[Ifeffit] continuous scan data

Bruce Ravel ravel at phys.washington.edu
Wed Jul 17 08:17:51 CDT 2002

 CS> First of all, thanks!  I just put athena and ifeffit on our
 CS> beamline computer at MR-CAT for our users to assess data as they
 CS> take it.  It proved to be very easy for the students and newbies
 CS> to figure out.

That's just splendid.  I am very pleased and quite flattered by the
praise.  Thanks!

If there are features of the program the MR-CAT and its users would
like to see, please let me know.

 CS> The question that I have is how does IFEFFIT handle continuous
 CS> scan data, where the point spacing is almost but not quite
 CS> uniform in energy all the way through the scan.

Well, I am sure that Matt will correct me if I am wrong, but I think I
can answer this.  mu(E) data is almost never on an even grid,
regardless of how it is measured.  The background function is
evaluated on the energy array of the data using knots that are evenly
spaced in wavenumber.  When the background is removed, chi(E) -- which
is on the original energy grid -- is interpolated onto an even grid in

 CS> I have noticed the error of misordered data which can often show
 CS> up in the continuous scans.  We will probably try to fix that
 CS> ourselves.

Misordered data is handled in a sensible manner in recent versions of
athena.  It may even be handled correctly ;-)

 CS> What is done with the high density data when converting to
 CS> k-space?  Do you rebin (averaging both E and mu data) or do you
 CS> use a smoothing fit to take advantage of the statistics present
 CS> in the excess data points, or do you just interpolate and throw
 CS> away the extra statistics?

I think I answered this above.  I suppose you might say that ifeffit
"interpolates and throws", but even that depends on what advantage you
claim to be getting by measuring on a finer grid.  There are physical
limits to the resolution, so in that sense a finer grid does not help.
However, mesuring for one second per point on, say, a 0.25 eV grid is
similar in a counting statistics sense to two measurements of one
second per point on a 0.5 eV grid.  That counting statistics
improvement is not lost in the interpolation of chi(E) to chi(k).

Or, perhaps, I'm missing your point entirely... That happens ;-)


 Bruce Ravel  ----------------------------------- ravel at phys.washington.edu
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 Naval Research Laboratory                          phone: (1) 202 767 5947
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 NRL Synchrotron Radiation Consortium (NRL-SRC)
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 National Synchrotron Light Source
 Brookhaven National Laboratory, Upton, NY 11973

 My homepage:    http://feff.phys.washington.edu/~ravel 
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