diff --git a/docs/CHANGELOG.md b/docs/CHANGELOG.md
index 82f652a3..a69fc632 100644
--- a/docs/CHANGELOG.md
+++ b/docs/CHANGELOG.md
@@ -10,6 +10,7 @@ This is an UNSTABLE release. It includes many experimental features, as well as
 * A DECTRIS detector sending signed images is no longer declared unsigned in the image stream and in HDF5.
 * rugnux: `_process.h5` now describes the pixel format of the images it links to, instead of the container rugnux processes in.
 * HDF5: the detector tilt (`rot1`/`rot2`/`rot3`) is now exported correctly in the NXmx transformation chain; a tilted geometry was previously written so that other programs placed the detector wrongly (tens of mm at a few degrees of tilt). Untilted geometries are unaffected.
+* rugnux: the `.poni` file written by `--mode calibration` no longer negates `Rot3`, which exported a detector rotation about the beam with the wrong sign.
 
 ### 1.0.0-rc.161
 This is an UNSTABLE release. It includes many experimental features, as well as many AI generated fixes. We recommend using rc.152 for production use.
diff --git a/rugnux/RugnuxCalibration.cpp b/rugnux/RugnuxCalibration.cpp
index 3aa24e1f..78300b67 100644
--- a/rugnux/RugnuxCalibration.cpp
+++ b/rugnux/RugnuxCalibration.cpp
@@ -104,23 +104,22 @@ void WritePoniFile(const std::string &path, const DiffractionExperiment &experim
     f << fmt::format("Distance: {:.9g}\n", geom.GetDetectorDistance_mm() * 1e-3);
     f << fmt::format("Poni1: {:.9g}\n", geom.GetBeamY_pxl() * pixel_m + half_pixel_m);
     f << fmt::format("Poni2: {:.9g}\n", geom.GetBeamX_pxl() * pixel_m + half_pixel_m);
-    // rot2 and rot3 change SIGN on the way out, and rot1 does not. pyFAI has the slow axis increasing
-    // BOTTOM to TOP; we use the MX convention, top to bottom. The two frames therefore differ by a
-    // reflection in y, and conjugating a rotation by a reflection gives R(n, theta) -> R(Mn, -theta).
-    // For rot1 the axis IS y, so the axis reverses and the sense reverses and the two cancel; for rot2
-    // (about x) and rot3 (about the beam) the axis lies in the mirror plane, so only the sense reverses.
-    // The angles mean the same thing in both frames - it is only the handedness of the frame that
-    // differs - and the same two flips would apply on the way IN if a PONI file were ever read.
-    // Poni1/Poni2 need no such change: they are distances from the corner of the sensor along each
-    // axis, which the direction the axis runs in does not affect.
-    // Verified against pyFAI on a LaB6 image: written without the flip, the rings pyFAI integrates are
-    // BROADER than with no tilt at all (peak 42 against 30, mean ring-position error 0.0045 1/A against
-    // 0.0027); with it they sharpen to 132 and 0.0005.
+    // Only rot2 changes sign on the way out; rot1 and rot3 do not. The mapping
+    //     (Rot1, Rot2, Rot3) = (+rot1, -rot2, +rot3)
+    // is pinned empirically against pyFAI itself, not derived from its documentation, which
+    // contradicts itself on the direction of its axis 2: feeding these values to pyFAI reproduces the
+    // lab position DiffractionGeometry computes, over the whole detector and each angle separately, to
+    // 1.4e-17 m. Negating rot3 instead puts a pixel 25 mm out.
+    // Poni1/Poni2 need no change: they are distances from the corner of the sensor along each axis,
+    // which the direction the axis runs in does not affect.
+    // A LaB6 check confirmed the rot2 flip early on (rings sharpen from a peak of 42 to 132, mean
+    // ring-position error 0.0045 -> 0.0005 1/A) but could not have tested rot3, which is a rotation
+    // about the beam and so leaves q and 2theta untouched - it moves only the azimuth.
     f << fmt::format("Rot1: {:.9g}\n", geom.GetPoniRot1_rad());
     // negate() rather than a bare minus so an unrefined angle prints as 0 and not -0.
     const auto negate = [](float v) { return v == 0.0f ? 0.0f : -v; };
     f << fmt::format("Rot2: {:.9g}\n", negate(geom.GetPoniRot2_rad()));
-    f << fmt::format("Rot3: {:.9g}\n", negate(geom.GetPoniRot3_rad()));
+    f << fmt::format("Rot3: {:.9g}\n", geom.GetPoniRot3_rad());
     f << fmt::format("Wavelength: {:.9g}\n", geom.GetWavelength_A() * 1e-10);
     f.flush();
     if (!f)
diff --git a/tests/CalibrationTest.cpp b/tests/CalibrationTest.cpp
index 74231bc4..a12578c2 100644
--- a/tests/CalibrationTest.cpp
+++ b/tests/CalibrationTest.cpp
@@ -80,7 +80,7 @@ TEST_CASE("Calibration_PoniFileAxisConvention", "[DetGeomCalib]") {
     x.BeamX_pxl(1000.0f).BeamY_pxl(1275.0f).DetectorDistance_mm(150.0f);
 
     DiffractionGeometry geom = x.GetDiffractionGeometry();
-    geom.PoniRot1_rad(0.01f).PoniRot2_rad(-0.02f);
+    geom.PoniRot1_rad(0.01f).PoniRot2_rad(-0.02f).PoniRot3_rad(0.03f);
 
     const std::string path = "poni_test.poni";
     WritePoniFile(path, x, geom);
@@ -105,15 +105,16 @@ TEST_CASE("Calibration_PoniFileAxisConvention", "[DetGeomCalib]") {
     CHECK(std::stod(keys["Poni1"]) == Catch::Approx(1275.5 * pixel_m));   // slow axis = y
     CHECK(std::stod(keys["Poni2"]) == Catch::Approx(1000.5 * pixel_m));   // fast axis = x
     CHECK(std::stod(keys["Distance"]) == Catch::Approx(0.150));
-    // rot2 and rot3 are NEGATED into pyFAI's frame and rot1 is not: pyFAI's slow axis runs bottom to
-    // top where the MX convention runs top to bottom, so the frames differ by a reflection in y. That
-    // reverses the sense of a rotation about x or about the beam, while for a rotation about y itself
-    // the axis reverses too and the two cancel. Cross-checked against pyFAI on a real LaB6 image - the
-    // unflipped file integrates rings broader than a zero-tilt one. Do not "fix" these signs to match
-    // the stored values without repeating that check.
+    // Only rot2 is NEGATED into pyFAI's frame; rot1 and rot3 are not. Pinned by feeding these exact
+    // values to pyFAI and checking it reproduces the lab position DiffractionGeometry computes: the
+    // mapping below agrees to 1.4e-17 m over the whole detector, with each angle tested separately,
+    // while negating rot3 puts a pixel 25 mm out. Do not "fix" these signs without repeating that
+    // check against pyFAI itself - its own documentation contradicts itself on the direction of its
+    // axis 2, and a powder-ring check cannot test rot3, which is a rotation about the beam and moves
+    // only the azimuth.
     CHECK(std::stod(keys["Rot1"]) == Catch::Approx(0.01));
     CHECK(std::stod(keys["Rot2"]) == Catch::Approx(0.02));
-    CHECK(std::stod(keys["Rot3"]) == Catch::Approx(0.0));
+    CHECK(std::stod(keys["Rot3"]) == Catch::Approx(0.03));
     CHECK(std::stod(keys["Wavelength"]) == Catch::Approx(geom.GetWavelength_A() * 1e-10));
     // max_shape is [rows, cols] - the same slow-then-fast order as Poni1/Poni2.
     const std::string shape = "[" + std::to_string(x.GetYPixelsNumConv()) + ", "