The centre in the file is often a placeholder, and nothing measures it until post-refinement
has already indexed the sweep - by which time a wrong centre has chosen the lattice. Two exact
facts about a rotation sweep give it from spot positions alone, with no cell, no orientation
matrix and nothing indexed.
Rotating 180 degrees about the spindle and taking -h negates a reflection's component along the
spindle and leaves the rest, so with the spindle perpendicular to the beam the Laue condition is
preserved and the spots recorded half a turn apart are mirror images along the spindle. Those are
Friedel mates, not the same reflection. The same reflection appears twice for a different reason:
it meets the Ewald sphere on two crossings, generally not half a turn apart, differing only in the
sign of the component perpendicular to both the spindle and the beam. The first observable gives
the coordinate along the spindle, the second the coordinate across it. Each candidate pairing
votes and the true value accumulates while wrong pairings scatter.
Both observables need guarding, because a vote is a comb and the tallest tooth is not always the
right one. Along the spindle a false pairing cannot fake the equality of Friedel amplitudes.
Across it, the two crossings of one reflection are separated by a sweep angle its own position
fixes, which no accidental pair reproduces.
The mirror is exact in the laboratory frame, so it is only as good as the rotation axis. Every
file here states an ideal axis and none of them has one; a skew about the beam spreads the vote
instead of shifting it, and past a milliradian it moves an otherwise correct answer by pixels
while every internal statistic still looks healthy. It is therefore fitted, not assumed. A tilt of
the axis towards the beam is measured and reported but not applied, being confounded with the
detector rotation until that is fitted too.
Nothing inside the fit can see a wrong tooth - when the vote flips, every frame pair flips with
it - so the answer is checked from outside, by asking whether it depends on where the search
began. That, and a floor on the angular span the pairs cover, are what refuse the cases this
cannot measure: a sweep barely past half a turn is the dangerous one, not the short one, because
at exactly half a turn there is nothing to fit and just past it there is almost nothing.
Where the sweep is too short for any of this the radial background profile gives a coarser centre
from a handful of images, and where neither can measure it the file's value is kept.
The beam-stop projection now takes its own frames rather than sharing the sample, so turning this
on cannot change the mask; and both samples keep away from the ends of the sweep, where shutter
synchronisation spoils an image. Reading twice as many frames as before costs a few seconds once,
and is what makes the answer independent of which frames were drawn.
Off by default. Over the 38-crystal rotation battery it serves every dataset, agrees with XDS's
refined direct beam to 0.116 px in the median against 0.135 for the value in the file, and changes
no space group.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
