[Ifeffit] Some more questions

Bruce Ravel bravel at anl.gov
Thu Mar 22 17:48:02 CDT 2007

On Thursday 22 March 2007 17:18, Shantanu Behera wrote:
> 1. There is a k-weight button in the background removal column in
> Athena. How's that used and what's its significance?

The background removal algorithm involves a Fourier transform to
R-space in an effort to optimize the low-frequency components by
varying the parameters of the spline.  The k-weight you are asking
about is the k-weight used in that FT.

Setting that k-weight to 2 or 3 can sometimes help with data that has
very little or no signal at high k by penalizing the spline for not
staying close to the data.  A word of caution, though -- high
k-weight values for the background removal can have surprising
interactions with large values of the high-end spline clamp.

> 2. Assume that I have collected data up to 14k, and I see that the
> signal beyond 12k is very noisy. I want to use the data up to 12k and
> within that data range pick a F-filter window of 2-10k or 3-10k. How
> do I go about this? Can I do  that with the spline range (in k or E)
> options in bkg removal (Athena) column?

You can restrict the range of the background removal then Fourier
transform over the entire remaining range.  Or you can remove the
background over the entire data range and restrict the FT using the FT
kmax parameter.  In principle, those should be equivalent although the
latter seems like a better and more flexible solution.  In practice,
real data sometimes works better with a restricted background removal
range.  The only way to know is to try both and see.  Hopefully,
Athena does a good job of allowing you to try looking at your data in
several different ways quickly and efficiently.


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

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

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