[Ifeffit] Re: How to change the spline knots in Athena

[Bruce Ravel]

On Tuesday 21 February 2006 03:08, you wrote:
> I am using Athena (0.8.048) software for XAFS analysis.
> It is greatly helpful to me for XAFS characterization.
>
> I have a question on the background estimation.
> Spline knots is displayed by "About current group" menu.
> Could you tell me how to change the spline knots, if possible.
> Changing the knots is useful to suppress the long-range variation.

This is the kind of question that is better sent to the mailing list, so I am 
CCing my answer there.

The number of knots is determined by the values of the k-range of the spline 
fit and by the value of Rbkg.  The number of knots is delta_k * Rbkg * 2 / pi 
and the knots are evenly spaced in k.  Ifeffit (and so Athena) does not allow 
you to move the knots to different locations nor does it allow you add or 
remove knots.  The AUTOBK algorithm works by making a distinction between 
data and background in terms of Fourier components.  Consequently it *cannot* 
allow you to move the knots or use more knots, since doing so would change 
the Fourier components included in the background spline.

B

-- 
 Bruce Ravel  ---------------------------------------------- bravel at anl.gov

 Molecular Environmental Science Group, Building 203, Room E-165
 MRCAT, Sector 10, Advanced Photon Source, Building 433, Room B007

 Argonne National Laboratory         phone and voice mail: (1) 630 252 5033
 Argonne IL 60439, USA                                fax: (1) 630 252 9793

 My homepage:    http://cars9.uchicago.edu/~ravel 
 EXAFS software: http://cars9.uchicago.edu/~ravel/software/