--- Forwarded to Chandra-Users ----
In chandra-users, you wrote:
>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
Generally speaking, if different energy bands are chosen for
different chips (to reflect instrument specific effective area
curves) one would have different sets of weights for each chip.
But in this case, the FI and BI chips behave similarly enough
that one could use the same energy bands for both, and the
weights should be the same for each chip.
Aside from that, the weights should normally be derived from
the same spectrum model and are instrument-independent.
[snip]
>
>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 a
>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.
I suspect that the response files are used only to provide a
grid for Sherpa to compute the model spectrum.
The thread should make it clear that the weights are mostly
independent of the instrument response.
> 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.
>
You're right -- apart from some dependence on the specific
bands selected, the weights should not vary from chip to chip.
They should all be computed using the same model spectrum.
John Houck
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