[Ifeffit] Purple & Red E Derivatives

[Bruce Ravel]

On Tuesday, February 22, 2011 05:03:02 pm Andrew Korinda wrote:
> While working in Athena today I noticed something peculiar while
> looking at the derivative plots of mu(E) data. When I plot the current
> group (red button) the plot can be spiky. However, when I plot as
> marked group (purple button) everything is much smoother and more
> rounded. Smoothed data will do the same thing. It always happens
> whether I have mu(E) or norm(E) selected. I have never noticed a
> difference in how any of the other plots current when switching
> between marked and active plotting. I didn't see anything in the
> documentation about how the derivatives are calculated so I'm turning
> to all of you. Have any of you noticed this? Does anyone know if there
> is a difference in how the derivatives are calculated in each plotting
> function?

Hi Andy,

You have uncovered a very interesting mistake that I have never
noticed before.  First some background info.

There is a preference that defined how many smoothing iterations are
made when plotting a derivative spectrum.  If you edit the preferences
(Settings menu -> Edit preference), then click down to the plot ->
smoothderiv parameter, you will see that it is set to 3.  (Well, by
default it should be, it could be different on your computer.)

The plot->smoothderiv parameter defines the munber of smoothing
iterations before plotting a derivative spectrum.  If you set this to
0, then your data will not be smoothed.  The default value of three
means to run your data through the smoothing filter 3 times before
plotting.

When plotting with the purple button, the derivative spectrum is
computed then smoothed.  When plotting with the red button the mu(E)
spectrum is smoothed then the derivative is computed.  In an example
that I just looked at, I can see a difference similar to what you
describe due to the order of operation.

The behavior of the red button does not seem right to me.  I don't
recall a reason why I made these behave differently.  It seems to be a
mistake that no one has noticed and reported to me until now.

Good eye!

I'll put it on my list of things to fix.

B

P.S. The order of operations is clearly documented in the Ifeffit
buffer, which you can find in the Edit menu.  The numerical derivative
is computed as the numerical differential of the mu(E) spectrum
divided by the numerical differential of the energy array.  Smoothing
happens as explained above.  Justw atch the lines that get printed to
the buffer when you click the plot buttons.

-- 

 Bruce Ravel  ------------------------------------ bravel at bnl.gov

 National Institute of Standards and Technology
 Synchrotron Methods Group at NSLS --- Beamlines U7A, X24A, X23A2
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