Re: [Ifeffit] Ifeffit Digest, Vol 162, Issue 17

Topic 3: Incorrect dark current (offset) is just one of many ways that glitches can escape being normalized out, and one of the few you can correct in software. Others include but are not limited to 1. Any detection-system nonlinearity in I0 or detected channel (F or T). This includes deadtime in fluorescence detectors. 2. Inadequate detection speed in QXAS so that the recording of I0 lags behind that of fluorescence (or vice versa if using a Lytle detector or similar). 3. Harmonic/stray beam content varies while going through the glitch. 4. Beam position or shape varies while going through the glitch. If the crystals are in a diverging beam, then each part of the crystal sees a slightly different incidence angle, so the exact energy at which the two reflections contributing to the glitch both satisfy the Bragg condition varies across the crystal surface, resulting in non-uniform reflection. There are undoubtely many more. mam On 8/12/2016 2:36 PM, Christopher Thomas Chantler wrote:

Topic 1: We have a routine /edit within ifeffit (our modified version) which propagates and fits uncertainty. Working on a couple of minor details before passing it on to Matt and Bruce for general use.

Topic 3: Monochromator glitches should be eliminated in absorption spectra under normal circumstances if the dark current estimation is well defined. I recommend careful measurement and recording of this for all spectra.

------------------------------------------------------------ Christopher Chantler, Professor, FAIP Editor-in-Chief, Radiation Physics and Chemistry Chair, International IUCr Commission on XAFS President, International Radiation Physics Society School of Physics, University of Melbourne Parkville Victoria 3010 Australia +61-3-83445437 FAX +61-3-93474783 chantler@unimelb.edu.au chantler@me.com http://optics.ph.unimelb.edu.au/~chantler/xrayopt/xrayopt.html http://optics.ph.unimelb.edu.au/~chantler/home.html

________________________________________ From: Ifeffit [ifeffit-bounces@millenia.cars.aps.anl.gov] on behalf of ifeffit-request@millenia.cars.aps.anl.gov [ifeffit-request@millenia.cars.aps.anl.gov] Sent: Friday, 12 August 2016 11:22 PM To: ifeffit@millenia.cars.aps.anl.gov Subject: Ifeffit Digest, Vol 162, Issue 17

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Today's Topics:

1. Statistical errors in linear combination fits (Joshua Kas) 2. Re: Statistical errors in linear combination fits (Bruce Ravel) 3. Athena interpolation when removing mono glitches (Michael Gaultois) 4. Re: Athena interpolation when removing mono glitches (Bruce Ravel)

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Message: 1 Date: Thu, 11 Aug 2016 13:31:45 -0700 From: Joshua Kas To: ifeffit@millenia.cars.aps.anl.gov Subject: [Ifeffit] Statistical errors in linear combination fits Message-ID: Content-Type: text/plain; charset="utf-8"

Hi All, I was wondering if it is possible to pass a value for the uncertainty in the data to Athena when doing linear combination fits. We have calculated chi^2 values using a simple estimate of the statistical uncertainty, and find that our values differ by several factors of 10 when compared to the reported values from Athena. I assume that this has to do with value of the uncertainty that Athena is using, but I certainly could be mistaken. In any case, the reduced chi^2 reported by Athena is much smaller than 1, while the fit is off by fairly large amounts compared to any reasonable estimate of the statistical error. Thanks in advance for any help and sorry if this info is already available in the archives, Josh Kas