[Ifeffit] Rebinning

[Carlo U. Segre]

Matt:

Let me see if I can explain my reasoning adequately

The reason we want to rebin continuous scan data is because in our
continuous scan, we oversample the data significantly so reducing it to a
step size in k of 0.05 really costs nothing.  With the rebinning, it is
possible to make use of the statistical weight of all the data.  A simnple
interpolation would throw this away.  I know that you have a spline fit in
IFEFFIT and perhaps that is an equivalent method to make use of all the
counts in the data but this is slightly different.  We have been using
this for a while with MR-CAT data and it seems to work well for us.

Yes, for this algorithm, we select an E0 and a distance above that at
which to begin rebinning.  You could just as well just set an energy at
which to begin rebinning but the conventional idea of using energies
relative to E0 make sense to me.  The bin size just starts at this
E0+Offset, whatever it is.  There is no regard for an even value in
k-space since the data needs to be kept in terms of energy anyway.  (If
you have suggestions about this, please let me know).

If E0 changes, interpolation will be required for analysis but this is
something that has to be done anyway when summing multiple spectra where
there could be a small shift in E0 from one to the next.  We just have to
live with that, I suppose.  This code fragment is also incorporated in a
bigger "filter" that Ken has written which fits multiple spectra to each
other to determine shifts, then interpolates the spectra to a common grid,
sums or averages them and then performs the rebinning on the final
product.  We plan to use this in a GUI to assess the quality of data as we
take it in order to figure out when to stop collecting on a sample.

I suppose that it is possible to do what you suggest in "2" below but the
boxcar gives a better representation of where the center of mass of the
actual data is.  As I mentioned before, the goal is not to get data on an
even grid but just to remove the oversampling without losing statistics.
Interpolation to an even grid is left for later, in the analysis software.


Typically, in a step scan you would set the delta-k to some value like
0.05 or more so your original data would be no better than the rebinned
data.  If it is important to have smaller steps then perhaps the default
should be smaller than 0.05k?  The way I see it, we have not yet pushed
the data collection as far as we will eventually go.  Right now we have an
EPICS limitation of no more than 4000 data points in a scan but once we
break past this limit, I expect that there is plenty of flux at APS to
take data even more densely.

Your question about weighted averaging is a good one.  I would have to
think about the convolution a bit more.  What do others think?

Carlo

On Tue, 11 Mar 2003, Matt Newville wrote:

>Hi Carlo,
>
>Thanks!!  That's wonderful.  It seems like this should be an
>option in spline() and bkg_cl(), and possibly in pre_edge().  I
>admit to being still a little confused by the goal of the
>rebinning, especially with respect to loss of resolution.  It
>seems like to use this script, you need to do two things:
>  1. select an E0.   The data will be put at energies that
>     will give an even k-grid with this e0.  I think it's
>     inevitable that if e0 changes, you might want to re-bin.
>
>  2. assume that the k-grid is small enough that the
>     measurements at independent energies are within the
>     acceptable energy resolution that they can be
>     simply summed.
>
>Is that right?
>
>Would it be OK with you (and everyone else) to have a weighted
>average replace the boxcar average?  For example, at k=12,
>[E(k+0.05) - E(k)] = 5eV, which is larger than normal energy
>resolutions, so the boxcar average might wash out the high-k
>EXAFS, no?  It might be better to convolve the spectra with a
>lorenztian reflecting the incident energy resolution (probably
>defaulting to 1eV).  Does that seem OK or is there something eles
>going on?
>
>Anyway, thanks!!
>
>--Matt
>
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-- 
Carlo U. Segre -- Professor of Physics
Associate Dean for Research, Armour College
Illinois Institute of Technology
Voice: 312.567.3498            Fax: 312.567.3494
Carlo.Segre at iit.edu    http://www.iit.edu/~segre