A reflection was accepted onto an image when |delta_phi| * zeta was within the
mosaicity window, where delta_phi is the offset from the frame's mid-exposure
angle to the exact diffracting condition. That asks whether the frame's CENTRE
lies inside the rocking curve, which is a stricter question than the one that
matters: whether any of the curve lies inside the frame's exposure. The two
differ by half a wedge, and the partiality computed a few lines further down
already integrates over that half wedge on both sides - so the acceptance test
and the quantity it gates disagreed about where the frame is.
The consequence is not a clipped intensity but a lost reflection. Consecutive
frame centres are one wedge apart, so the nearest centre can be half a wedge
away; once the window is narrower than that, the reflection fails the test on
its best frame and on every other, and is never predicted at all. That happens
when sigma_eff < zeta * wedge / (2 * mosaicity_multiplier) - coarse slicing on a
sharp crystal at high zeta, which is where a reflection is fully recorded on one
image and measured best.
Subtracting the half wedge from the tested offset restores the intended
question. On a crystal that reaches the regime (0.4 deg per image, fitted
sigma_M 0.051 deg) low-resolution R_meas goes 6.8% -> 5.4% and ISa 13.3 -> 14.1.
Elsewhere the window merely widens by half a wedge, which admits partials whose
partiality is a few parts in a thousand; those are correctly measured and
correctly down-weighted, and four of the six crystals tested do not move, while
one loses 1.2 ISa. Both engines carry the same test and both are changed.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
leonarski_fandClaude Opus 5
e0bd208666