[Ifeffit] Rbkg value in Athena

[Matt Newville]

Ann,

My generic recommendations are to start with
   Rbkg = 1.0
   k-weight = 1
   Spline range:  k = 0  to whatever
 
in Athea's 'Background removal' section -- and take 
Athena's defaults for everything else.

> I am a little bit confused on what the Rbkg value should be when
> I do the background subtraction.  I have read some previous
> posts which stated that Rbkg can be determined after the fit.  

Well, Rbkg should be significantly below  the first shell 
distance.  If the fist neighbor distance is much below 2.0Ang, 
it might be important to re-try the background subtraction with 
a smaller Rbkg.

> However, I always think that the fit depends strongly on how the
> background being subtracted.  

It shouldn't.  Are you finding something different?

> How does the initial spline value affect the background
> subtraction?  Should it be 1eV or 25 eV above the edge?  I
> appreciate your help.

I'm not sure I understand what you mean by 'the initial spline
value'.  If you're having difficulty with background subtraction,
I recommend starting with the above values then trying these
approaches (in roughly this order):
  1. increase the high 'Spline clamp' 
  2. increase the low end of the k-range from 0 up to 0.5 or 1.0
  3. try k-weight of 0, 1, and 2.
  4. alter e0 by up to  +/- 5eV (maybe more for heavy elements).

Shelly also wrote up some useful suggestions at:
   http://cars9.uchicago.edu/cgi-bin/ifeffit/faqwiz?req=show&file=faq05.008.htp

Hope that's a good start,

--Matt