diff --git a/tests/XtalOptimizerTest.cpp b/tests/XtalOptimizerTest.cpp
index 5a809a87..f200693a 100644
--- a/tests/XtalOptimizerTest.cpp
+++ b/tests/XtalOptimizerTest.cpp
@@ -538,10 +538,14 @@ TEST_CASE("XtalOptimizer_rotation") {
         }
     }
 
-    // Seed slightly perturbed geometry and lattice; provide rotation axis for refinement
+    // Seed slightly perturbed geometry and lattice; provide rotation axis for refinement.
+    // The beam-centre component PARALLEL to the spindle (here X, the rotation axis) is a gauge-weak
+    // direction that XtalOptimizer now deliberately restrains toward the header rather than refining
+    // freely (see the BeamComponentPrior in XtalOptimizer.cpp). So only the perpendicular component
+    // (Y) is seeded off-truth to exercise beam-centre recovery; X is seeded at its true value.
     XtalOptimizerData xtal_opt{};
     xtal_opt.latt = CrystalLattice(39.7f, 50.6f, 79.6f, 90.0f, 94.5f, 90.5f);
-    xtal_opt.geom.BeamX_pxl(1003).BeamY_pxl(997).DetectorDistance_mm(203.0)
+    xtal_opt.geom.BeamX_pxl(1000).BeamY_pxl(997).DetectorDistance_mm(203.0)
             .PoniRot1_rad(0.01).PoniRot2_rad(0.02);
     xtal_opt.crystal_system = gemmi::CrystalSystem::Monoclinic;
     xtal_opt.axis = axis;