[Ifeffit] continuous scan data
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
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
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
Code 6134, Building 3, Room 222
Naval Research Laboratory phone: (1) 202 767 5947
Washington DC 20375, USA fax: (1) 202 767 1697
NRL Synchrotron Radiation Consortium (NRL-SRC)
Beamlines X11a, X11b, X23b, X24c, U4b
National Synchrotron Light Source
Brookhaven National Laboratory, Upton, NY 11973
My homepage: http://feff.phys.washington.edu/~ravel
EXAFS software: http://feff.phys.washington.edu/~ravel/software/exafs/
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