The detector tilt is refined by default, and on a pattern that cannot separate
it from the beam centre the fit returns one anyway - there was nothing to stop
it. Both displace a ring's radius as cos(phi), and only how that amplitude grows
with the ring's radius tells them apart, which takes two well-sampled rings. At
500 mm on the LaB6 series only two rings reach the detector and the outer one is
barely there: the tilt came out at the opposite sign to every shorter distance,
dragged the PONI 28 px, and bought a residual of 0.960 px against 0.962 pinned.
The covariance says so plainly - 0.1 sigma, and a beam centre quoted to +-180 px.
So ask it. A tilt is kept only where the fit had it free AND it stands at least
three times its own uncertainty; otherwise rot1/rot2 go back to the header's
values and the beam centre and distance are refitted around them. Over the
series the tilt stands at 50, 33, 15 and 8 sigma at 110 to 300 mm and 0.1 at
500 mm, so any threshold between 2 and 5 gives the same verdict on all five -
this says which regime a fit is in, not where a line was drawn. The declined
500 mm fit lands on a direct beam of 773.53 px, against 773.56 for the pinned
fit measured independently.
It is a rejection criterion and nothing more. Clearing it does not certify a
tilt: that estimator is limited by systematics rather than by this sigma, and a
coherent half-pixel error in the ring positions fakes a tilt of the usual size
while leaving sigma small. The report says "refined", never "verified".
Writing the gate turned up a related fault in the pass loop. RingOptimizer pins
the tilt by itself when every point it is given lies on one ring, and on a
barely-sampled pattern a later pass lands in exactly that state - which froze
the tilt at whatever the FIRST pass had produced and returned it with sigma
zero, an unmeasured tilt wearing the appearance of a fixed one. The gate reads
that as "not measured" and refits pinned, which is why it is stated over the
geometry that gets reported rather than over what the last fit happened to do.
Both paths are covered, profile and spots; the spots path was reporting a
refined tilt as declined for the same reason.
Two things measured and NOT taken:
A robust loss. A Cauchy loss scaled to the previous pass's median residual
changed nothing on the series - rms 0.415 to 0.421 at 110 mm, no case improved,
every direct beam within 0.06 px. Ring points are per-sector peaks that already
had to stand 3 sigma clear of their own background, so there are no gross
outliers left to reject. Recorded at the call site rather than left as an unused
option.
A quality gate that refuses a bad calibration. Three candidate signals, all
measured against naming the wrong standard on LaB6 data: sigma(PONI) does not
see it at all (0.52-0.65 px, indistinguishable from healthy); the residual only
half sees it (3.2-3.6 px wrong against 0.4-1.0 right, but a correct run from a
wrong header sits at 1.0-2.4 and would be caught too); and the seed's match
score is dominated by how many rings the calibrant lists, scoring 0.29 for a
perfect LaB6 fit against 0.21 for a wrongly named silicon. None of the three
separates, so no gate is shipped. What the run does say is the recovered
distance against the header, and a wrong standard moves that to 446 mm on a
110 mm exposure - unmissable, and the operator's call rather than a threshold's.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NfuDvf5ipV3Hi8TiCUKD27
leonarski_fandClaude Opus 5
01ad1e1743