Single-axis rotation has a gauge degeneracy: rotating the whole experiment
about the spindle leaves every spot position unchanged, so the beam-centre
component parallel to the spindle is a null / under-determined direction.
Refining it freely lets it wander a few pixels and absorb centroid
systematics into a wrong beam that the co-refined orientation keeps
position-consistent.
Add a soft header prior on that one component in the shared XtalOptimizer:
residual = w*(beam[parallel] - header), with w set so the restraint behaves
like a sigma_px-pixel prior competing with the positional residuals. The
gauge direction has ~zero data sensitivity, so the prior pins it near the
LaB6-monitored header, while a real, well-supported drift can still overcome
it (the beam does drift). One restraint covers all three refinement sites at
once: the noisy accumulated-spot primary, the per-frame filter, and
post-refine.
Validated against independent XDS processing: neutral on well-determined
data (the gauge direction has no effect there), and a real low-resolution
accuracy gain where the free refinement had otherwise wandered off the
header - the merged intensities move back into agreement with XDS while the
internal CC1/2, which is blind to this, barely changes. No space-group
determinations change.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
leonarski_fandClaude Opus 4.8
c140b3567d