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DM-53812-v30: Remove overcorrection to ra covariance #817

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Merged
cmsaunders merged 1 commit into from
Jan 17, 2026
Merged

DM-53812-v30: Remove overcorrection to ra covariance #817

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11 changes: 4 additions & 7 deletions src/table/wcsUtils.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -112,13 +112,10 @@ Eigen::Matrix2f calculateCoordCovariance(geom::SkyWcs const &wcs, lsst::geom::Po
Eigen::Matrix2f d = localMatrix.cast<float>() * scale * (lsst::geom::PI / 180.0);

Eigen::Matrix2f skyCov = d * err * d.transpose();

// Multiply by declination correction matrix in order to get sigma(RA) * cos(Dec) for the uncertainty
// in RA, and cov(RA, Dec) * cos(Dec) for the RA/Dec covariance:
float cosDec = std::cos(skyCenter.getDec().asRadians());
Eigen::Matrix2f decCorr{{cosDec, 0}, {0, 1.0}};
Eigen::Matrix2f skyCovCorr = decCorr * skyCov * decCorr;
return skyCovCorr;
// Because the transform from pixels to RA/Dec was done at a local
// gnomonic, the RA and Dec covariance are already commensurate, and
// multiplying the RA covariance by cos(Dec) is not necessary.
return skyCov;
}

template <typename SourceCollection>
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6 changes: 1 addition & 5 deletions tests/test_sourceTable.py
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Expand Up @@ -235,12 +235,8 @@ def testCoordErrors(self):

centroidErr = self.record.getCentroidErr()
coordErr = radMatrix.dot(centroidErr.dot(radMatrix.T))
# multiply RA by cos(Dec):
cosDec = np.cos(skyCenter.getDec().asRadians())
decCorr = np.array([[cosDec, 0], [0, 1]])
coordErrCorr = decCorr.dot(coordErr.dot(decCorr))
catCoordErr = self.record.get(self.coordErrKey)
np.testing.assert_almost_equal(coordErrCorr, catCoordErr, decimal=16)
np.testing.assert_almost_equal(coordErr, catCoordErr, decimal=16)

def testSorting(self):
self.assertFalse(self.catalog.isSorted())
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