Re: using fakeit

On Tue, Aug 18, 2009 at 18:31 -0400, Herbert Pablo wrote:
>
> Hey all,
>
> so the default on fakeit is to load in an arf and an rmf and then a function.
> I have 2 questions.
>
> 1.) when i use fakeit (with a  function and parameters that I already used to
> fit data) nothing comes out. I just get a straight line. I'll put the code
> below:
>
> load_arf("/export/lecanard/6601/combined/6601_HEG_1.arf");
> load_rmf("/export/lecanard/6601/rmf/acis_1.0_HEG_-1_rmf.fits");
> assign_rmf(1,1);
> assign_arf(1,1);
>
> require("xspec");
> fit_fun("phabs(1)*pcfabs(2)*diskbb(3)");
>
> set_par("phabs(1)", [2.683428 ]);
> set_par("pcfabs(2)",[2.711441 ,0.6840924]);
> set_par("diskbb(3)",[52.20151 ,1.897251]);
>
> fakeit;
> plot_unit("kev");
> plot_data_counts(1);

Hi Herbert,

This sequence works for me.  I loaded the MEG arf and rmf that
are packaged in the isis distribution (see isis/test/data), and
then did a cut-and-paste of the rest of your script, and got a
reasonable looking fake spectrum.  This worked both in
isis-1.4.9-51 and in the current release, isis-1.5.0.

What version of isis are you using?

>
>
> 2.) Eventually what I would like to do is just take the model that I had
> before and extend it to cover wavelengths that my data can't see...(up to
> about 25 keV).  However, if I put in an arf I'll just be cutting the effective
> area away to match what I already have. Can fakeit do this or should I look
> for other avenues?
>

If you just want to evaluate the model, you can use eval_fun
and provide an arbitrary grid:

   (lo, hi) = linear_grid (ylo, yhi, num_bins);
   f = eval_fun (lo, hi);

If the model should be computed beyond the ARF grid during the
spectral fit, you can use 'set_eval_grid_method' to do that.
For details, see the help page for set_eval_grid_method,
especially the bit about 'USER_GRID'.

Thanks,
-John

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