The expected-variance weights decompose an observation's sigma^2 into a
background part and a Poisson signal part, then rebuild the signal part
at the reflection's merged mean. The decomposition subtracted corr*I with
I taken as-is, so a negative I ADDED to the background part: an
observation at I = -1.5 with sigma^2 = 1 came out with a base variance of
2.7 rather than 1.
That inflates the variance of precisely the down-fluctuated observations
the correction exists for. Below about one photon they are then
under-weighted and the merged mean is biased high - the same direction of
error, in the same regime, that weighting by the observation's own sigma
produces. Subtract max(0, I) instead: a negative intensity has no Poisson
signal to remove.
Both users of the decomposition are fixed - the stills merge, where
expected-variance weighting is now the default, and the rotation combine
it was mirrored from, which had it first.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
leonarski_fandClaude Opus 5
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