[Ifeffit] calibration/alignment

[Matt Newville]

Dear Ornel,

Alignment here means to make sure that the energy scale has not
drifted between different scans.  At many (especially older)
facilities, it is not too unusual to have the apparent energy scale
drift around a bit over time.

Sometimes a reference sample is measured simultaneously specifically
for detecting such drifts.  Generally one asserts that the recorded
energies were offset from one scan to another by a constant amount
that happened between one scan and other (ie, not during a scan).
This assumption might be justified from a mechanical point-of-view,
but is rarely tested in practice.

It is easy for the data processing to simply add a constant to all
energies of one scan until it looks like the energies align with
another scan.  For small shifts (a few eV or so), this is probably
fine.  Consequently, this easy procedure is commonly done even though
the underlying assumptions are rarely tested and the procedure is
loses accuracy for large shifts.

If you don't think you need to do this, then you're probably right.
The corollary is that if you do think you need to do this all the
time, and that the energy shift is larger than a few eV, then there is
probably something wrong, and you should look carefully at whether
"shift by a constant energy" is even reasonable.

If you're just getting started, I would say to not worry about energy
alignment until it becomes an obvious problem.

--Matt


On Thu, Oct 14, 2010 at 4:46 AM, ornella smila castro
<sornella at hotmail.com> wrote:
> Hi everyone,
>
> I am trying to do some data processing with Athena but I am already stuck at
> the first step. The thing is: I read the worked example section of the
> "Athena's user guide" and on the example on the iron foil, it is mentioned
> to calibrate the data at the right energy (until here evrything is fine) but
> then it is said to align the data. Can anyone explain to me what does
> "alignment" exactly means, and what is the aim of "aligning the data".   The
> data that I have collected were through a channel through which a solution
> flow (~200 microliters/hr) so I am not convinced that alignment makes
> sense.
>
> Many thanks,
> Ornel
>
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