An image integrated in pyFAI through our .poni came out with every chi 180 degrees from where it
belongs. pyFAI's in-plane axes are the negatives of ours, so Rot3 needs a half turn on top of the
sign flip. Being a rotation about the beam it leaves 2theta alone - which is why radial integration
was right all along and only the azimuth was wrong, and why a powder-ring check could never have
caught it.
The half turn is needed for the orientation-3 form written before rc.162 as well, so it is not an
artefact of declaring the orientation - the file has been 180 degrees out for as long as it has been
written.
Verified against pyFAI 2026.5.0 on a tilted detector with an off-centre beam, against the lab
positions of the NXmx chain: 2theta to 3.6e-15 deg and chi to 2.8e-14 deg. Then end to end, by
integrating an image in jfjoch's own layout through a .poni the code actually writes: chi lands
within 0.15 deg of physical truth on a 0.5 deg cake bin.
Withdraws two changelog claims. The .poni does negate Rot3, and declaring orientation did not fix
the azimuth: pyFAI's orientation is numerically inert here, so the file was relabelled and not
corrected.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VfYvJT5Nb71suJCowRBn5z
leonarski_fandClaude Opus 5
c1b6030c4f