Re: fit_flux

John Houck <houck_at_email.domain.hidden>On Thu, Feb 22, 2007 at 10:47 -0500, John E. Davis wrote:
>> What does "fit_flux" do?
>> 
>> Suppose I load a grating spectrum C(h) grating ARF A(E), and RMF
>> R(h,E), where in this context h represents the grating wavelength.
>> If I flux_corr the spectrum I will get some quantity denoted by F(h).
>> Then does fit_flux assume that F(h) related to the model s(E) by
>> 
>>    F(h) = \int dE R(h,E) s(E) ,
>> 
>> independent of the kernel?
>
>Yes and no.  The standard kernel is linear, so in that case,
>your interpretation (which is the one isis uses) seems
>reasonable.

  I realise that "flux_corr" depends upon the kernel.  However, is
"fit_flux" kernel-dependent?

[...]
>  isis> flux_corr(1);            
>  Not Implemented:operation not supported by assigned kernel
>  
>That should strongly suggest that it's not a good idea to
>use fit_flux with piled-up CCD data.

  Why?  Couldn't I use "put_data_flux" to assign a flux to a dataset
  and then fit that, independent of the kernel?
  
>The isis distribution does also contain the gpileup kernel 
>(no doubt you're familiar with it, since you wrote it).
>However, because that kernel has remained experimental, the
>issue of how it should interact with 'fit_flux' has never been
>fully resolved.  Is this a good time to revisit that?

I think fit_flux should be independent of the kernel and assume

    F(h) = \int dE R(h,E) s(E)

is the relation between the model-flux s(E) and the data-flux F(h).
Of course R(h,E) should not include the ARF.

[...]
>Did you try looking at the help page?:
>
>   isis> help fit_flux

Yes, but that did not tell me anything about the kernel or provide an
equation such as the above one.

[...]
>> 
>> Finally, I noticed that the mailinglist archives are not publically
>> available.  Are there any plans to make them accessable?
>> 
>
>I hadn't considered it, but it seems like a good idea.

I think that would also give isis a somewhat larger web presence.

Thanks,
--John
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