[Ifeffit] k weight in Athena

[Matt Newville]

Dear Mahendra,

The background removal in Athena(/Ifeffit) uses a Fourier transform
algorithm, and the k-weight in the Athena panel sets the k-weight for
that background-subtraction FT, and so sets how much  to weight the
low- and high-k range of the data during the background subtraction.

This k-weight does not need to be the same as you would normally use
to display or fit the chi(k) data, and is one of the parameters that
can have a big effect on the resulting chi(k) (the main parameter is
Rbkg.  The values for E0, kmin, kmax, and the spline clamps also have
real impacts on the results).

I recommend starting with a kweight of 1 or 2 -- one of those will
probably be fine.  A kweight of 0 is sometimes OK, but is often not so
good, and I've never seen a case where kweight>3 was helpful for
background removal.

Athena gives a great way to play with these background parameters.....
Hope that helps,

--Matt