[Ifeffit] What differences are there between Athena and AUTOBK backgrounds?

Matt Newville newville at cars.uchicago.edu
Tue Aug 5 22:22:50 CDT 2003


Scott,

There are some differences between autobk 2.61 and Ifeffit's
spline() command.  I consider autobk 2.61 to be pretty old (it's
from 1995):  I found a version 2.63 (1996) this evening:  I can
probably find older versions, but I don't have complete records of
all changes to these codes (autobk 2.0 and feffit 2.0 were from
1993, and there were 1.0 versions!).  I do fondly remember autobk
2.0 and feffit2.05 (the first to really do multiple-data-sets!) but
I'm pretty sure I wouldn't want to run them.

Anyway, enough nostalgia: the main differences between Ifeffit and
autobk earlier than 2.92 (year 2000, at which point autobk became
part of ifeffit) are:
 1. fixed an 'off by k=0.05Ang (1 grid point)' error.
 2. changes in E->k interpolation schemes: now using the
    'rolling boxcar' average discussd here earlier this year.
 3. switch to fully double precision.

Minor changes were made in other pieces: methods for choosing the
spline knots, initial guesses for the free parameters, the actual
spline evaluation, Fourier transforms, and fitting.  There are some
differences in normalization, too.  I think none of these are
significant changes, but the 'noise level' is pretty high for
background subtraction, making it hard to tell if a change make
things better or worse.

You might look if there is an 'off by one grid point' difference
between the chi(k) spectra.

--Matt



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