[ESSSANS] Apply the SANS beam center as a beam direction

packages/esssans/docs/user-guide/common/beam-center-finder.ipynb

@@ -188,6 +188,10 @@
    "id": "13",
    "metadata": {},
    "source": [
+    "The result is the beam center relative to the sample.\n",
+    "Its component along the beam is the distance from the sample at which the transverse offset was determined, here the distance of the detector panel.\n",
+    "That distance is part of the result because the same beam direction corresponds to a proportionally smaller transverse offset on a detector closer to the sample.\n",
+    "\n",
     "We can now update our previous figure with the new position of the beam center (pink dot),\n",
     "which is clearly in the center of the beam/beam stop."
    ]
@@ -449,11 +453,20 @@
     "workflow[DimsToKeep] = ()\n",
     "workflow[WavelengthMask] = None\n",
     "workflow[WavelengthBands] = None\n",
+    "\n",
     "kwargs = dict(  # noqa: C408\n",
     "    workflow=workflow,\n",
     "    detector=detector['data'],\n",
     "    norm=workflow.compute(CleanDirectBeam),\n",
-    ")"
+    ")\n",
+    "\n",
+    "\n",
+    "def beam_center(x, y):\n",
+    "    \"\"\"Beam center from transverse offsets, at the distance found by method 1.\"\"\"\n",
+    "    # Transverse offsets define a beam direction only together with the distance from\n",
+    "    # the sample at which they are given. The SANS2D beam runs along Z, so the plane\n",
+    "    # normal to the beam is the XY plane, here at the Z of the center of mass.\n",
+    "    return sc.vector([x, y, com.fields.z.value], unit='m')"
    ]
   },
   {
@@ -474,7 +487,7 @@
     "from ess.sans.beam_center_finder import _iofq_in_quadrants\n",
     "\n",
     "results = _iofq_in_quadrants(\n",
-    "    xy=[0, 0],\n",
+    "    beam_center=beam_center(x=0.0, y=0.0),\n",
     "    **kwargs,\n",
     ")"
    ]
@@ -541,9 +554,8 @@
     "bounds = [(x.min().value, x.max().value), (y.min().value, y.max().value)]\n",
     "\n",
     "res = minimize(\n",
-    "    sans.beam_center_finder._cost,\n",
+    "    lambda xy: sans.beam_center_finder._cost(beam_center(*xy), **kwargs),\n",
     "    x0=[0, 0],\n",
-    "    args=tuple(kwargs.values()),\n",
     "    bounds=bounds,\n",
     "    method='Powell',\n",
     "    tol=0.01,\n",
@@ -590,7 +602,7 @@
    "outputs": [],
    "source": [
     "results = _iofq_in_quadrants(\n",
-    "    xy=[res.x[0], res.x[1]],\n",
+    "    beam_center=beam_center(*res.x),\n",
     "    **kwargs,\n",
     ")\n",
     "\n",

packages/esssans/pyproject.toml

@@ -36,6 +36,7 @@ dependencies = [
   "numpy>=1.26.4",
   "pandas>=2.1.2",
   "plopp>=26.5.0",
+  "scippneutron>=26.9.1",
 ]
 
 [project.optional-dependencies]

packages/esssans/src/ess/isissans/general.py

@@ -12,7 +12,6 @@
 import scippneutron as scn
 import scippnexus as snx
 from ess.sans.types import (
-    BeamCenter,
     DetectorIDs,
     DetectorPixelShape,
     DetectorPositionOffset,
@@ -81,9 +80,9 @@ def default_parameters() -> dict:
 
 
 def to_detector_position_offset(
-    global_offset: DetectorBankOffset, beam_center: BeamCenter
+    global_offset: DetectorBankOffset,
 ) -> DetectorPositionOffset[RunType]:
-    return DetectorPositionOffset[RunType](global_offset - beam_center)
+    return DetectorPositionOffset[RunType](global_offset)
 
 
 def to_monitor_position_offset(

packages/esssans/src/ess/isissans/sans2d.py

@@ -6,7 +6,7 @@
 import scipp as sc
 from ess.sans import SansWorkflow
 from ess.sans.parameters import typical_outputs
-from ess.sans.types import BeamCenter, DetectorMasks, EmptyDetector, SampleRun
+from ess.sans.types import DetectorMasks, EmptyDetector, SampleRun
 
 from ess.reduce.workflow import register_workflow
 
@@ -25,33 +25,26 @@
 """Sample holder mask"""
 
 
-def detector_edge_mask(
-    beam_center: BeamCenter, sample: EmptyDetector[SampleRun]
-) -> DetectorEdgeMask:
-    # These values were determined by hand before the beam center was available.
-    # We therefore undo the shift introduced by the beam center.
-    raw_pos = sample.coords['position'] + beam_center
-    mask_edges = (sc.abs(raw_pos.fields.x) > sc.scalar(0.48, unit='m')) | (
-        sc.abs(raw_pos.fields.y) > sc.scalar(0.45, unit='m')
+def detector_edge_mask(sample: EmptyDetector[SampleRun]) -> DetectorEdgeMask:
+    pos = sample.coords['position']
+    mask_edges = (sc.abs(pos.fields.x) > sc.scalar(0.48, unit='m')) | (
+        sc.abs(pos.fields.y) > sc.scalar(0.45, unit='m')
     )
     return DetectorEdgeMask(mask_edges)
 
 
 def sample_holder_mask(
-    beam_center: BeamCenter,
     sample: EmptyDetector[SampleRun],
     low_counts_threshold: LowCountThreshold,
 ) -> SampleHolderMask:
-    # These values were determined by hand before the beam center was available.
-    # We therefore undo the shift introduced by the beam center.
-    raw_pos = sample.coords['position'] + beam_center
+    pos = sample.coords['position']
     summed = sample.hist()
     holder_mask = (
         (summed.data < low_counts_threshold)
-        & (raw_pos.fields.x > sc.scalar(0, unit='m'))
-        & (raw_pos.fields.x < sc.scalar(0.42, unit='m'))
-        & (raw_pos.fields.y < sc.scalar(0.05, unit='m'))
-        & (raw_pos.fields.y > sc.scalar(-0.15, unit='m'))
+        & (pos.fields.x > sc.scalar(0, unit='m'))
+        & (pos.fields.x < sc.scalar(0.42, unit='m'))
+        & (pos.fields.y < sc.scalar(0.05, unit='m'))
+        & (pos.fields.y > sc.scalar(-0.15, unit='m'))
     )
     return SampleHolderMask(holder_mask)
 

packages/esssans/src/ess/loki/larmor_workflow.py

@@ -108,12 +108,14 @@ def larmor_detector_coord_transform_graph(
     *,
     sample_position: Position[snx.NXsample, RunType],
     source_position: Position[snx.NXsource, RunType],
+    beam_center: BeamCenter,
     gravity: GravityVector,
 ) -> ElasticCoordTransformGraph[RunType]:
     graph = sans_elastic(
         correct_for_gravity=correct_for_gravity,
         sample_position=sample_position,
         source_position=source_position,
+        beam_center=beam_center,
         gravity=gravity,
     )
     return ElasticCoordTransformGraph[RunType]({**graph, **tof.elastic_Q('tof')})
@@ -186,5 +188,9 @@ def LokiAtLarmorTutorialWorkflow() -> sciline.Pipeline:
         data.loki_tutorial_run_60392()
     )
     workflow[Filename[EmptyBeamRun]] = str(data.loki_tutorial_run_60392())
-    workflow[BeamCenter] = sc.vector(value=[-0.02914868, -0.01816138, 0.0], unit='m')
+    # Beam center determined with `beam_center_from_center_of_mass`, the Z component
+    # being the distance from the sample at which it was determined.
+    workflow[BeamCenter] = sc.vector(
+        value=[-0.02914868, -0.01816138, 4.11610], unit='m'
+    )
     return workflow