Dear all
Rebinning is always fraught; I would normally not allow a student to do it...
You might be interested in our paper [Schalken, M et al., JSR, 2018] which explicitly avoids rebinning and propagates uncertainty to ifeffit-like [Matt modified] analysis, efeffit.
This was an outcome from Q2XAFS.
We also include code in supplementary information.
Note that this also discusses interpolation and how to preserve information content if that is needed.
Best wishes
Chris
PS looking forward to great discussions at Poland
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Christopher Chantler, Professor, FAIP, Fellow American Physical Society
Editor-in-Chief, Radiation Physics and Chemistry
Chair, International IUCr Commission on XAFS CIT, CCN
President, International Radiation Physics Society
School of Physics, University of Melbourne
Parkville Victoria 3010 Australia
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From: Ifeffit
Yes, we measure fast and have taken as many as 20000 points. The problem is not in the shifts that you mention. This is normal and expected. the problem is specificallly in the rebinning algorithm in Demeter. It seems to be different than the one in the old Horae package. I have done a test of this and I attache a coule of figures that show the difference.
I have used 10 continuous scans for this test. The data were taken at the MRCAT beamline, Sector 10 at the APS. The data are for the Fe K-edge and there are about 3400 points per scan with a point density of about 0.35 eV/step. I used both versions of Athena and performed the following steps to give the data groups shown in the plots
new_athena.png Fe_new_rebin_merge - (blue) all 10 scans rebinned at input and then merged Fe_new_merge - (red) all 10 scans merged only Fe_new_merge_rebin - (green) all 10 scans merged then rebinned
old_athena.png Fe_old_rebin_merge - (blue) all 10 scans rebinned at input and then merged Fe_old_merge - (red) all 10 scans merged only Fe_old_merge_rebin - (green) all 10 scans merged then rebinned
comp_athena.png Fe_old_rebin_merge - (blue) Fe_new_rebin_merge - (red)
It is clear that the new Athena (Demeter) is not rebinning the same way as the old one (Horae). The contrast is particularly evident with the last plot. The new rebinning algorithm is introducing more noise. For the moment, I recommend only merging and perhaps smoothing if you can tolerate a bit of amplitude reduction.
I have been thinking that it might even be better to have the data acquisition software do the rebinning on the fly so the data does not have to be manipulated in Athena. I am not sure if this is a good idea yet but I think it would help my users.
Carlo
On Wed, 27 Jun 2018, Edmund Welter wrote:
Dear Carlo,
do you also measure as fast as possible in the sense that for two consecutive scans the points on the energy axis are not at the same positions? This is what happens at my beamline. The differences are typically very small but there are differences and one should not just add all the first points and all the second points and so on because they are not necessarily exactly at the same energy. Sometimes the beamline computer is doing something else in parallel (whatever that might be) and the distance between points A and B is significantly larger than the distance between B and C.
So, the problem is, at which point does it make sense to merge several spectra of the same sample? I presume that Athena is taking care of this when I use it to merge spectra, but it can only do so by interpolating the points in the spectrum onto a common grid before summing up the spectra.
The best solution might be to rebin/interpolate the spectra onto a fixed grid before they are imported into Athena (or any other program), depends on what Athena is exactly doing when it is rebinning data.
Another aspect is that Athena is not very happy about 8600 points/spectrum anyway, at least as long as it using Ifeffit.
Cheers,
Edmund
On 27.06.2018 15:14, Carlo Segre wrote:
Hi Ilya:
We always take data in this mode at APS Sector 10 and I have also find
the rebinning function is not working satisfactorily at this time. I find that for the current version of the software it is better to merge your data and let IFEFFIT interpolate to the dk=0.05 grid that it uses.
Carlo
On Wed, 27 Jun 2018, Ilya Sinev wrote:
Hi all,
I have a question regarding the chi(k) function isolation and rebinning processes. I have some data recorded in ?quasi channel-cut? modus, i.e. with the mono constantly moving and the data points collected with the highest possible rate. With 180 sec measurement in yields to a spectrum of ca. 8600 point, which obviously needs to be rebinned. The rebinned data, however, does not look good in k-space even if multiple data are merged. Moreover, I have an impression that the raw spectrum in k-space does not have those 8000+ points anymore but significantly less. Is there any reduction of
that the
data points number that is not seen (e.g. as a preparation step for FT)? Since the unbinned data has higher quality, does it then make more sense to keep using it for EXAFS analysis?
Thank you
Ilya Sinev
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