MAINT: Warn when fitting cHPI amplitudes on Maxwell filtered data

mne/chpi.py

@@ -624,6 +624,16 @@ def _setup_hpi_amplitude_fitting(
     on_missing = "raise" if not allow_empty else "ignore"
     hpi_freqs, hpi_pick, hpi_ons = get_chpi_info(info, on_missing=on_missing)
 
+    # check for maxwell filtering
+    for ent in info["proc_history"]:
+        for key in ("sss_info", "max_st"):
+            if len(ent["max_info"]["sss_info"]) > 0:
+                warn(
+                    "Fitting cHPI amplutudes after Maxwell filtering may not to work, "
+                    "consider fitting on the original data"
+                )
+                break
+
     _validate_type(t_window, (str, "numeric"), "t_window")
     if info["line_freq"] is not None:
         line_freqs = np.arange(

mne/tests/test_chpi.py

@@ -422,6 +422,15 @@ def test_calculate_chpi_positions_artemis():
     )
 
 
+@testing.requires_testing_data
+def test_warn_maxwell_filtered():
+    """Test that trying to compute locations on Maxwell filtered data warns."""
+    raw = read_raw_fif(sss_fif_fname).crop(0, 1)
+    with pytest.warns(RuntimeWarning, match="Maxwell filter"):
+        amps = compute_chpi_amplitudes(raw)
+    assert len(amps["times"]) > 0  # but for this file, it does work!
+
+
 @testing.requires_testing_data
 def test_initial_fit_redo():
     """Test that initial fits can be redone based on moments."""