diff --git a/viewer/JFJochImageReadingWorker.cpp b/viewer/JFJochImageReadingWorker.cpp
index a1ce8a26d..55661f511 100644
--- a/viewer/JFJochImageReadingWorker.cpp
+++ b/viewer/JFJochImageReadingWorker.cpp
@@ -352,8 +352,17 @@ void JFJochImageReadingWorker::LoadFile_i(const QString &filename, qint64 image_
         }
 
         current_image.reset();
+        current_image_ptr.reset();   // the previous file's image must not be re-emitted against this dataset
         current_summation = 1;
         current_file = filename;
+        // An image number carried over from the previous dataset (e.g. a D-Bus caller keeping its
+        // position) may not exist in this one: open the file at its first image instead of showing none.
+        if (!http_mode && image_number >= 0 && image_number + summation > total_images) {
+            logger.Warning("Image {} out of range for {} ({} images), showing image 0",
+                           image_number, filename.toStdString(), total_images);
+            image_number = 0;
+            summation = 1;
+        }
         if (dataset) {
             curr_experiment = dataset->experiment;
             roi_override_.reset();   // new file: use its ROIs, forget any earlier edits
@@ -581,7 +590,11 @@ void JFJochImageReadingWorker::UpdateDataset_i(const std::optional<DiffractionEx
 
 void JFJochImageReadingWorker::UpdateDataset(const DiffractionExperiment &experiment) {
     QMutexLocker ul(&m);
-    UpdateDataset_i(experiment);
+    try {
+        UpdateDataset_i(experiment);
+    } catch (const std::exception &e) {
+        logger.Error("Dataset update failed: {}", e.what());
+    }
 }
 
 void JFJochImageReadingWorker::ReanalyzeImage_i() {
@@ -602,7 +615,12 @@ void JFJochImageReadingWorker::ReanalyzeImage_i() {
     // Azimuthal profile for the analysis/display pipeline. AzimuthalIntegrationProfile
     // holds a mutex (non-copyable), so keep it via unique_ptr re-created each analysis.
     last_profile_ = std::make_unique<AzimuthalIntegrationProfile>(*azint_mapping);
-    image_analysis->Analyze(new_image->ImageData(), *last_profile_, spot_finding_settings);
+    try {
+        image_analysis->Analyze(new_image->ImageData(), *last_profile_, spot_finding_settings);
+    } catch (const std::exception &e) {
+        logger.Error("Analysis of image failed: {}", e.what());
+        return;
+    }
 
     current_image_ptr = new_image;
     auto end_time = std::chrono::high_resolution_clock::now();
@@ -624,27 +642,28 @@ void JFJochImageReadingWorker::FindCenter(QVector<float> ring_q, bool guess, boo
 
     DiffractionGeometry geom = current_image_ptr->Dataset().experiment.GetDiffractionGeometry();
     const std::vector<float> ring_Q(ring_q.begin(), ring_q.end());
+    // A diverged fit is rejected by the setters (NaN beam, sub-mm distance), so they sit in the try too.
     try {
         if (guess)
             GuessGeometry(geom, current_image_ptr->ImageData().spots, ring_Q, refine_tilt);
         else
             OptimizeGeometry(geom, current_image_ptr->ImageData().spots, ring_Q, refine_tilt);
-    } catch (const JFJochException &e) {
-        logger.ErrorException(e);
+
+        logger.Info("Geometry found X: {} pxl Y: {} pxl Dist: {} mm", geom.GetBeamX_pxl(), geom.GetBeamY_pxl(),
+                    geom.GetDetectorDistance_mm());
+
+        DiffractionExperiment new_experiment = current_image_ptr->Dataset().experiment;
+        new_experiment.BeamX_pxl(geom.GetBeamX_pxl()).BeamY_pxl(geom.GetBeamY_pxl())
+                .DetectorDistance_mm(geom.GetDetectorDistance_mm())
+                .PoniRot1_rad(geom.GetPoniRot1_rad())
+                .PoniRot2_rad(geom.GetPoniRot2_rad())
+                .PoniRot3_rad(geom.GetPoniRot3_rad());
+        UpdateDataset_i(new_experiment);
+    } catch (const std::exception &e) {
+        logger.Error("Finding beam center failed: {}", e.what());
         return;
     }
 
-    logger.Info("Geometry found X: {} pxl Y: {} pxl Dist: {} mm", geom.GetBeamX_pxl(), geom.GetBeamY_pxl(),
-                geom.GetDetectorDistance_mm());
-
-    DiffractionExperiment new_experiment = current_image_ptr->Dataset().experiment;
-    new_experiment.BeamX_pxl(geom.GetBeamX_pxl()).BeamY_pxl(geom.GetBeamY_pxl())
-            .DetectorDistance_mm(geom.GetDetectorDistance_mm())
-            .PoniRot1_rad(geom.GetPoniRot1_rad())
-            .PoniRot2_rad(geom.GetPoniRot2_rad())
-            .PoniRot3_rad(geom.GetPoniRot3_rad());
-    UpdateDataset_i(new_experiment);
-
     QVector<float> rings;
     for (int i = 0; i < 15 && i < ring_Q.size(); i++) {
         rings.push_back(2 * PI / ring_Q[i]);
@@ -695,14 +714,29 @@ void JFJochImageReadingWorker::ReanalyzeImages(bool input) {
 
 void JFJochImageReadingWorker::UpdateAzintSettings(const AzimuthalIntegrationSettings &settings) {
     QMutexLocker locker(&m);
+    // Checked here, not only where the mapping is built: settings that cannot be mapped would
+    // otherwise be kept and fail every later file open and dataset update.
+    if (settings.GetBinCount() >= UINT16_MAX) {
+        logger.Error("Azimuthal integration settings ignored: {} bins, at most 65534 supported",
+                     settings.GetBinCount());
+        return;
+    }
     azint_settings = settings;
-    UpdateDataset_i(std::nullopt);
+    try {
+        UpdateDataset_i(std::nullopt);
+    } catch (const std::exception &e) {
+        logger.Error("Azimuthal integration update failed: {}", e.what());
+    }
 }
 
 void JFJochImageReadingWorker::UpdateBraggIntegrationSettings(BraggIntegrationSettings settings) {
     QMutexLocker locker(&m);
     bragg_settings = settings;
-    UpdateDataset_i(std::nullopt);
+    try {
+        UpdateDataset_i(std::nullopt);
+    } catch (const std::exception &e) {
+        logger.Error("Bragg integration update failed: {}", e.what());
+    }
 }
 
 void JFJochImageReadingWorker::UpdateScalingSettings(ScalingSettings settings) {
@@ -778,7 +812,11 @@ void JFJochImageReadingWorker::EmitReferenceInfo_i() {
 
 void JFJochImageReadingWorker::SetROIDefinition(const ROIDefinition &rois) {
     QMutexLocker locker(&m);
-    SetROIDefinition_i(rois);
+    try {
+        SetROIDefinition_i(rois);
+    } catch (const std::exception &e) {
+        logger.Error("ROI update failed: {}", e.what());
+    }
 }
 
 void JFJochImageReadingWorker::MaskFromSelectedROI(QString name, bool add) {
@@ -879,25 +917,30 @@ void JFJochImageReadingWorker::ApplyROIOverrideToImage_i() {
 
 void JFJochImageReadingWorker::UpdateUserMask_i(const std::vector<uint32_t> &mask) {
     std::shared_ptr<const JFJochReaderDataset> dataset;
-    if (http_mode) {
-        http_reader.UpdateUserMask(mask);
-        dataset = http_reader.GetDataset();
-    } else {
-        file_reader->UpdateUserMask(mask);
-        dataset = file_reader->GetDataset();
-    }
+    try {
+        if (http_mode) {
+            http_reader.UpdateUserMask(mask);
+            dataset = http_reader.GetDataset();
+        } else {
+            file_reader->UpdateUserMask(mask);
+            dataset = file_reader->GetDataset();
+        }
 
-    if (!dataset) {
-        logger.Error("UpdateUserMask_i: dataset is null (http_mode={}) - skipping update to avoid crash", http_mode);
+        if (!dataset) {
+            logger.Error("UpdateUserMask_i: dataset is null (http_mode={}) - skipping update to avoid crash", http_mode);
+            return;
+        }
+
+        UpdateAzint_i(dataset.get());
+
+        EmitDatasetLoaded_i(dataset);
+
+        current_image_ptr = std::make_shared<JFJochReaderImage>(current_image_ptr->ImageData(), dataset);
+    } catch (const std::exception &e) {
+        logger.Error("User mask update failed: {}", e.what());
         return;
     }
 
-    UpdateAzint_i(dataset.get());
-
-    EmitDatasetLoaded_i(dataset);
-
-    current_image_ptr = std::make_shared<JFJochReaderImage>(current_image_ptr->ImageData(), dataset);
-
     if (current_image.has_value())
         LoadImage_i(current_image.value(), current_summation);
 }
@@ -918,7 +961,11 @@ void JFJochImageReadingWorker::SaveUserMaskTIFF(QString filename) {
         return;
     auto user_mask = current_image_ptr->Dataset().pixel_mask->GetUserMask();
     CompressedImage mask_image(user_mask, current_image_ptr->Dataset().experiment.GetXPixelsNum(), current_image_ptr->Dataset().experiment.GetYPixelsNum());
-    WriteTIFFToFile(filename.toStdString(), mask_image);
+    try {
+        WriteTIFFToFile(filename.toStdString(), mask_image);
+    } catch (const std::exception &e) {
+        logger.Error("Could not write mask TIFF {}: {}", filename.toStdString(), e.what());
+    }
 }
 
 void JFJochImageReadingWorker::LoadUserMaskTIFF(QString filename, bool replace) {
@@ -961,8 +1008,15 @@ void JFJochImageReadingWorker::UploadUserMask() {
         return;
 
     auto user_mask = current_image_ptr->Dataset().pixel_mask->GetUserMask();
-    if (http_mode)
+    if (!http_mode)
+        return;
+    try {
         http_reader.UploadUserMask(user_mask);
+    } catch (const HttpUnauthorized &) {
+        HandleUnauthorized_i(current_file);
+    } catch (const std::exception &e) {
+        logger.Error("Upload of user mask failed: {}", e.what());
+    }
 }
 
 void JFJochImageReadingWorker::LoadCalibration(QString dataset) {
@@ -1265,7 +1319,11 @@ void JFJochImageReadingWorker::RemoveRun(QString id) {
     }
     run_labels_.erase(id.toStdString());
     // Refresh the (possibly now-Original) active view; ActivateSnapshot_i also re-emits runsChanged.
-    ActivateSnapshot_i(QString::fromStdString(ActiveRun_i()));
+    try {
+        ActivateSnapshot_i(QString::fromStdString(ActiveRun_i()));
+    } catch (const std::exception &e) {
+        logger.Error("Could not activate run after removal: {}", e.what());
+    }
 }
 
 void JFJochImageReadingWorker::RegisterProcessingSnapshot(QString id, QString label, QString master_path) {
diff --git a/viewer/JFJochViewerMenu.cpp b/viewer/JFJochViewerMenu.cpp
index ac5699c89..f25a4f804 100644
--- a/viewer/JFJochViewerMenu.cpp
+++ b/viewer/JFJochViewerMenu.cpp
@@ -327,7 +327,7 @@ void JFJochViewerMenu::openHttpSelected() {
 
     char *port_std = std::getenv("JUNGFRAUJOCH_HTTP_PORT");
     if (port_std != nullptr) {
-        const int tmp = std::stoi(std::string(port_std));
+        const int tmp = QString(port_std).toInt();   // 0 for a non-number, unlike std::stoi which throws
         if (tmp < 65536 && tmp > 0)
             portSpinBox->setValue(tmp);
     } else
diff --git a/viewer/image_viewer/JFJochAzIntImage.cpp b/viewer/image_viewer/JFJochAzIntImage.cpp
index 7ccf3ce45..ac69df000 100644
--- a/viewer/image_viewer/JFJochAzIntImage.cpp
+++ b/viewer/image_viewer/JFJochAzIntImage.cpp
@@ -107,7 +107,11 @@ void JFJochAzIntImage::mouseDoubleClickEvent(QMouseEvent *event) {
         float phi = image->Dataset().az_int_bin_to_phi[idx];
 
         auto geom = image->Dataset().experiment.GetDiffractionGeometry();
-        auto coord = geom.ResPhiToPxl(2 * PI / q, phi / 180.0 * PI);
-        emit zoomOnBin(QPointF(coord.first, coord.second));
+        try {
+            auto coord = geom.ResPhiToPxl(2 * PI / q, phi / 180.0 * PI);
+            emit zoomOnBin(QPointF(coord.first, coord.second));
+        } catch (const std::exception &) {
+            // a bin beyond 2 theta = 180 deg (manual High Q above 4 pi / lambda) is on no pixel
+        }
     }
 }
\ No newline at end of file
diff --git a/viewer/image_viewer/JFJochDiffractionImage.cpp b/viewer/image_viewer/JFJochDiffractionImage.cpp
index 479fec07d..37a14a8c1 100644
--- a/viewer/image_viewer/JFJochDiffractionImage.cpp
+++ b/viewer/image_viewer/JFJochDiffractionImage.cpp
@@ -834,11 +834,14 @@ void JFJochDiffractionImage::roiEditMove(const QPointF &scenePos) {
         case RoiEdit::ResizeCircle:
             edit_radius_ = std::max(1.0, QLineF(edit_center_, scenePos).length());
             break;
+        // PxlToRes is infinite on the beam centre, which ROIAzimuthal rejects: keep the last finite value
         case RoiEdit::AzimInner:
-            edit_d_max_ = geom.PxlToRes(cursor_r);
+            if (const float d = geom.PxlToRes(cursor_r); std::isfinite(d))
+                edit_d_max_ = d;
             break;
         case RoiEdit::AzimOuter:
-            edit_d_min_ = geom.PxlToRes(cursor_r);
+            if (const float d = geom.PxlToRes(cursor_r); std::isfinite(d))
+                edit_d_min_ = d;
             break;
         case RoiEdit::RotatePhiMin:
             edit_phi_min_ = geom.Phi_rad(scenePos.x(), scenePos.y()) * 180.0f / static_cast<float>(PI);
diff --git a/viewer/image_viewer/JFJochGridScanImage.cpp b/viewer/image_viewer/JFJochGridScanImage.cpp
index ea6601f90..cfcf5adb0 100644
--- a/viewer/image_viewer/JFJochGridScanImage.cpp
+++ b/viewer/image_viewer/JFJochGridScanImage.cpp
@@ -155,7 +155,10 @@ void JFJochGridScanImage::mouseDoubleClickEvent(QMouseEvent *event) {
 }
 
 void JFJochGridScanImage::setImage(int64_t val) {
-    if (settings) {
+    // The image can lie outside the grid: live, the broker's image and dataset are fetched apart and
+    // can belong to different collections, and a re-emitted image can meet a newer dataset's grid.
+    // Such an image has no cell to mark (GetElementPos*_step would throw on the GUI thread).
+    if (settings && val >= 0 && val < settings->GetNElem()) {
         current_image_W = settings->GetElementPosX_step(val);
         current_image_H = settings->GetElementPosY_step(val);
     } else {
diff --git a/viewer/widgets/JFJochViewerROIList.cpp b/viewer/widgets/JFJochViewerROIList.cpp
index ab4a701b3..cf3dbf23d 100644
--- a/viewer/widgets/JFJochViewerROIList.cpp
+++ b/viewer/widgets/JFJochViewerROIList.cpp
@@ -114,29 +114,35 @@ void JFJochViewerROIList::ApplyEditor() {
     const std::string sel = SelectedName().toStdString();
     bool changed = false;
 
-    for (auto &b : rois_.boxes)
-        if (b.GetName() == sel) {
-            b = ROIBox(sel, fedit_[0]->text().toLongLong(), fedit_[1]->text().toLongLong(),
-                            fedit_[2]->text().toLongLong(), fedit_[3]->text().toLongLong());
-            changed = true; break;
-        }
-    for (auto &c : rois_.circles)
-        if (c.GetName() == sel) {
-            c = ROICircle(sel, fedit_[0]->text().toDouble(), fedit_[1]->text().toDouble(),
-                               std::max(0.1, fedit_[2]->text().toDouble()));
-            changed = true; break;
-        }
-    for (auto &a : rois_.azimuthal)
-        if (a.GetName() == sel) {
-            const float qmin = fedit_[0]->text().toFloat();
-            const float qmax = fedit_[1]->text().toFloat();
-            const float phimin = fedit_[2]->text().toFloat();
-            const float phimax = fedit_[3]->text().toFloat();
-            const float d_min = (qmax > 0) ? 2.0f * static_cast<float>(PI) / qmax : a.GetDMin_A();
-            const float d_max = (qmin > 0) ? 2.0f * static_cast<float>(PI) / qmin : a.GetDMax_A();
-            a = ROIAzimuthal(sel, d_min, d_max, phimin, phimax);
-            changed = true; break;
-        }
+    // The ROI constructors reject what QString::toDouble lets through ("inf", "nan"); an exception
+    // escaping a slot terminates the viewer, so such an entry is just not applied.
+    try {
+        for (auto &b : rois_.boxes)
+            if (b.GetName() == sel) {
+                b = ROIBox(sel, fedit_[0]->text().toLongLong(), fedit_[1]->text().toLongLong(),
+                                fedit_[2]->text().toLongLong(), fedit_[3]->text().toLongLong());
+                changed = true; break;
+            }
+        for (auto &c : rois_.circles)
+            if (c.GetName() == sel) {
+                c = ROICircle(sel, fedit_[0]->text().toDouble(), fedit_[1]->text().toDouble(),
+                                   std::max(0.1, fedit_[2]->text().toDouble()));
+                changed = true; break;
+            }
+        for (auto &a : rois_.azimuthal)
+            if (a.GetName() == sel) {
+                const float qmin = fedit_[0]->text().toFloat();
+                const float qmax = fedit_[1]->text().toFloat();
+                const float phimin = fedit_[2]->text().toFloat();
+                const float phimax = fedit_[3]->text().toFloat();
+                const float d_min = (qmax > 0) ? 2.0f * static_cast<float>(PI) / qmax : a.GetDMin_A();
+                const float d_max = (qmin > 0) ? 2.0f * static_cast<float>(PI) / qmin : a.GetDMax_A();
+                a = ROIAzimuthal(sel, d_min, d_max, phimin, phimax);
+                changed = true; break;
+            }
+    } catch (const std::exception &) {
+        return;
+    }
 
     if (changed)
         emit roisChanged(rois_);
diff --git a/viewer/widgets/JFJochViewerSettingsDock.cpp b/viewer/widgets/JFJochViewerSettingsDock.cpp
index 2b0364779..ea037df2b 100644
--- a/viewer/widgets/JFJochViewerSettingsDock.cpp
+++ b/viewer/widgets/JFJochViewerSettingsDock.cpp
@@ -230,9 +230,9 @@ QWidget *JFJochViewerSettingsDock::BuildMXPage() {
     cellA_ = new NumberLineEdit(1.0, 2000.0, 79.0, 3, "", page);
     cellB_ = new NumberLineEdit(1.0, 2000.0, 79.0, 3, "", page);
     cellC_ = new NumberLineEdit(1.0, 2000.0, 38.0, 3, "", page);
-    cellAlpha_ = new NumberLineEdit(1.0, 179.0, 90.0, 2, "", page);
-    cellBeta_ = new NumberLineEdit(1.0, 179.0, 90.0, 2, "", page);
-    cellGamma_ = new NumberLineEdit(1.0, 179.0, 90.0, 2, "", page);
+    cellAlpha_ = new NumberLineEdit(5.0, 179.0, 90.0, 2, "", page);
+    cellBeta_ = new NumberLineEdit(5.0, 179.0, 90.0, 2, "", page);
+    cellGamma_ = new NumberLineEdit(5.0, 179.0, 90.0, 2, "", page);
     spaceGroup_ = new NumberLineEdit(0.0, 230.0, 0.0, 0, "", page);
     spaceGroup_->setToolTip("Space group number (1–230); 0 = unset.");
     spaceGroupName_ = new QLabel("—", page);
@@ -682,12 +682,20 @@ QWidget *JFJochViewerSettingsDock::BuildAzIntSection() {
     az->addRow("Azimuthal bins", azimBins);
     azSection->setContentLayout(az);
 
+    // A combination can still be invalid (Low Q above High Q): the setters throw, and an exception
+    // escaping a slot terminates the viewer. Build on a copy and keep the previous settings instead.
     auto emitAz = [=, this] {
-        azint_.QRange_recipA(static_cast<float>(lowQ->value()),
-                             autoHighQ->isChecked() ? std::nullopt
-                                                    : std::optional<float>(highQ->value()));
-        azint_.QSpacing_recipA(static_cast<float>(spacing->value()));
-        azint_.AzimuthalBinCount(azimBins->currentData().toInt());
+        AzimuthalIntegrationSettings s = azint_;
+        try {
+            s.QRange_recipA(static_cast<float>(lowQ->value()),
+                            autoHighQ->isChecked() ? std::nullopt
+                                                   : std::optional<float>(highQ->value()));
+            s.QSpacing_recipA(static_cast<float>(spacing->value()));
+            s.AzimuthalBinCount(azimBins->currentData().toInt());
+        } catch (const std::exception &) {
+            return;
+        }
+        azint_ = s;
         emit azintChanged(azint_);
     };
     connect(lowQ, &SliderPlusBox::valueChanged, this, [emitAz] { emitAz(); });
@@ -792,7 +800,7 @@ QWidget *JFJochViewerSettingsDock::BuildBraggSection() {
     auto *gaussian = new QCheckBox("Gaussian profile fit", this);
     gaussian->setChecked(bragg_.GetIntegrator() != IntegratorMode::BoxSum);
     gaussian->setToolTip("Profile-fit the spots (more accurate intensities); off = classical box-sum.");
-    auto *r1 = new NumberLineEdit(1.0f, 30.0f, bragg_.GetR1(), 1, "", this);
+    auto *r1 = new NumberLineEdit(1.0f, 20.0f, bragg_.GetR1(), 1, "", this);
     auto *r2 = new NumberLineEdit(1.0f, 30.0f, bragg_.GetR2(), 1, "", this);
     auto *r3 = new NumberLineEdit(1.0f, 30.0f, bragg_.GetR3(), 1, "", this);
     auto *radii = new QHBoxLayout();
@@ -817,15 +825,23 @@ QWidget *JFJochViewerSettingsDock::BuildBraggSection() {
     form->addRow(trimRow);
     section->setContentLayout(form);
 
+    // The radii must grow r1 < r2 < r3, which a single typed value can break: the setters throw, and an
+    // exception escaping a slot terminates the viewer. Build on a copy and keep the previous settings.
     auto emitBragg = [=, this] {
-        bragg_.Integrator(gaussian->isChecked() ? IntegratorMode::ProfileGaussian : IntegratorMode::BoxSum);
-        bragg_.R1(static_cast<float>(r1->value())).R2(static_cast<float>(r2->value()))
-              .R3(static_cast<float>(r3->value()));
-        // Either estimator clears the other, so unchecking has to put the default clip back explicitly.
-        if (bkgTrim->isChecked())
-            bragg_.BackgroundTrimFraction(static_cast<float>(bkgTrimFrac->value()));
-        else
-            bragg_.BackgroundClipNSigma(BraggIntegrationSettings().GetBackgroundClipNSigma());
+        BraggIntegrationSettings s = bragg_;
+        try {
+            s.Integrator(gaussian->isChecked() ? IntegratorMode::ProfileGaussian : IntegratorMode::BoxSum);
+            s.R1(static_cast<float>(r1->value())).R2(static_cast<float>(r2->value()))
+             .R3(static_cast<float>(r3->value()));
+            // Either estimator clears the other, so unchecking has to put the default clip back explicitly.
+            if (bkgTrim->isChecked())
+                s.BackgroundTrimFraction(static_cast<float>(bkgTrimFrac->value()));
+            else
+                s.BackgroundClipNSigma(BraggIntegrationSettings().GetBackgroundClipNSigma());
+        } catch (const std::exception &) {
+            return;
+        }
+        bragg_ = s;
         emit braggChanged(bragg_);
     };
     connect(gaussian, &QCheckBox::toggled, this, [emitBragg] { emitBragg(); });
@@ -909,27 +925,35 @@ QWidget *JFJochViewerSettingsDock::BuildScalingSection() {
 void JFJochViewerSettingsDock::EmitExperiment() {
     if (!have_experiment_)
         return;
-    experiment_.IncidentEnergy_keV(static_cast<float>(energy_->value()));
-    experiment_.DetectorDistance_mm(static_cast<float>(distance_->value()));
-    experiment_.BeamX_pxl(static_cast<float>(beamX_->value()));
-    experiment_.BeamY_pxl(static_cast<float>(beamY_->value()));
-    experiment_.PoniRot1_rad(static_cast<float>(rot1_->value() * PI / 180.0));
-    experiment_.PoniRot2_rad(static_cast<float>(rot2_->value() * PI / 180.0));
-    experiment_.PolarizationFactor(polarizationOn_->isChecked()
-        ? std::optional<float>(static_cast<float>(polarization_->value())) : std::nullopt);
-    experiment_.Goniometer(GoniometerFromFields());
-    experiment_.GridScan(GridScanFromFields());
-    if (cellKnown_->isChecked()) {
-        experiment_.SetUnitCell(UnitCell{
-            static_cast<float>(cellA_->value()), static_cast<float>(cellB_->value()),
-            static_cast<float>(cellC_->value()), static_cast<float>(cellAlpha_->value()),
-            static_cast<float>(cellBeta_->value()), static_cast<float>(cellGamma_->value())});
-        const int sg = static_cast<int>(std::lround(spaceGroup_->value()));
-        experiment_.SpaceGroupNumber(sg > 0 ? std::optional<int64_t>(sg) : std::nullopt);
-    } else {
-        experiment_.SetUnitCell(std::nullopt);
-        experiment_.SpaceGroupNumber(std::nullopt);
+    // The setters validate and throw; an exception escaping a slot terminates the viewer, so
+    // an invalid entry keeps the previous experiment instead.
+    DiffractionExperiment e = experiment_;
+    try {
+        e.IncidentEnergy_keV(static_cast<float>(energy_->value()));
+        e.DetectorDistance_mm(static_cast<float>(distance_->value()));
+        e.BeamX_pxl(static_cast<float>(beamX_->value()));
+        e.BeamY_pxl(static_cast<float>(beamY_->value()));
+        e.PoniRot1_rad(static_cast<float>(rot1_->value() * PI / 180.0));
+        e.PoniRot2_rad(static_cast<float>(rot2_->value() * PI / 180.0));
+        e.PolarizationFactor(polarizationOn_->isChecked()
+            ? std::optional<float>(static_cast<float>(polarization_->value())) : std::nullopt);
+        e.Goniometer(GoniometerFromFields());
+        e.GridScan(GridScanFromFields());
+        if (cellKnown_->isChecked()) {
+            e.SetUnitCell(UnitCell{
+                static_cast<float>(cellA_->value()), static_cast<float>(cellB_->value()),
+                static_cast<float>(cellC_->value()), static_cast<float>(cellAlpha_->value()),
+                static_cast<float>(cellBeta_->value()), static_cast<float>(cellGamma_->value())});
+            const int sg = static_cast<int>(std::lround(spaceGroup_->value()));
+            e.SpaceGroupNumber(sg > 0 ? std::optional<int64_t>(sg) : std::nullopt);
+        } else {
+            e.SetUnitCell(std::nullopt);
+            e.SpaceGroupNumber(std::nullopt);
+        }
+    } catch (const std::exception &) {
+        return;
     }
+    experiment_ = e;
     emit experimentChanged(experiment_);
 }