[Ifeffit] Bug in Athena?

[Matt Newville]

Hi Scott


On Thu, Nov 19, 2009 at 9:50 AM, Scott Calvin <SCalvin at slc.edu> wrote:
> Hi Matt,
>
> I'm the one who requested the merged reference channel.
>
> If the data is ideal, of course only one reference scan is needed. But there
> are two common ways it can be nonideal that are relevant:
>
> 1) The monochromator does not hold calibration; i.e. there is an energy
> shift between scans
>
> 2) The reference channel is very noisy, perhaps because of an inherently
> thick sample
>
> If  1) is a significant problem and 2) is not, then it makes sense to align
> the scans using the reference, at which point any reference scan will do for
> determination of the chemical shift of the merged data from the sample.
>
> If 2) is a significant problem and 1) is not, then it makes sense to merge
> the references along with the sample data, because that will make it easier
> to determine the chemical shift.

For this case, wouldn't it be better to measure the reference
separately to determine the chemical shift, and not rely on the
reference channel for this purpose?

How often is the reference channel both noisy AND improved by merging?
 That would imply a transmission measurement
that was poor due to low flux.  But if this is because the sample is
thick as you suggest, the x-rays hitting the reference could be
dominated by harmonics, and the reference data may just be bad, not
noisy due to counting statistics.

--Matt

PS: I think that means I agree with Anatoly!