The background belongs to the beam and the shadow to the stop, and the two are
not concentric - fitting the stop edge per azimuth gives offsets of 13.4 px on
an 85.8 px disk, 22.2 px on 67.3 px and 6.9 px on 23.7 px, 8 to 33 per cent of
the stop radius on every crystal measured. The finder bridged that gap with a
radial envelope, the largest ring background over an outward window, used as the
reference for an individual pixel. That quantity exceeds the local background
wherever the background rises outward, so sound pixels near the stop scored below
the penumbra threshold and were masked. Measured against the fitted edge on a
long-distance disk stop, the mask was displaced rather than mis-sized: short by
up to 20 px on one side, over-reaching by up to 45 px on the other, with eight of
twenty-four azimuth sectors falling short.
The ring median is already the right reference wherever a ring still has
unshadowed pixels to measure, which is every ring except those lying wholly
inside the disk - and it needs no assumption about where the stop sits. So the
envelope is gone from the per-pixel test, and the rings it existed to cover are
handled directly: walking outward, a ring whose background is a fraction of the
background further out is shadow in its entirety. That comparison is only ever
asked whether a whole ring is inside the stop, never to judge a pixel, which is
where its failure mode lives. Blockage is deliberately not a counting test - on a
bright dataset the shadow interior is still well counted.
Detection is now one channel instead of two, and 113 lines shorter.
Measured: no azimuth sector falls short by more than 3.4 px, over-reach drops on
all three fitted crystals, and mask area moves by at most 0.04 per cent of the
detector on six crystals, so this corrects the shape rather than resizing.
Battery: space-group agreement with XDS unchanged at 34/37, median change in
R_meas and in the lowest shell 0.000 pp. The crystal that suffered worst when
masking was introduced recovers to its unmasked quality - R_meas 25.1 -> 17.2 per
cent, ISa 4.45 -> 10.04 - which is what removing the over-masking should do.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
leonarski_fandClaude Opus 5
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