First of all, thanks to Matt for the info--this seems to me to be a very sensible way for Ifeffit to handle the issue. In terms of Grant's comments, the interesting part to me (and I admit I am getting esoteric here) is that zero-padding has the potential to change the cutoff effects in some circumstances. If you're using a window that goes to zero anyway, then I think it has exactly the effect that Grant describes. But not all windows do this. As a particularly simple example, consider a perfect cosine function over an integer number of periods, with no window. Even though the transform is applied to a finite interval, the result is a spike to within the resolution discussed in Grant's post. (As I recall, the finite sampling also causes the transform to be periodic, but the period is large enough to be disregarded in EXAFS analysis.) But now consider the identical function over the identical interval with zero-padding. This introduces a sharp discontinuity, and causes lots of sidebands on the Fourier transform. Of course, I chose a very special case. Windows reduce this effect considerably, so I would certainly agree that zero-padding does not "botch up" the data. But I wonder if some reduction in sidebands could be achieved by, for example, using a window that goes to the average of the values of the function at the two ends of the interval rather than to 0 (and adjusting the windowing function accordingly). It's on my list of things to try when I get the chance...I don't expect it will make a dramatic difference, but it might make an incremental one. --Scott Calvin Sarah Lawrence College At 12:34 PM 6/8/2004 -0500, Grant wrote:

You might ask whether zero padding botches up the data. It can be shown (I worked it out once) that zero padding is exactly equivalent to an interpolation of the unpadded data witha specific interpolation kernel (a sinc function or something as I recall). What happens is that as you pad with more zeros into a longer array, the sampling in r-space increases, and ultimately the curve looks just like what you would get with a continuous fourier transform (with cutoff effects from the finite k-space window).