Hmm. Another possible way to do it is to delete the bad data points and then do a
"slow" FT, which would be a fit
of the data, at the points given with no interpolation, to a sum of sines and
cosines. This would have the
nice feature of using the data as it is and ignoring the bad stuff. Filtering
would involve multiplying the
sines and cosines by some window function (in R space) and evaluating them *at
the given k-points*, not on a regular
grid. This of course means that evaluating FEFF paths and the like is likely
to be slow because you don't get to
use recursion relations to evaluate sin(2*R*k(i)+delta) as you would if k(i)
are uniformly tabulated. Now that computers are
a bazillion times faster than they were when EXAFS analysis traditions were
established, maybe that's the way to go. What do you think?
mam
On 12/12/2012 2:16 PM, Bruce Ravel wrote:
On Wednesday, December 12, 2012 11:09:29 PM Dr. Dariusz A. Zając wrote:
coming back to the problem with Bragg peaks, does anyone knows if it is
possible to analyse EXAFS with more than 1 interest regions? like 2 or
more FT windows - before, between and after Bragg peaks, or is possible
to introduce one own FT window function in Athena?
Interesting question. Some days I really love this mailing list.
A similar question came up recently on the mailing list, although I
think here you are asking whether regions can be chosen in k-space.
(The previous discussion was about regions in R space.)
I am commenting off the top of my head here, so I may be neglecting
something. But I think what you are suggesting to have a windowing
function that goes up, then down, then up again, then down again. I
think the problem with that is that it would introduce low frequency
components into the transformed data. Granted, the transform of the
theory would have the same effect, but I worry about how that kind of
windowing function would be correlated with the parameters of the fit.
As Scott suggested in the earlier conversation, another option is to
do a multiple data set fit where the data is imported twice. For one
instance of the data, the FT window is set over the first range. For
the other instance, the window is set over the second range. Again, I
worry about how the choice of windows may be correlated with the
parameters of the fit, given the limited information content used in
each part of the MDS fit.
I don't plan on exploring this myself. But if someone wanted to do so
and attempt to convince me that it should be implemented in the
software, I am -- as always -- willing to listen.
Cheers,
B
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