Hi Regina, It uses R = [15, 25] Ang by default, though these can be changed with the "rwgt1" and "rwgt2" parameters to the chi_noise() command.
The normalization definitions can be confusing for discrete transforms and also given that for the XAFS sin(2kR) one could either choose conjugate variable pairs of 2k and R or k and 2R (ifeffit uses the latter). See http://cars9.uchicago.edu/~ifeffit/refman/node141.html for more details. Using the chi_noise() command, you will typically see small influences on the exact value of the estimated uncertainty in chi(k) as you change the FT k-ranges and especially k-weight. Typically (or at least, when I look) the order of magnitude (and even the first significant digit) is generallly preserved. To be honest, reading further than 1 significant digit of an automated estimation of noise based on an assumption of white noise may not be very robust. Hope that helps, --Matt On Thu, Feb 17, 2011 at 8:23 AM, <kirshst...@googlemail.com> wrote: > Dear Matt and dear other readers, > > I tested the method Matt suggested to make Athena put out the epsilon_k > value for my data. > As of itself, the methods works (I get a value), but this value is similar > but not exactly the same value as I obtain from calculating by myself > according to parseval's theorem. > Which makes me think that the small discrepancy might be due to the fact > that Athena considers a different R-range from what I used (all other > parameters being the same)? > I used R 15 to 30 Å. Does someone know what R-range Athena uses? > Also, just to be sure: so the normalization used for FFT by Athena is > sqrt(delta_k / Pi)? > > thank you for your kind help, > > regina > > > On Fri, Feb 4, 2011 at 8:34 PM, Matt Newville <newvi...@cars.uchicago.edu> > wrote: >> >> Regina, >> >> On Fri, Feb 4, 2011 at 10:02 AM, <kirshst...@googlemail.com> wrote: >> > Hello, >> > >> > I would like to use parseval's theorem (as described in a document >> > called: >> > Error Reporting Recommendations: A Report of the Standards and Criteria >> > Committee, Adopted by the IXS Standards and Criteria Committee July 26, >> > 2000 >> > ) to quantify the statistical noise in my spectra. >> > >> > In section 3 (as shown in the attachment), it says that for the formula >> > to >> > work, the forward FT has to be normalized by sqrt(deltak/Pi). (delta k >> > is >> > the spacing of points in k space) >> > >> > What is the normalization used for FFT in athena? if it is not >> > sqrt(deltak/Pi), what is it? how does the parseval theorem formula need >> > to >> > be modified as a function of FT normalization? >> > >> > Many thanks for answers! >> > >> > Regina >> >> It's even easier than that. Ifeffit / Artemis do (and Athena can) >> report the epsilon_k and epsilon_r as defined in that report. The >> 2000 report actually codified work done with the earlier feffit code >> to estimate the noise in the data in both k and R space. Again, >> Artemis does this automatically, and you can see the values for >> epsilon_k and epsilon_r when looking at fit results. Athena >> doesn't directly do this calculation, but if you open the Ifeffit >> Buffer (Edit -> Display Ifeffit Buffer), you can do the following: >> >> 1. choose FFT parameters in the Forward Fourier transform part of the >> main Athena window. >> >> 2. Hit the [R] Plot button for the group of data you're looking at to >> make sure the FFT parameters are up to date. >> >> 3. Open up the Ifeffit Buffer and look for the "Group Name" (Athena >> uses a 4 letter sequence which looks random). You'll see something >> like >> newplot(jukc.k, "(1*jukc.chi*jukc.k^2)+0.0000", ....) >> >> All you're looking for is the 4-letter "Group Name" / prefix for the >> data set: in this case "jukc" >> >> 4. In the Ifeffit> input line at the bottom of the buffer, type >> chi_noise(jukc.chi) >> 5. Then type >> show epsilon_k, epsilon_r >> >> This will show the values estimated for the noise in chi(k) and chi(R) >> using the FFT parameters you input. For most data and FFT parameters, >> epsilon_k should not change significantly, though epsilon_r will >> change significantly with the k-weight. >> >> Cheers, >> >> --Matt >> _______________________________________________ >> Ifeffit mailing list >> Ifeffit@millenia.cars.aps.anl.gov >> http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit > > > _______________________________________________ > Ifeffit mailing list > Ifeffit@millenia.cars.aps.anl.gov > http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit > > _______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit