[Ifeffit] Larch flattening

[Kirill Lomachenko]

Dear Matt,
I have a question and a comment regarding Larch. I would appreciate if 
you could have a look.
1) The flattening algorithm for XAS in Larch  works differently with 
respect to Athena when linear post-edge function is used for 
normalization. In Larch the normalized spectrum is always fitted by a 
parabola for flattening, no matter which function was used for 
normalization. It works fine when the post-edge function is also a 
parabola, but when it is a straight line, the resulting flattened 
spectrum does not stick to Y=1, as it is expected to do (and as it does 
in Athena). My question is: is it a bug or a feature? Just in case, I 
made some minor modifications in the pre_edge.py file that allow to get 
the same results as in Athena when linear post-edge is used. Maybe these 
corrections are a bit clumsy, but it seems to work. If anyone needs, I 
can share.

2) There is a typo in the larch web-manual in the description of the 
nnorm parameter of the pre_edge() function. It is stated that it is the 
number of terms in the fitting polynomial (i.e., 1+degree) whereas it 
seems that it is just its degree. So, nnorm=1 corresponds to a linear 
function and nnorm=2 to quadratic. In the gui help it is partially 
corrected, but the phrase "Default=3 (quadratic)" stays. It is not 
critical at all, but may be misleading for beginners...

Using the possibility, I would like to thank you for all the great work 
you are doing on Larch. It really helps for large datasets!

All the best,
Kirill

-- 
Dr. Kirill A. Lomachenko
Scientist at BM23/ID24 beamlines
European Synchrotron Radiation Facility
71 avenue des Martyrs
CS 40220
38043 Grenoble Cedex 9, France
Tel: +33 438 88 19 14
www.esrf.eu