diff --git a/common/DiffractionExperiment.cpp b/common/DiffractionExperiment.cpp
index fa535919b..db0bdfa26 100644
--- a/common/DiffractionExperiment.cpp
+++ b/common/DiffractionExperiment.cpp
@@ -654,6 +654,9 @@ void DiffractionExperiment::FillMessage(StartMessage &message) const {
     message.images_per_file = GetImagesPerFile();
     message.beam_center_x = GetBeamX_pxl();
     message.beam_center_y = GetBeamY_pxl();
+    const auto direct_beam = GetDiffractionGeometry().GetDirectBeam_pxl();
+    message.direct_beam_x = direct_beam.first;
+    message.direct_beam_y = direct_beam.second;
     message.detector_distance = GetDetectorDistance_mm() * 1e-3f;
     message.incident_wavelength = GetWavelength_A();
     // NXmx incident_wavelength_spread is the FWHM of the wavelength distribution,
diff --git a/common/JFJochMessages.h b/common/JFJochMessages.h
index 5e3360708..dae9b38a2 100644
--- a/common/JFJochMessages.h
+++ b/common/JFJochMessages.h
@@ -210,6 +210,14 @@ struct StartMessage {
     float detector_distance;
     float beam_center_x;
     float beam_center_y;
+    // Where the undeflected beam lands on the detector, in pixels. This is NOT beam_center_x/y above:
+    // that is the PONI, the foot of the perpendicular from the sample, and the two part company as
+    // soon as the detector is tilted. Derived from the rest of the geometry
+    // (DiffractionGeometry::GetDirectBeam_pxl), and carried so a consumer that wants the beam
+    // position - which is what most of them mean, XDS among them - does not have to redo the tilt
+    // arithmetic. Absent in a stream written before these fields existed.
+    std::optional<float> direct_beam_x;
+    std::optional<float> direct_beam_y;
 
     uint64_t number_of_images;
 
diff --git a/frame_serialize/CBORStream2Deserializer.cpp b/frame_serialize/CBORStream2Deserializer.cpp
index d6e8a753e..22ee26e4f 100644
--- a/frame_serialize/CBORStream2Deserializer.cpp
+++ b/frame_serialize/CBORStream2Deserializer.cpp
@@ -1254,6 +1254,10 @@ namespace {
                 message.beam_center_x = GetCBORFloat(value);
             else if (key == "beam_center_y")
                 message.beam_center_y = GetCBORFloat(value);
+            else if (key == "direct_beam_x")
+                message.direct_beam_x = GetCBORFloat(value);
+            else if (key == "direct_beam_y")
+                message.direct_beam_y = GetCBORFloat(value);
             else if (key == "detector_distance")
                 message.detector_distance = GetCBORFloat(value);
             else if (key == "number_of_images")
diff --git a/frame_serialize/CBORStream2Serializer.cpp b/frame_serialize/CBORStream2Serializer.cpp
index a37c135df..e8c0cc006 100644
--- a/frame_serialize/CBORStream2Serializer.cpp
+++ b/frame_serialize/CBORStream2Serializer.cpp
@@ -689,6 +689,10 @@ void CBORStream2Serializer::SerializeSequenceStart(const StartMessage& message)
     CBOR_ENC_AXIS(mapEncoder, "detector_translation", message.detector_translation);
     CBOR_ENC(mapEncoder, "beam_center_x", message.beam_center_x);
     CBOR_ENC(mapEncoder, "beam_center_y", message.beam_center_y);
+    // Not a DECTRIS field, and skipped by a consumer that does not know it. beam_center_x/y above is
+    // the PONI, which is not where the beam lands once the detector is tilted; these two are.
+    CBOR_ENC(mapEncoder, "direct_beam_x", message.direct_beam_x);
+    CBOR_ENC(mapEncoder, "direct_beam_y", message.direct_beam_y);
     CBOR_ENC(mapEncoder, "countrate_correction_enabled", message.countrate_correction_enabled);
     CBOR_ENC(mapEncoder, "flatfield_enabled", message.flatfield_enabled);
     CBOR_ENC(mapEncoder, "number_of_images", message.number_of_images);
diff --git a/tests/CBORTest.cpp b/tests/CBORTest.cpp
index 33a677382..5e5abca64 100644
--- a/tests/CBORTest.cpp
+++ b/tests/CBORTest.cpp
@@ -17,6 +17,8 @@ TEST_CASE("CBORSerialize_Start", "[CBOR]") {
         .detector_distance = 0.0005,
         .beam_center_x = 456.6,
         .beam_center_y = 124.3,
+        .direct_beam_x = 461.2,
+        .direct_beam_y = 129.9,
         .number_of_images = 34567,
         .image_size_x = 456,
         .image_size_y = 457,
@@ -89,6 +91,12 @@ TEST_CASE("CBORSerialize_Start", "[CBOR]") {
     CHECK(output_message.detector_distance == Catch::Approx(message.detector_distance));
     CHECK(output_message.beam_center_x == Catch::Approx(message.beam_center_x));
     CHECK(output_message.beam_center_y == Catch::Approx(message.beam_center_y));
+    // The PONI (beam_center_*) and the direct beam are different points on a tilted detector, so the
+    // round trip has to keep them apart, not merely keep two floats.
+    REQUIRE(output_message.direct_beam_x);
+    REQUIRE(output_message.direct_beam_y);
+    CHECK(output_message.direct_beam_x.value() == Catch::Approx(message.direct_beam_x.value()));
+    CHECK(output_message.direct_beam_y.value() == Catch::Approx(message.direct_beam_y.value()));
     CHECK(output_message.number_of_images == message.number_of_images);
     CHECK(output_message.image_size_x == message.image_size_x);
     CHECK(output_message.image_size_y == message.image_size_y);