The value of years: good stories and advice from M. H. J. Koch

Kochpaper

One paper I managed to miss for my review paper on data corrections is a paper by M. H. J. Koch from Hamburg. The paper is written in a nice informal way, replete with good quotes, where he talks about his experiences in instrument development for SAXS and WAXS beamlines. In particular the paper details the development of delay line (wire) detectors, and may form a good introduction into this topic. It seems that wire detectors may still have their uses: their rapid response to incoming photons still puts them among the fastest 2D detectors out there. Read More

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Everybody hates statistics…

Everybody hates statistics [1]…

 

… but it can be of major importance in our small angle world. While very few papers on small-angle scattering discuss statistics, they can tell you whether your observations are real or just imaginary. In addition, statistics will let you know whether you have been able to describe your scattering pattern with your model or not. All in all, nice to have.

 

I will briefly discuss two statistical concepts which could be of great use, as they have been for me. While I never really could understand all the concepts during statistics lectures at university (a situation which may sound familiar), I can try to explain some simple concepts. By the by, if you (dear reader) are a statistician, I would be happy to get in touch with you. The first concept is the most straightforward, and involves the uncertainties on fitting parameters. Secondly I will discuss statistics on collected intensity and how to retrieve them for a variety of detectors.

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Baby, and the perfect measurement made easier

I apologise for my low activity in the last few weeks. Those of you who are following my Twats (Twitter messages) will probably be able to link this to our newborn baby. Now growing strongly for two weeks, having to wake up every four hours in what seems like the longest beamtime ever have left me feeling like a pinball at night.

Despite that, the research continues, and I am happy to say that getting the Perfect measurement (or an approximation thereof at least, as described in the document of a few posts ago) will become maybe a little easier with some new software I wrote. The software draft is written in Matlab, but I am trying to learn Python to recode the essentials in free software (interested in helping? Let me know!).

Why all this focus on the perfect measurement, you may ask? Well, it turns out that for most of the SAXS analyses to give you the correct answer (as opposed to just an answer), your data should be correct to within 1%. Most beamlines will not give you this accuracy out of the box (and some beamlines have more problems that will come to light if you run a standard sample), and thus advanced data correction is necessary.

This software will, after entering the right information, do most of the corrections necessary to get good data. This means background subtraction, corrections for flatfield, distortion, spherical correction, polarisation and darkcurrent corrections, and corrections for variables such as incoming flux, transmission and scaling to absolute intensity. Furthermore, it will do error calculation and propagation and integration/binning of the data. The end result is ascii file with columns for q, I and error in I, ready for fitting in one of the existing software packages. For anisotropic patterns, a 2D binning method will be implemented shortly, until then the corrected images can be used.

So that was just to wet your appetite, more in the near future after more testing and debugging. Bye!

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Work in Progress: How to do a SAXS measurement well

Hi all.

For a work-related project, I have been developing some software and writing some documentation. This documentation is still a work in progress (as is the software), and it lacks much graphics. However, I think it could serve as a good introduction to those who want to do a good SAXS measurement, or those who will join on an expedition to a SAXS beamline. Let me know what you think! The appendix is a chapter from my thesis.

imp_userguide

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