[ESSSPECTROSCOPY] Source the BIFROST Bragg peak monitor from the NXmonitor

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

@@ -29,7 +29,6 @@
     "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",
-    "from ess.reduce.unwrap import DetectorLtotal\n",
     "from ess.spectroscopy.types import *"
    ]
   },
@@ -113,24 +112,7 @@
     "    return McStasRawDetector[RunType](monitor)\n",
     "\n",
     "\n",
-    "def detector_ltotal_from_straight_line_approximation(\n",
-    "        detector_beamline: RawDetector[RunType],\n",
-    "        source_position: Position[snx.NXsource, RunType],\n",
-    "        sample_position: Position[snx.NXsample, RunType],\n",
-    "        gravity: GravityVector,\n",
-    ") -> DetectorLtotal[RunType]:\n",
-    "    from ess.reduce.unwrap import to_wavelength\n",
-    "    return to_wavelength.detector_ltotal_from_straight_line_approximation(\n",
-    "        detector_beamline,  # type: ignore[arg-type]\n",
-    "\n",
-    "        source_position=source_position,\n",
-    "        sample_position=sample_position,\n",
-    "        gravity=gravity,\n",
-    "    )\n",
-    "\n",
-    "\n",
-    "workflow.insert(assemble_detector_data_flatten)\n",
-    "workflow.insert(detector_ltotal_from_straight_line_approximation)"
+    "workflow.insert(assemble_detector_data_flatten)"
    ]
   },
   {

packages/essspectroscopy/src/ess/bifrost/detector.py

@@ -11,7 +11,6 @@
 from ess.spectroscopy.indirect.conversion import add_spectrometer_coords
 from ess.spectroscopy.types import (
     Analyzer,
-    DetectorPositionOffset,
     EmptyDetector,
     NeXusComponent,
     NeXusTransformation,
@@ -86,7 +85,6 @@ def get_calibrated_detector_bifrost(
     analyzer: Analyzer[RunType],
     *,
     transform: NeXusTransformation[snx.NXdetector, RunType],
-    offset: DetectorPositionOffset[RunType],
     primary_graph: PrimarySpecCoordTransformGraph[RunType],
     secondary_graph: SecondarySpecCoordTransformGraph[RunType],
 ) -> EmptyDetector[RunType]:
@@ -107,8 +105,6 @@ def get_calibrated_detector_bifrost(
         Loaded analyzer parameters.
     transform:
         Transformation that determines the detector position.
-    offset:
-        Offset to add to the detector position.
     primary_graph:
         Coordinate transformation graph for the primary spectrometer.
     secondary_graph:
@@ -122,9 +118,7 @@ def get_calibrated_detector_bifrost(
         Detector geometry and spectrometer coordinates.
     """
 
-    da = get_base_calibrated_detector_bifrost(
-        detector, analyzer, transform=transform, offset=offset
-    )
+    da = get_base_calibrated_detector_bifrost(detector, analyzer, transform=transform)
     da = da.rename(dim_0='tube', dim_1='length')
 
     arc, channel = arc_and_channel_from_detector_number(da.coords['detector_number'])
@@ -147,7 +141,6 @@ def get_base_calibrated_detector_bifrost(
     analyzer: Analyzer[RunType],
     *,
     transform: NeXusTransformation[snx.NXdetector, RunType],
-    offset: DetectorPositionOffset[RunType],
 ) -> sc.DataArray:
     """Extract the data array corresponding to a detector's signal field.
 
@@ -163,8 +156,6 @@ def get_base_calibrated_detector_bifrost(
         Loaded analyzer parameters.
     transform:
         Transformation that determines the detector position.
-    offset:
-        Offset to add to the detector position.
 
     Returns
     -------
@@ -173,9 +164,14 @@ def get_base_calibrated_detector_bifrost(
     """
 
     from ess.reduce.nexus import compute_detector_position, extract_signal_data_array
+    from ess.reduce.nexus.workflow import no_offset
 
     da = extract_signal_data_array(detector)
-    position = compute_detector_position(da, transform=transform, offset=offset)
+    # BIFROST's detectors ride the rotating tank, so a lab-frame offset added after
+    # the transform cannot express any correction one would actually want; the
+    # transformation chain is the handle. compute_detector_position requires the
+    # argument, so pin it to zero here.
+    position = compute_detector_position(da, transform=transform, offset=no_offset)
     return _assign_detector_position(da, position)
 
 

packages/essspectroscopy/src/ess/bifrost/single_crystal/detector.py

@@ -1,29 +1,100 @@
 # SPDX-License-Identifier: BSD-3-Clause
 # Copyright (c) 2026 Scipp contributors (https://github.com/scipp)
 
-"""Bragg peak detector handling for BIFROST."""
+"""Bragg peak monitor handling for BIFROST."""
 
+import scipp as sc
 import scippnexus as snx
 from ess.spectroscopy.types import (
     Analyzer,
-    DetectorPositionOffset,
+    ElasticMonitor,
     EmptyDetector,
     NeXusComponent,
+    NeXusData,
     NeXusTransformation,
+    RawDetector,
     RunType,
 )
 
-from ..detector import get_base_calibrated_detector_bifrost
+from ..detector import _assign_detector_position, get_base_calibrated_detector_bifrost
+
+
+def get_calibrated_bragg_peak_monitor(
+    monitor: NeXusComponent[ElasticMonitor, RunType],
+    *,
+    transform: NeXusTransformation[ElasticMonitor, RunType],
+) -> EmptyDetector[RunType]:
+    """Extract the data array corresponding to the Bragg peak monitor's signal field.
+
+    BIFROST's Bragg peak monitor is the elastic monitor, named ``elastic_monitor``
+    in NeXus files (``cbm5`` in instrument documentation) and written as an
+    ``NXmonitor``. It has no pixel offsets, so its position is the transformed origin
+    as in :func:`ess.reduce.nexus.workflow.get_calibrated_monitor`, rather than the
+    per-pixel computation used for detectors. The position is assigned with the
+    BIFROST-specific broadcasting because the monitor is mounted on the detector tank
+    and therefore moves with the instrument angle.
+
+    Parameters
+    ----------
+    monitor:
+        Loaded NeXus monitor.
+    transform:
+        Transformation that determines the monitor position.
+
+    Returns
+    -------
+    :
+        Monitor with geometry coordinates.
+    """
+    from ess.reduce.nexus import extract_signal_data_array
+
+    da = extract_signal_data_array(monitor)
+    unit = transform.value.unit
+    position = transform.value * sc.vector([0.0, 0.0, 0.0], unit=unit)
+    return EmptyDetector[RunType](_assign_detector_position(da, position))
+
+
+def assemble_bragg_peak_monitor_data(
+    monitor: EmptyDetector[RunType],
+    data: NeXusData[ElasticMonitor, RunType],
+) -> RawDetector[RunType]:
+    """Combine the Bragg peak monitor's geometry with its event data.
+
+    Assembled as a monitor, not as a detector: ``assemble_detector_data`` groups
+    events by ``event_id`` onto a ``detector_number`` grid, and the Bragg peak
+    monitor is a single pixel with nothing to group by. Its geometry is therefore
+    assigned straight onto the events. The result is the same whether or not the
+    events carry an ``event_id`` -- the event group in a file does, a stream may
+    not -- because the coordinate is simply left untouched.
+
+    Parameters
+    ----------
+    monitor:
+        Monitor geometry from :func:`get_calibrated_bragg_peak_monitor`.
+    data:
+        Monitor event data.
+
+    Returns
+    -------
+    :
+        Events with geometry coordinates.
+    """
+    from ess.reduce.nexus.workflow import assemble_monitor_data
+
+    return RawDetector[RunType](assemble_monitor_data(monitor, data))
 
 
 def get_calibrated_bragg_peak_detector(
     detector: NeXusComponent[snx.NXdetector, RunType],
     analyzer: Analyzer[RunType],
     *,
     transform: NeXusTransformation[snx.NXdetector, RunType],
-    offset: DetectorPositionOffset[RunType],
 ) -> EmptyDetector[RunType]:
-    """Extract the data array corresponding to the Bragg peak detector's signal field.
+    """Extract the data array corresponding to a detector's signal field.
+
+    Simulated data contains no Bragg peak monitor, so a bank ('triplet') of the
+    regular inelastic detector stands in for it. Real data uses
+    :func:`get_calibrated_bragg_peak_monitor` instead.
 
     Parameters
     ----------
@@ -33,17 +104,14 @@ def get_calibrated_bragg_peak_detector(
         Loaded analyzer parameters.
     transform:
         Transformation that determines the detector position.
-    offset:
-        Offset to add to the detector position.
 
     Returns
     -------
     :
         Detector with geometry coordinates.
     """
-    return get_base_calibrated_detector_bifrost(
-        detector, analyzer, transform=transform, offset=offset
-    )
+    return get_base_calibrated_detector_bifrost(detector, analyzer, transform=transform)
 
 
-providers = (get_calibrated_bragg_peak_detector,)
+providers = (get_calibrated_bragg_peak_monitor, assemble_bragg_peak_monitor_data)
+simulation_providers = (get_calibrated_bragg_peak_detector,)

packages/essspectroscopy/src/ess/bifrost/single_crystal/time_of_flight.py

@@ -6,7 +6,10 @@
 import scippnexus as snx
 from ess.spectroscopy.types import (
     DataGroupedByRotation,
+    EmptyDetector,
     ErrorLimitedLookupTable,
+    GravityVector,
+    Position,
     PulseStrideOffset,
     RawDetector,
     RunType,
@@ -17,6 +20,36 @@
 from ess.reduce.unwrap.types import DetectorLtotal
 
 
+def detector_ltotal(
+    sample_data: DataGroupedByRotation[RunType],
+    source_position: Position[snx.NXsource, RunType],
+    sample_position: Position[snx.NXsample, RunType],
+    gravity: GravityVector,
+) -> DetectorLtotal[RunType]:
+    """
+    Compute Ltotal from the straight-line approximation.
+
+    The Bragg peak monitor views the beam direct from the sample, without an analyzer
+    in between, so a straight line is the correct flight path.
+
+    Computed after grouping so that ``Ltotal`` carries the dimensions of the grouped
+    data. BIFROST's tank rotates and the Bragg peak monitor is mounted on it, so the
+    monitor position -- and hence ``Ltotal`` -- is time-dependent; ``group_by_rotation``
+    turns that 'time' dimension into 'a4'. Deriving ``Ltotal`` from the ungrouped
+    geometry instead would leave it labelled with a dimension the data no longer has.
+
+    This is a wrapper around
+    :func:`ess.reduce.unwrap.detector_ltotal_from_straight_line_approximation`
+    for different input types.
+    """
+    return reduce_unwrap.to_wavelength.detector_ltotal_from_straight_line_approximation(
+        detector=EmptyDetector[RunType](sample_data),
+        source_position=source_position,
+        sample_position=sample_position,
+        gravity=gravity,
+    )
+
+
 def detector_wavelength_data(
     sample_data: DataGroupedByRotation[RunType],
     lookup: ErrorLimitedLookupTable[RunType, snx.NXdetector],
@@ -40,4 +73,4 @@ def detector_wavelength_data(
     )
 
 
-providers = (detector_wavelength_data,)
+providers = (detector_ltotal, detector_wavelength_data)

packages/essspectroscopy/src/ess/bifrost/single_crystal/workflow.py

@@ -10,7 +10,7 @@
     SampleRun,
 )
 
-from ess.reduce import unwrap as reduce_unwrap
+from ess.reduce.nexus.types import NeXusName
 
 from ..cutting import group_by_rotation
 from ..io import nexus
@@ -30,7 +30,7 @@
 _SIMULATION_PROVIDERS = (
     *nexus.providers,
     *conversion.providers,
-    *detector.providers,
+    *detector.simulation_providers,
     *q_map.providers,
     *time_of_flight.providers,
     convert_simulated_time_to_event_time_offset,
@@ -43,8 +43,7 @@ def BifrostBraggPeakMonitorWorkflow() -> sciline.Pipeline:
         run_types=(SampleRun,),
         monitor_types=(ElasticMonitor, NormalizationMonitor),
     )
-    # Use the vanilla implementation instead of the indirect geometry one:
-    workflow.insert(reduce_unwrap.to_wavelength.detector_wavelength_data)
+    workflow[NeXusName[ElasticMonitor]] = 'elastic_monitor'
     for provider in _PROVIDERS:
         workflow.insert(provider)
     for key, val in default_parameters().items():
@@ -57,8 +56,7 @@ def BifrostSimulationBraggPeakMonitorWorkflow() -> sciline.Pipeline:
         run_types=(SampleRun,),
         monitor_types=(ElasticMonitor, NormalizationMonitor),
     )
-    # Use the vanilla implementation instead of the indirect geometry one:
-    workflow.insert(reduce_unwrap.to_wavelength.detector_wavelength_data)
+    workflow[NeXusName[ElasticMonitor]] = 'elastic_monitor'
     for provider in _SIMULATION_PROVIDERS:
         workflow.insert(provider)
     for key, val in simulation_default_parameters().items():