diff --git a/rugnux/RugnuxDefaults.h b/rugnux/RugnuxDefaults.h
index 4ea9a6fa8..84ad9e7b0 100644
--- a/rugnux/RugnuxDefaults.h
+++ b/rugnux/RugnuxDefaults.h
@@ -45,7 +45,8 @@ void ApplyRugnuxExperimentDefaults(DiffractionExperiment &experiment);
 // search start de novo. A stored cell otherwise resolves the indexing algorithm to ffbidx and drives
 // it to that lattice, and a stored space group skips the search outright. The CLI calls this and then
 // applies -C / -S (or a reference MTZ) on top; the viewer instead takes the crystal straight from its
-// settings panel, which clears both fields when "Unit cell known" is unticked.
+// settings panel, which clears both fields when "Unit cell known" is unticked - as it is when a file is
+// opened.
 void ClearStoredCrystal(DiffractionExperiment &experiment);
 
 // Scaling/merging defaults for the workflow. Several differ between rotation and stills because the
diff --git a/viewer/JFJochViewerWindow.cpp b/viewer/JFJochViewerWindow.cpp
index 0b4cc8bf1..2f85ebd3b 100644
--- a/viewer/JFJochViewerWindow.cpp
+++ b/viewer/JFJochViewerWindow.cpp
@@ -627,6 +627,8 @@ JFJochViewerWindow::JFJochViewerWindow(QWidget *parent, bool dbus, const QString
             reading_worker, &JFJochImageReadingWorker::FindCenter);
     connect(this, &JFJochViewerWindow::datasetReady,
             settingsPanel, &JFJochViewerSettingsDock::datasetLoaded);
+    connect(reading_worker, &JFJochImageReadingWorker::fileOpened,
+            settingsPanel, &JFJochViewerSettingsDock::fileOpened);
     connect(this, &JFJochViewerWindow::imageReady,
             settingsPanel, &JFJochViewerSettingsDock::loadImage);
     connect(settingsPanel, &JFJochViewerSettingsDock::ringsFromCalibration,
diff --git a/viewer/widgets/JFJochViewerSettingsDock.cpp b/viewer/widgets/JFJochViewerSettingsDock.cpp
index 8ab69f720..1a7dc456a 100644
--- a/viewer/widgets/JFJochViewerSettingsDock.cpp
+++ b/viewer/widgets/JFJochViewerSettingsDock.cpp
@@ -1022,6 +1022,10 @@ void JFJochViewerSettingsDock::datasetLoaded(std::shared_ptr<const JFJochReaderD
     ApplyProcessingMode();   // the mode just read back decides rotation vs stills
 }
 
+void JFJochViewerSettingsDock::fileOpened() {
+    cellKnown_->setChecked(false);   // a change emits the experiment without the crystal
+}
+
 void JFJochViewerSettingsDock::ApplyProcessingMode() {
     // Rotation is the only mode that changes how the data is processed: the rotation good-path
     // (rotation indexing + Ewald partiality + rot3d combine + scale-fulls). A grid scan is a raster
diff --git a/viewer/widgets/JFJochViewerSettingsDock.h b/viewer/widgets/JFJochViewerSettingsDock.h
index 5cd774391..f6f9faf58 100644
--- a/viewer/widgets/JFJochViewerSettingsDock.h
+++ b/viewer/widgets/JFJochViewerSettingsDock.h
@@ -42,6 +42,9 @@ public:
 
 public slots:
     void datasetLoaded(std::shared_ptr<const JFJochReaderDataset> dataset);
+    // A new file: the crystal it stores is shown but not used until "Known unit cell" is ticked, as a
+    // bare `rugnux` forgets it (ClearStoredCrystal).
+    void fileOpened();
     void loadImage(std::shared_ptr<const JFJochReaderImage> image);
     void referenceLoaded(ReferenceMtzInfo info);   // worker reports a (un)loaded reference MTZ
     // Dataset re-processing reads a stored HDF5 file, so it is unavailable on a live HTTP stream:
diff --git a/viewer/windows/JFJochProcessingJobsWindow.cpp b/viewer/windows/JFJochProcessingJobsWindow.cpp
index 2a91d6725..d722e3010 100644
--- a/viewer/windows/JFJochProcessingJobsWindow.cpp
+++ b/viewer/windows/JFJochProcessingJobsWindow.cpp
@@ -432,10 +432,10 @@ void JFJochProcessingJobsWindow::newJob(ProcessMode mode, CalibrationSelection c
     //
     // The cell and space group come from the settings panel and nowhere else: with "Unit cell known"
     // ticked they are indexed and merged with, unticked the panel clears both and the run determines
-    // them from the data (-C / -S versus bare `rugnux`). The panel is filled from the file when one is
-    // opened, so what a run will use is always the crystal shown on screen - including after a finished
-    // job's _process.h5 becomes the active snapshot, where a run that ended in P1 shows P1 and can be
-    // cleared, rather than silently pinning every later run to it.
+    // them from the data (-C / -S versus bare `rugnux`). A newly opened file starts unticked - its stored
+    // crystal is shown but not used, as `rugnux` forgets it - so what a run uses is always the crystal
+    // shown on screen. A finished job's _process.h5 becoming the active snapshot does tick it with the
+    // run's crystal (a run that ended in P1 shows P1), which can be cleared before the next run.
     DiffractionExperiment experiment = inputs.experiment;
     const auto polarization = experiment.GetPolarizationFactor();
     ApplyRugnuxExperimentDefaults(experiment);