Re: fit_flux

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.

Currently, the only other "fully supported" kernel is the CCD
pileup kernel.  It is nonlinear and does not support flux_corr.
If the pileup kernel has been assigned, an attempt to use
flux_corr generates an error message:

  isis> set_kernel (1, "pileup");
  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.

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 "googled" this but did not find the answer except for this comment:
> 
>     % Now do the fit...  ('fit_counts' folds the model
>     % through the instrument response; 'fit_flux'
>     % would fit the model directly to flux-corrected
>     % data if such is available -- the latter is sometimes
>     % appropriate for high-resolution data.)

Did you try looking at the help page?:

   isis> help fit_flux
   
   fit_flux
   
    SYNOPSIS
       search for best fit parameters
   
    USAGE
       s = fit_flux ([response_type][, &info_struct])
   
    DESCRIPTION
       Analogous to fit_counts except that the model is compared with
       the flux-corrected histogram (see flux_corr).
   
       If an ARF has been assigned it will not be re-applied. If the
       assigned redistribution function includes the effective area,
       an error will be generated.  Use factor_rsp to factor such a
       response matrix into an ARF and a normalized RMF.
   ...

> 
> 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.

Thanks,
-John
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