Are you using the SAXS constraints, with rot1, rot2, rot3 fixed ?
https://github.com/silx-kit/pyFAI/blob/30cfe1d78ebe2325dc46164503d35b35d1252266/src/pyFAI/gui/tasks/GeometryTask.py#L661
In the pyFAI geometry model, the poni and rot1 + rot2 parameters are strongly
correlated at small angles. If the detector tilts are fixed, the poni (= beam
center) and the distance should stabilize.
Best,
Jon
On 17/03/2026 05:14, "Time" (via pyfai Mailing List) wrote:
> Dear pyFAI Developers,
>
> I hope this email finds you well.
>
> My name is Linjun He, and I am a graduate student working on SAXS-related
> scattering data calibration and detector geometry analysis. During our use of
> pyFAI for calibrating a large number of experimental datasets, we observed a
> certain degree of instability in the fitted geometric parameters. I am therefore
> writing to seek your advice and would sincerely appreciate any guidance you may
> be able to provide.
>
> Our experimental conditions are as follows, and here I use a single SAXS image
> as an example for illustration:
>
> Data type: SAXS image
>
> X-ray energy: 10 keV
>
> Standard sample: AgBH
>
> Detector: Pilatus 2M
>
> Exposure time: 5 s
>
> During the calibration process in pyFAI, we consistently used the same raw image
> （Attachment1）and applied the same mask file（Attachment2） throughout the
> entire workflow. In the peak-picking interface, we manually selected the peaks
> and then performed calibration under different ring number settings (1, 2, 3, 4,
> and 5).
>
> We observed that, although the raw imageand the mask file were exactly the same,
> the final fitted geometric parameters still showed noticeable differences when
> different numbers of rings were used for calibration (the settings interface is
> shown in Attachment 3, and the corresponding results are summarized in
> Attachment 4). The parameters that changed include:
>
> poni1
>
> poni2
>
> rotation1
>
> rotation2
>
> rotation3
>
> distance
>
> In addition, we noticed that even for the same diffraction ring, the number of
> valid points identified in the peak-picking interface could sometimes differ
> (see the comparison in Attachment 5; the corresponding data are highlighted in
> red in Attachment 4).
>
> Therefore, we would like to ask the following questions:
>
> 1.Under the condition of using the same image, the same mask, and the same
> manually selected peaks, why can the number of valid points on the same ring
> still change?
>
> 2.When the ring number is set to 1, 2, 3, 4, and 5, the fitted values of poni1,
> poni2, rotation1, rotation2, rotation3, and distance show certain differences.
> Is this behavior considered normal in pyFAI?
>
> 3.Are these variations mainly caused by differences in the number of points
> selected during the peak-picking stage, or are they primarily due to the
> different constraints introduced into the geometric fitting when different ring
> number settings are used?
>
> 4.For SAXS image calibration, would you recommend a more stable peak-picking or
> ring-selection strategy to reduce this type of parameter fluctuation?
>
> We have organized the fitting results obtained from the same image under
> different ring number settings into a table, and it can be clearly seen that
> these parameters do vary to a certain extent. I would like to understand whether
> this behavior is expected in the pyFAI fitting process, or whether there are
> additional settings or precautions that we should pay particular attention to.
>
> Thank you very much for taking the time to read this email. I would greatly
> appreciate your guidance and reply.
>
> Sincerely,
> Linjun He
>
> --------------------------------------------------------------------------------
> *从QQ邮箱发来的超大附件*
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