mkinstmap (ahelp mkinstmap) and the
quoted article by John Houck. Let me explain my problems in the following:
The definition of the weighted instrument map is
weightedinstmap = SUM(over i) W_i * QE(E_i) * Area (E_i)
W_i= S_F(i) /[SUM(over i) S_F(i)]
where S_F(i) is the summed number of PHOTONS (or fluxes) within a narrow
sub-band deltaE_i. It is easy to prove that deviding the counts map with
this weighted instrument map will produce a flux map with the right units.
The instrumental effects are in QE and Area maps (Here the RMF effect
is negelected; otherwise a convolution is required in the above definition).
Therefore, the weights are instrument-independent!!!
The thread requires the use of both RMF and ARF files, in addition to a
spectral data file. I am not exactly sure how these files are incorporated
in the calculation of the weights, since I don't use Sherpa. It seems to me
that none of these files should be needed to calculate the weights, if they are
instrument-independent. However, the files might be required in order to
get the model spectrum, although the results don't depend on the files at all.
If this is the case, the thread should be designed in a much simpler way and
should explain clearly that the results are independent of the input files.
Then, I still don't understand why the weights should vary from one
chip to another. Am I missing something here? I'd very much appreciate
comments from experts on the issue.
Daniel Wang
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