diff --git a/reader/HDF5MetadataSource.cpp b/reader/HDF5MetadataSource.cpp
index a99e92f5c..f6c35c52d 100644
--- a/reader/HDF5MetadataSource.cpp
+++ b/reader/HDF5MetadataSource.cpp
@@ -14,6 +14,43 @@
 #include "../common/Logger.h"
 #include "../common/ROIDefinition.h"
 
+// The image orientation the file itself states, in its NXdetector_module pixel directions. NXmx gives
+// those in the McStas frame, which is the internal frame turned 180 degrees about z.
+//
+// Only an exact match against one of the eight discrete orientations is taken. Anything else is a
+// continuous rotation of the detector in its own plane, which belongs in rot1/rot2/rot3 and cannot be
+// separated from the tilt by looking at the module alone - so it is left as it is rather than
+// approximated. Every real file examined here is exactly discrete.
+static std::optional<DetectorOrientation> ReadModuleOrientation(HDF5Object *file) {
+    const std::string base = "/entry/instrument/detector/module/";
+    if (!file->IsDataSet(base + "fast_pixel_direction") || !file->IsDataSet(base + "slow_pixel_direction"))
+        return {};
+
+    HDF5DataSet fast_dataset(*file, base + "fast_pixel_direction");
+    HDF5DataSet slow_dataset(*file, base + "slow_pixel_direction");
+    if (!fast_dataset.AttrExists("vector") || !slow_dataset.AttrExists("vector"))
+        return {};
+
+    const auto f = fast_dataset.ReadAttrVec("vector");
+    const auto s = slow_dataset.ReadAttrVec("vector");
+    if ((f.size() != 3) || (s.size() != 3))
+        return {};
+
+    const Coord fast(-f[0], -f[1], f[2]);
+    const Coord slow(-s[0], -s[1], s[2]);
+
+    for (int64_t quarter_turns = 0; quarter_turns < 4; quarter_turns++) {
+        for (bool mirror_y: {false, true}) {
+            const DetectorOrientation orientation(mirror_y, quarter_turns);
+            const RotMatrix m = orientation.Matrix();
+            if (((m.Column(0) - fast).Length() < 1e-3f) && ((m.Column(1) - slow).Length() < 1e-3f))
+                return orientation;
+        }
+    }
+    return {};
+}
+
+
 inline std::pair<gemmi::CrystalSystem, char> parse_bravais_lattice(const std::string &val) {
     if (val.empty())
         return {gemmi::CrystalSystem::Triclinic, 'P'};
@@ -821,13 +858,17 @@ HDF5MetadataSource::OpenResult HDF5MetadataSource::Open(const std::string &filen
         // How the stored image sits in the detector plane. A different setting from mirror_y above,
         // recorded separately by the writer; absence means the identity, which is what a file written
         // before it existed - or by anything else - describes.
-        detector.ImageOrientation(DetectorOrientation(
-                master_file->GetOptBool(
-                        "/entry/instrument/detector/detectorSpecific/detector_orientation_mirror_y")
-                        .value_or(false),
-                master_file->GetOptInt(
-                        "/entry/instrument/detector/detectorSpecific/detector_orientation_quarter_turns")
-                        .value_or(0)));
+        // NXmx states this in the module's pixel directions, which is where it is read from first;
+        // detectorSpecific carries the same setting for a file this system wrote, and is the fallback
+        // for one whose module group says nothing usable. Absence of both means the identity.
+        detector.ImageOrientation(ReadModuleOrientation(master_file.get()).value_or(
+                DetectorOrientation(
+                        master_file->GetOptBool(
+                                "/entry/instrument/detector/detectorSpecific/detector_orientation_mirror_y")
+                                .value_or(false),
+                        master_file->GetOptInt(
+                                "/entry/instrument/detector/detectorSpecific/detector_orientation_quarter_turns")
+                                .value_or(0))));
         // Sensor thickness/material drive the parallax/absorption model, so take them from the file
         // rather than the DetectorSetup default (NXmx stores thickness in metres).
         if (master_file->Exists("/entry/instrument/detector/sensor_thickness"))