[ESSSPECTROSCOPY] Update BIFROST NeXus loader for angles

packages/essspectroscopy/docs/user-guide/bifrost/bifrost-bragg-peak-monitor.ipynb

@@ -25,6 +25,7 @@
     "    simulated_elastic_incoherent_with_phonon,\n",
     "    lookup_table_simulation,\n",
     ")\n",
+    "from ess.bifrost.io.mcstas import load_sample_angle\n",
     "from ess.bifrost.single_crystal import BifrostSimulationBraggPeakMonitorWorkflow, make_q_map\n",
     "from ess.bifrost.single_crystal.types import *\n",
     "from ess.bifrost.types import McStasRawDetector\n",
@@ -58,8 +59,11 @@
    "outputs": [],
    "source": [
     "workflow = BifrostSimulationBraggPeakMonitorWorkflow()\n",
+    "\n",
     "workflow[Filename[SampleRun]] = simulated_elastic_incoherent_with_phonon()\n",
     "workflow[LookupTableFilename] = lookup_table_simulation()\n",
+    "workflow.insert(load_sample_angle)  # Needed for simulation data\n",
+    "\n",
     "workflow[LookupTableRelativeErrorThreshold] = {\n",
     "    'detector': np.inf,\n",
     "    'normalization_monitor': np.inf,\n",

packages/essspectroscopy/src/ess/bifrost/io/mcstas.py

@@ -8,14 +8,37 @@
 from ess.spectroscopy.types import (
     EmptyDetector,
     NeXusData,
+    NeXusFileSpec,
     PulsePeriod,
     RawDetector,
     RunType,
+    SampleAngle,
 )
 
+from ess.reduce.nexus import open_component_group
+from ess.reduce.nexus.types import NeXusLocationSpec
+
 from ..types import McStasRawDetector
 
 
+def load_sample_angle(
+    file_spec: NeXusFileSpec[RunType],
+) -> SampleAngle[RunType]:
+    """Load the rotation angle of a sample from a McStas BIFROST NeXus file."""
+    return SampleAngle[RunType](_load_experiment_parameter(file_spec, "a3"))
+
+
+def _load_experiment_parameter(
+    file_spec: NeXusFileSpec[RunType], param_name: str
+) -> sc.DataArray:
+    with open_component_group(
+        NeXusLocationSpec(filename=file_spec.value),
+        nx_class=snx.NXparameters,
+        parent_class=snx.NXentry,
+    ) as group:
+        return group[param_name][()]['value']
+
+
 def assemble_detector_data(
     detector: EmptyDetector[RunType],
     event_data: NeXusData[snx.NXdetector, RunType],
@@ -74,4 +97,5 @@ def wrap_event_time_offset(event_time_offset: sc.Variable) -> sc.Variable:
 providers = (
     assemble_detector_data,
     convert_simulated_time_to_event_time_offset,
+    load_sample_angle,
 )

packages/essspectroscopy/src/ess/bifrost/io/nexus.py

@@ -25,33 +25,57 @@
 from ess.reduce.nexus.types import NeXusLocationSpec, TransformationTimeFilter
 
 
-# See https://github.com/scipp/essreduce/issues/98
-def moderator_class_for_source() -> NeXusClass[snx.NXsource]:
-    """Select NXmoderator as the source."""
-    return NeXusClass[snx.NXsource](snx.NXmoderator)
-
-
 def load_sample_angle(
     file_spec: NeXusFileSpec[RunType],
 ) -> SampleAngle[RunType]:
-    return SampleAngle[RunType](_load_experiment_parameter(file_spec, "a3"))
+    """Load the rotation angle of a sample from a BIFROST NeXus file."""
+    return SampleAngle[RunType](
+        _load_rotation_angle(
+            file_spec,
+            "114_sample_stack/rotation_stage",
+        )
+    )
 
 
 def load_instrument_angle(
     file_spec: NeXusFileSpec[RunType],
 ) -> InstrumentAngle[RunType]:
-    return InstrumentAngle[RunType](_load_experiment_parameter(file_spec, "a4"))
+    """Load the rotation angle for the BIFROST detector from a NeXus file."""
+    return InstrumentAngle[RunType](
+        _load_rotation_angle(
+            file_spec,
+            "detector_tank_angle/transformations/detector_tank_angle_r0",
+        )
+    )
 
 
-def _load_experiment_parameter(
-    file_spec: NeXusFileSpec[RunType], param_name: str
-) -> sc.DataArray:
+def _load_rotation_angle(file_spec: NeXusFileSpec[RunType], path: str) -> sc.DataArray:
     with open_component_group(
         NeXusLocationSpec(filename=file_spec.value),
-        nx_class=snx.NXparameters,
+        nx_class=snx.NXinstrument,
         parent_class=snx.NXentry,
-    ) as group:
-        return group[param_name][()]['value']
+    ) as instrument:
+        log: snx.Group = instrument[path]  # type: ignore[assignment]
+        if isinstance(log['value'], snx.Group):
+            # The NXlog is nested in this group, e.g., the group can be an NXpositioner
+            # with children 'value', 'target_value', etc.
+            transform: snx.nxtransformations.Transform = log['value'][()]  # type: ignore[assignment]
+        else:
+            # `log` is the NXlog we are looking for without further nesting.
+            transform: snx.nxtransformations.Transform = log[()]  # type: ignore[assignment]
+
+        if transform.transformation_type != 'rotation':
+            raise ValueError(
+                "Expected the instrument angle at detector_tank_angle to be a rotation,"
+                f" got '{transform.transformation_type}' instead.'"
+            )
+        return transform.value  # type: ignore[return-value]
+
+
+# See https://github.com/scipp/ess/issues/120
+def moderator_class_for_source() -> NeXusClass[snx.NXsource]:
+    """Select NXmoderator as the source."""
+    return NeXusClass[snx.NXsource](snx.NXmoderator)
 
 
 def load_analyzer_for_detector(

packages/essspectroscopy/tests/bifrost/live_test.py

@@ -10,6 +10,7 @@
     lookup_table_simulation,
     simulated_elastic_incoherent_with_phonon,
 )
+from ess.bifrost.io.mcstas import load_sample_angle
 from ess.bifrost.live import BifrostQCutWorkflow, CutAxis, CutAxis1, CutAxis2, CutData
 from ess.spectroscopy.types import (
     Filename,
@@ -35,6 +36,10 @@ def qcut_workflow(
         self, simulation_detector_names: list[NeXusDetectorName]
     ) -> sciline.Pipeline:
         workflow = BifrostQCutWorkflow(simulation_detector_names)
+        # The test file is from a simulation and
+        # looks slightly different from production files.
+        workflow.insert(load_sample_angle)
+
         workflow[Filename[SampleRun]] = simulated_elastic_incoherent_with_phonon()
         workflow[LookupTableFilename] = lookup_table_simulation()
         workflow[LookupTableRelativeErrorThreshold] = {