Small fixes to DiskChopper.from_nexus

src/scippneutron/chopper/disk_chopper.py

@@ -334,9 +334,9 @@ def from_nexus(
             )
         return DiskChopper(
             axle_position=chopper['position'],
-            frequency=_get_1d_variable(chopper, 'rotation_speed'),
-            beam_position=_get_1d_variable(chopper, 'beam_position'),
-            phase=_get_1d_variable(chopper, 'phase'),
+            frequency=_get_0d_variable(chopper, 'rotation_speed'),
+            beam_position=_get_0d_variable(chopper, 'beam_position'),
+            phase=_get_0d_variable(chopper, 'phase'),
             slit_height=chopper.get('slit_height'),
             radius=chopper.get('radius'),
             **_get_edges_from_nexus(chopper),
@@ -557,6 +557,10 @@ def _source_phase_factor(self, pulse_frequency: sc.Variable) -> int:
         # of the slits, so use `max` here:
         return round(max(quot.value, 1))
 
+    def as_dict(self) -> dict[str, Any]:
+        """Return the DiskChopper fields as a dictionary."""
+        return dataclasses.asdict(self)
+
 
 def _field_eq(a: Any, b: Any) -> bool:
     if isinstance(a, sc.Variable | sc.DataArray):
@@ -627,13 +631,7 @@ def _get_edges_from_nexus(
     }
 
 
-def _len_or_1(x: sc.Variable) -> int:
-    if x.ndim == 0:
-        return 1
-    return len(x)
-
-
-def _get_1d_variable(
+def _get_0d_variable(
     dg: Mapping[str, sc.Variable | sc.DataArray], name: str
 ) -> sc.Variable:
     if (val := dg.get(name)) is None:
@@ -642,9 +640,11 @@ def _get_1d_variable(
     msg = (
         "Chopper field '{name}' must be a scalar variable, {got}. "
         "See the chopper user-guide for more information: "
-        "https://scipp.github.io/scippneutron/user-guide/chopper/pre-processing.html"
+        "https://scipp.github.io/scippneutron/user-guide/chopper/processing-nexus-choppers.html"
     )
 
+    if isinstance(val, sc.DataArray):
+        val = val.data
     if not isinstance(val, sc.Variable):
         raise TypeError(msg.format(name=name, got=f'got a {type(val)}'))
     if val.ndim != 0: