back_fun problems

Hi - 

I'm trying to create a model background for HRC-S for simulation
purposes. So I used tg_bkg (a ciao contrib script) and dmtype2split (a
ciao tool) to create PHA Type I files of the summed _up, _down
background from the default "bow-tie" LETGS extractions. These have a
constant ratio of background width to source width of 10, so the
BACKSCALE is 10, and the counts are scaled by 1/BACKSCALE when loaded.

So I then load the background spectrum for -1 order, and fit a
poly(1)+gauss(1) (and no responses assigned).  So the resulting model
defines the counts/s/A/area.  (where "area" means spectral region
width of 1 unit).  This model gives a reasonable reproduction of the
LETG/HRC-S background.

If I then try to use this model as a back_fun via:

  back_fun( 1, "get_data_exposure(1)*bin_width(1)*( poly(1)+gauss(1) )" ) ; 

which is what I think I need according to the B(h) term in eq. 7.35
of the ISIS manual:   s * A * (counts/s/A) = counts/bin.


To verify, I define a dummy model with zero normalization:

  fit_fun("Powerlaw(1)");
  set_par( "Powerlaw(1).norm", 0.0, 0, 0, 1);

I should recover the background model when I eval_counts.
I get the right shape, but it seems to be a factor of 8 too low.
(the data_backscale is 1.0).

This factor is supiciously 1 / bin_width / back_backscale ( 1./ 0.0125 / 10 ).

Am I misinterpreting some scaling in fitting the background pha and/or
in defining the background model?    I can renormalize this to derive
an empirical background model, but I'd like to understand the
discrepancy. 

Thanks,

-- Dave

David Huenemoerder  617-253-4283 (o); -253-8084 (f); http://space.mit.edu.ezproxyberklee.flo.org/home/dph
MIT Kavli Institute for Astrophysics and Space Research
70 Vassar St., NE80-6065,
Cambridge, MA  02139
[Admin. Asst.: Elaine Tirrell, 617-253-7480, egt_at_email.domain.hidden
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