[ESSREDUCE] Project chopper axle onto incident beam for flight distance

packages/essreduce/src/ess/reduce/unwrap/lut.py

@@ -429,6 +429,34 @@ def _to_component_reading(component) -> BeamlineComponentReading:
     )
 
 
+def chopper_distance_along_beam(
+    axle_position: sc.Variable,
+    source_position: sc.Variable,
+) -> sc.Variable:
+    """Flight distance from the source to a chopper along the incident beam.
+
+    A disk chopper's axle is offset from the beam (the disk sits above, below, or
+    to the side of it), so the straight-line distance
+    ``norm(axle_position - source_position)`` overestimates the flight distance,
+    and for closely-spaced choppers can even place them in the wrong order along
+    the cascade. The neutron crosses the disk where the incident beam intersects
+    it, so the flight distance is the projection of the axle position onto the
+    incident-beam direction. Assuming the incident beam runs along the z axis,
+    this is the z-component of the offset from the source. This matches the
+    straight-line ``Ltotal`` used for detectors and monitors, where the component
+    lies on the beam and the projection reduces to the norm.
+
+    Parameters
+    ----------
+    axle_position:
+        Position of the chopper's rotation axle.
+    source_position:
+        Position of the neutron source.
+    """
+    source_position = source_position.to(unit=axle_position.unit)
+    return (axle_position - source_position).fields.z
+
+
 def simulate_chopper_cascade_using_tof(
     choppers: DiskChoppers[RunType],
     source_position: Position[snx.NXsource, RunType],
@@ -471,9 +499,7 @@ def simulate_chopper_cascade_using_tof(
     tof_choppers = []
     for name, ch in choppers.items():
         chop = tof.Chopper.from_diskchopper(ch, name=name)
-        chop.distance = sc.norm(
-            ch.axle_position - source_position.to(unit=ch.axle_position.unit)
-        )
+        chop.distance = chopper_distance_along_beam(ch.axle_position, source_position)
         tof_choppers.append(chop)
 
     source = tof.Source(
@@ -657,9 +683,7 @@ def compute_frame_sequence(
 
     chops = {
         key: chopper_cascade.Chopper(
-            distance=sc.norm(
-                ch.axle_position - source_position.to(unit=ch.axle_position.unit)
-            ),
+            distance=chopper_distance_along_beam(ch.axle_position, source_position),
             time_open=ch.time_offset_open(
                 pulse_frequency=frequency_for_chopper_rotation
             ),

packages/essreduce/tests/unwrap/lut_test.py

@@ -8,6 +8,7 @@
 from ess.reduce import unwrap
 from ess.reduce.nexus.types import AnyRun, FrameMonitor0, Position
 from ess.reduce.unwrap import GenericUnwrapWorkflow, LookupTableWorkflow, SourceBounds
+from ess.reduce.unwrap.lut import chopper_distance_along_beam
 
 sl = pytest.importorskip("sciline")
 
@@ -167,6 +168,49 @@ def _make_choppers():
     }
 
 
+def _single_slit_chopper(axle_position: sc.Variable) -> DiskChopper:
+    return DiskChopper(
+        frequency=sc.scalar(-14.0, unit='Hz'),
+        beam_position=sc.scalar(0.0, unit='deg'),
+        phase=sc.scalar(0.0, unit='deg'),
+        axle_position=axle_position,
+        slit_begin=sc.array(dims=['cutout'], values=[0.0], unit='deg'),
+        slit_end=sc.array(dims=['cutout'], values=[10.0], unit='deg'),
+        slit_height=sc.scalar(10.0, unit='cm'),
+        radius=sc.scalar(30.0, unit='cm'),
+    )
+
+
+def test_chopper_distance_along_beam_projects_offset_axle_onto_beam():
+    source = sc.vector([0, 0, 0], unit='m')
+    # Axle sits 3 m above the beam and 10 m downstream of the source.
+    axle = sc.vector([0, 3, 10], unit='m')
+    distance = chopper_distance_along_beam(axle, source)
+    assert sc.isclose(distance, sc.scalar(10.0, unit='m'))
+    # The straight-line distance to the offset axle would overestimate this.
+    assert sc.norm(axle - source) > sc.scalar(10.0, unit='m')
+
+
+def test_chopper_cascade_orders_offset_axle_by_beam_projection():
+    # Two closely-spaced choppers; the upstream one's axle is offset far enough
+    # from the beam that ``norm(axle - source)`` (6.185 m) exceeds the downstream
+    # chopper's distance (6.155 m). Ordering must follow the beam-projected
+    # distance, otherwise the upstream chopper is wrongly applied last (or, for a
+    # narrow distance range, never at all).
+    wf = _make_workflow("analytical")
+    wf[Position[snx.NXsource, AnyRun]] = sc.vector([0, 0, 0], unit='m')
+    wf[unwrap.PulseStride[AnyRun]] = 1
+    wf[unwrap.DiskChoppers[AnyRun]] = {
+        'near': _single_slit_chopper(sc.vector([0, 0.7, 6.145], unit='m')),
+        'far': _single_slit_chopper(sc.vector([0, 0, 6.155], unit='m')),
+    }
+
+    frames = wf.compute(unwrap.ChopperFrameSequence[AnyRun])
+
+    distances = sorted(f.distance.to(unit='m').value for f in frames)
+    assert distances == pytest.approx([0.0, 6.145, 6.155])
+
+
 @pytest.mark.parametrize("detector_or_monitor", ["detector", "monitor"])
 @pytest.mark.parametrize("wavelength_from", ["analytical", "simulation"])
 def test_lut_workflow_computes_table_with_choppers(