diff --git a/docs/CHANGELOG.md b/docs/CHANGELOG.md
index 49aee755..067b1e34 100644
--- a/docs/CHANGELOG.md
+++ b/docs/CHANGELOG.md
@@ -16,7 +16,7 @@ This is an UNSTABLE release. It includes many experimental features, as well as
 * HDF5: a still or grid scan recorded with a Smargon head position is no longer read back as a single image.
 * The writer refuses a stream whose start message declares a different pixel format than its images carry, instead of writing a master that does not describe its own data.
 * HDF5: `module_offset` is written as a float with a proper unit vector, and every transformation offset declares `offset_units`, so a reader does not fall back to the axis's own units - degrees on a rotation - when interpreting a length.
-* The image stream can carry the sample transformation chain (`transformations`, in the END message) in mounting order, so a goniometer axis, the Smargon chi/phi and a grid stage are described together and unambiguously. It is optional - a producer that does not send it gets the same chain built by the writer - so no metadata version change is needed.
+* The image stream can carry the sample transformation chain (`transformations`, in the END message) in mounting order, so a goniometer axis, the Smargon chi/phi and a grid stage are described together and unambiguously. It is optional - a producer that does not send it gets the same chain built by the writer - so no metadata version change is needed. The rotation width and end angles are derived by the writer rather than sent.
 * Smargon chi/phi are written for a still as well, and are read back from HDF5; before, they were dropped unless the run also had a rotation axis or a grid scan, and nothing read them.
 * The image stream and HDF5 now record `mirror_y`, whether the assembled image is mirrored in Y relative to the detector's raw readout, and it is read back.
 * rugnux: an image integrated in pyFAI through the `.poni` file written by `--mode calibration` now comes out with the correct azimuth; it was 180 degrees out. Radial integration is unchanged.
diff --git a/tests/JFJochReaderTest.cpp b/tests/JFJochReaderTest.cpp
index 783cfad6..1c8393da 100644
--- a/tests/JFJochReaderTest.cpp
+++ b/tests/JFJochReaderTest.cpp
@@ -2991,6 +2991,20 @@ TEST_CASE("JFJochReader_TransformationChain_SentAndBuilt", "[HDF5][Full]") {
     CHECK(sent.GetDatasetSettings().GetSmargonPosition()->phi_deg
           == Catch::Approx(-7.25f).margin(1e-3));
 
+    // Compared on the files, not through the reader: the reader reads neither AXISNAME_end nor the
+    // rotation width, so it cannot see the two routes diverge - and it did, until the writer started
+    // deriving them for a chain it was handed.
+    {
+        HDF5ReadOnlyFile built_file("test_chain_built_master.h5");
+        HDF5ReadOnlyFile sent_file("test_chain_sent_master.h5");
+        CHECK(built_file.FindLeafs("/entry/sample/transformations")
+              == sent_file.FindLeafs("/entry/sample/transformations"));
+        CHECK(sent_file.ReadVector<double>("/entry/sample/transformations/omega_end").size()
+              == static_cast<size_t>(x.GetImageNum()));
+        CHECK(sent_file.ReadVector<double>("/entry/sample/transformations/omega_range_average").at(0)
+              == Catch::Approx(0.1).margin(1e-4));
+    }
+
     remove("test_chain_built_master.h5");
     remove("test_chain_sent_master.h5");
     REQUIRE(H5Fget_obj_count(H5F_OBJ_ALL, H5F_OBJ_ALL) == 0);
diff --git a/writer/HDF5NXmx.cpp b/writer/HDF5NXmx.cpp
index 36418408..e0f1f2ae 100644
--- a/writer/HDF5NXmx.cpp
+++ b/writer/HDF5NXmx.cpp
@@ -825,6 +825,27 @@ void NXmx::Sample(const StartMessage &start, const EndMessage &end) {
                                     axis.GetEquipment(), axis.GetEquipmentComponent(),
                                     type, vector, offset, "");
             depends_on = base + axis.GetName();
+
+            // AXISNAME_end and the rotation width are derived here rather than carried on the wire.
+            // They follow from the values themselves for a constant step, which is every case there
+            // is today, so sending them would cost another array per axis and say nothing new.
+            const auto &values = axis.GetValues();
+            // Over the endpoints rather than the first pair: the values arrive as floats, and a
+            // single difference carries that noise straight into the reported rotation width.
+            const double step = (values.size() > 1)
+                    ? (values.back() - values.front()) / static_cast<double>(values.size() - 1)
+                    : 0.0;
+            if (axis.IsRotation() && (step != 0.0)) {
+                std::vector<double> end(values.size());
+                for (size_t i = 0; i < values.size(); i++)
+                    end[i] = values[i] + step;
+                SaveVector(transformations, axis.GetName() + "_end", end)->Units(axis.GetUnits());
+                SaveScalar(transformations, axis.GetName() + "_range_average",
+                           static_cast<float>(step))->Units(axis.GetUnits());
+                SaveScalar(transformations, axis.GetName() + "_range_total",
+                           static_cast<float>(step * static_cast<double>(values.size())))
+                        ->Units(axis.GetUnits());
+            }
         }
         group.SaveScalar("depends_on", depends_on);
         return;