diff --git a/image_analysis/scale_merge/CMakeLists.txt b/image_analysis/scale_merge/CMakeLists.txt
index 62aefd28..b1ab0985 100644
--- a/image_analysis/scale_merge/CMakeLists.txt
+++ b/image_analysis/scale_merge/CMakeLists.txt
@@ -1,2 +1,4 @@
-ADD_LIBRARY(JFJochScaleMerge ScaleAndMerge.cpp ScaleAndMerge.h FrenchWilson.cpp FrenchWilson.h)
+ADD_LIBRARY(JFJochScaleMerge ScaleAndMerge.cpp ScaleAndMerge.h FrenchWilson.cpp FrenchWilson.h
+        SearchSpaceGroup.cpp
+        SearchSpaceGroup.h)
 TARGET_LINK_LIBRARIES(JFJochScaleMerge Ceres::ceres Eigen3::Eigen JFJochCommon)
\ No newline at end of file
diff --git a/image_analysis/scale_merge/SearchSpaceGroup.cpp b/image_analysis/scale_merge/SearchSpaceGroup.cpp
new file mode 100644
index 00000000..70883a2c
--- /dev/null
+++ b/image_analysis/scale_merge/SearchSpaceGroup.cpp
@@ -0,0 +1,426 @@
+#include "SearchSpaceGroup.h"
+
+#include <algorithm>
+#include <cmath>
+#include <cctype>
+#include <iomanip>
+#include <limits>
+#include <sstream>
+#include <tuple>
+#include <unordered_map>
+#include <unordered_set>
+#include <vector>
+
+namespace {
+    struct MergedHKLKey {
+        int h = 0;
+        int k = 0;
+        int l = 0;
+        bool plus = true;
+
+        bool operator==(const MergedHKLKey& o) const noexcept {
+            return h == o.h && k == o.k && l == o.l && plus == o.plus;
+        }
+    };
+
+    struct MergedHKLKeyHash {
+        size_t operator()(const MergedHKLKey& key) const noexcept {
+            auto mix = [](uint64_t x) {
+                x ^= x >> 33;
+                x *= 0xff51afd7ed558ccdULL;
+                x ^= x >> 33;
+                x *= 0xc4ceb9fe1a85ec53ULL;
+                x ^= x >> 33;
+                return x;
+            };
+            return static_cast<size_t>(
+                mix(static_cast<uint64_t>(key.h)) ^
+                (mix(static_cast<uint64_t>(key.k)) << 1) ^
+                (mix(static_cast<uint64_t>(key.l)) << 2) ^
+                (mix(static_cast<uint64_t>(key.plus ? 1 : 0)) << 3));
+        }
+    };
+
+    bool IsIdentityOp(const gemmi::Op& op) {
+        return op.rot == gemmi::Op::identity().rot;
+    }
+
+    bool IsParityChanging(const gemmi::Op& op) {
+        return op.det_rot() < 0;
+    }
+
+    MergedHKLKey CanonicalizeMergedHKL(int h, int k, int l, bool merge_friedel) {
+        MergedHKLKey key{h, k, l, true};
+        if (merge_friedel)
+            return key;
+
+        const std::tuple<int, int, int> pos{h, k, l};
+        const std::tuple<int, int, int> neg{-h, -k, -l};
+        if (neg < pos) {
+            key.h = -h;
+            key.k = -k;
+            key.l = -l;
+            key.plus = false;
+        }
+        return key;
+    }
+
+    bool ReflectionPassesFilters(const MergedReflection& r, const SearchSpaceGroupOptions& opt) {
+        if (!std::isfinite(r.I))
+            return false;
+        if (!std::isfinite(r.sigma) || r.sigma <= 0.0)
+            return false;
+        if (!std::isfinite(r.d) || r.d <= 0.0)
+            return false;
+        if (opt.d_min_limit_A > 0.0 && r.d < opt.d_min_limit_A)
+            return false;
+        if (opt.min_i_over_sigma > 0.0 && r.I / r.sigma < opt.min_i_over_sigma)
+            return false;
+        return true;
+    }
+
+    std::unordered_map<MergedHKLKey, const MergedReflection*, MergedHKLKeyHash>
+    BuildMergedLookup(const std::vector<MergedReflection>& merged, const SearchSpaceGroupOptions& opt) {
+        std::unordered_map<MergedHKLKey, const MergedReflection*, MergedHKLKeyHash> out;
+        out.reserve(merged.size());
+
+        for (const auto& r : merged) {
+            if (!ReflectionPassesFilters(r, opt))
+                continue;
+            const auto key = CanonicalizeMergedHKL(r.h, r.k, r.l, opt.merge_friedel);
+            out.emplace(key, &r);
+        }
+
+        return out;
+    }
+
+    double PearsonCC(const std::vector<double>& x, const std::vector<double>& y) {
+        if (x.size() != y.size() || x.size() < 2)
+            return std::numeric_limits<double>::quiet_NaN();
+
+        double sx = 0.0, sy = 0.0;
+        double sxx = 0.0, syy = 0.0, sxy = 0.0;
+
+        for (size_t i = 0; i < x.size(); ++i) {
+            sx += x[i];
+            sy += y[i];
+            sxx += x[i] * x[i];
+            syy += y[i] * y[i];
+            sxy += x[i] * y[i];
+        }
+
+        const double n = static_cast<double>(x.size());
+        const double cov = sxy - sx * sy / n;
+        const double vx = sxx - sx * sx / n;
+        const double vy = syy - sy * sy / n;
+
+        if (vx <= 0.0 || vy <= 0.0)
+            return std::numeric_limits<double>::quiet_NaN();
+
+        return cov / std::sqrt(vx * vy);
+    }
+
+    std::optional<SpaceGroupOperatorScore> ScoreOperator(
+        const gemmi::Op& op,
+        const std::vector<MergedReflection>& merged,
+        const std::unordered_map<MergedHKLKey, const MergedReflection*, MergedHKLKeyHash>& lookup,
+        const SearchSpaceGroupOptions& opt) {
+
+        std::vector<double> x;
+        std::vector<double> y;
+        x.reserve(merged.size() / 2);
+        y.reserve(merged.size() / 2);
+
+        std::unordered_set<MergedHKLKey, MergedHKLKeyHash> used;
+        used.reserve(merged.size() / 2);
+
+        for (const auto& r : merged) {
+            if (!ReflectionPassesFilters(r, opt))
+                continue;
+
+            const auto key1 = CanonicalizeMergedHKL(r.h, r.k, r.l, opt.merge_friedel);
+            if (used.find(key1) != used.end())
+                continue;
+
+            const gemmi::Op::Miller hkl{{r.h, r.k, r.l}};
+            const auto hkl2 = op.apply_to_hkl(hkl);
+            const auto key2 = CanonicalizeMergedHKL(hkl2[0], hkl2[1], hkl2[2], opt.merge_friedel);
+
+            if (key1.h == key2.h && key1.k == key2.k && key1.l == key2.l && key1.plus == key2.plus)
+                continue;
+
+            auto it = lookup.find(key2);
+            if (it == lookup.end())
+                continue;
+
+            const auto* mate = it->second;
+            if (mate == nullptr)
+                continue;
+
+            x.push_back(r.I);
+            y.push_back(mate->I);
+
+            used.insert(key1);
+            used.insert(key2);
+        }
+
+        if (x.empty())
+            return std::nullopt;
+
+        SpaceGroupOperatorScore out;
+        out.op_triplet_hkl = op.as_hkl().triplet('h');
+        out.compared = static_cast<int>(x.size());
+        out.cc = PearsonCC(x, y);
+        out.accepted = (out.compared >= opt.min_pairs_per_operator &&
+                        std::isfinite(out.cc) &&
+                        out.cc >= opt.min_operator_cc);
+        return out;
+    }
+
+    SpaceGroupAbsenceScore ScoreSystematicAbsences(
+        const gemmi::GroupOps& gops,
+        const std::vector<MergedReflection>& merged,
+        const SearchSpaceGroupOptions& opt) {
+
+        SpaceGroupAbsenceScore out;
+
+        if (!opt.test_systematic_absences)
+            return out;
+
+        double sum_i_over_sigma = 0.0;
+
+        for (const auto& r : merged) {
+            if (!ReflectionPassesFilters(r, opt))
+                continue;
+
+            const gemmi::Op::Miller hkl{{r.h, r.k, r.l}};
+            if (!gops.is_systematically_absent(hkl))
+                continue;
+
+            out.violating_reflections += 1;
+            sum_i_over_sigma += std::max(0.0, r.I / r.sigma);
+        }
+
+        if (out.violating_reflections > 0)
+            out.mean_i_over_sigma = sum_i_over_sigma / out.violating_reflections;
+        else
+            out.mean_i_over_sigma = 0.0;
+
+        out.accepted = (out.violating_reflections <= opt.max_absent_violations) &&
+                       (out.mean_i_over_sigma <= opt.max_mean_absent_i_over_sigma);
+        return out;
+    }
+
+    bool IsCenteringCompatible(char requested, char candidate) {
+        if (requested == '\0')
+            return true;
+        return std::toupper(static_cast<unsigned char>(requested)) ==
+               std::toupper(static_cast<unsigned char>(candidate));
+    }
+
+    bool IsCandidateSpaceGroup(const gemmi::SpaceGroup& sg,
+                               const std::optional<gemmi::CrystalSystem>& crystal_system,
+                               char centering) {
+        if (!sg.is_reference_setting())
+            return false;
+        if (!sg.is_sohncke())
+            return false;
+        if (crystal_system.has_value() && sg.crystal_system() != crystal_system.value())
+            return false;
+        if (!IsCenteringCompatible(centering, sg.centring_type()))
+            return false;
+        return true;
+    }
+
+    std::vector<gemmi::SpaceGroup> EnumerateCandidateSpaceGroups(
+        const std::optional<gemmi::CrystalSystem>& crystal_system,
+        char centering) {
+
+        std::vector<gemmi::SpaceGroup> out;
+
+        for (const auto& sg : gemmi::spacegroup_tables::main) {
+            if (!IsCandidateSpaceGroup(sg, crystal_system, centering))
+                continue;
+            out.push_back(sg);
+        }
+
+        std::sort(out.begin(), out.end(),
+                  [](const gemmi::SpaceGroup& a, const gemmi::SpaceGroup& b) {
+                      const int order_a = a.operations().derive_symmorphic().order();
+                      const int order_b = b.operations().derive_symmorphic().order();
+                      if (order_a != order_b)
+                          return order_a < order_b;
+                      return a.number < b.number;
+                  });
+        return out;
+    }
+}
+
+SearchSpaceGroupResult SearchSpaceGroup(
+    const std::vector<MergedReflection>& merged,
+    const SearchSpaceGroupOptions& opt) {
+
+    SearchSpaceGroupResult result;
+    if (merged.empty())
+        return result;
+
+    const auto lookup = BuildMergedLookup(merged, opt);
+    const auto candidates = EnumerateCandidateSpaceGroups(opt.crystal_system, opt.centering);
+
+    for (const auto& sg : candidates) {
+        SpaceGroupCandidateScore score{.space_group = sg};
+
+        const gemmi::GroupOps gops_full = sg.operations();
+        const gemmi::GroupOps gops_rot = gops_full.derive_symmorphic();
+
+        double cc_sum = 0.0;
+        int cc_count = 0;
+        int compared_total = 0;
+        double min_cc = std::numeric_limits<double>::infinity();
+
+        for (const auto& op : gops_rot.sym_ops) {
+            if (IsIdentityOp(op))
+                continue;
+            if (IsParityChanging(op))
+                continue;
+
+            auto op_score = ScoreOperator(op, merged, lookup, opt);
+            if (!op_score.has_value())
+                continue;
+
+            compared_total += op_score->compared;
+            score.operator_scores.push_back(*op_score);
+
+            if (op_score->compared >= opt.min_pairs_per_operator && std::isfinite(op_score->cc)) {
+                cc_sum += op_score->cc;
+                min_cc = std::min(min_cc, op_score->cc);
+                cc_count += 1;
+                if (op_score->accepted)
+                    score.accepted_operators += 1;
+            }
+        }
+
+        score.absence_score = ScoreSystematicAbsences(gops_full, merged, opt);
+        score.tested_operators = static_cast<int>(score.operator_scores.size());
+        score.compared_total = compared_total;
+        score.mean_cc = (cc_count > 0) ? (cc_sum / cc_count) : 0.0;
+        score.min_cc = std::isfinite(min_cc) ? min_cc : 0.0;
+
+        const bool trivial_group = (gops_rot.order() <= 1);
+        const bool rotationally_accepted =
+            trivial_group ||
+            ((score.tested_operators > 0) &&
+             (score.accepted_operators == score.tested_operators) &&
+             (score.compared_total >= opt.min_total_compared));
+
+        score.accepted = rotationally_accepted && score.absence_score.accepted;
+
+        result.candidates.push_back(std::move(score));
+    }
+
+    std::sort(result.candidates.begin(), result.candidates.end(),
+              [](const SpaceGroupCandidateScore& a, const SpaceGroupCandidateScore& b) {
+                  if (a.accepted != b.accepted)
+                      return a.accepted > b.accepted;
+
+                  if (a.absence_score.mean_i_over_sigma != b.absence_score.mean_i_over_sigma)
+                      return a.absence_score.mean_i_over_sigma < b.absence_score.mean_i_over_sigma;
+
+                  if (a.absence_score.violating_reflections != b.absence_score.violating_reflections)
+                      return a.absence_score.violating_reflections < b.absence_score.violating_reflections;
+
+                  const int order_a = a.space_group.operations().derive_symmorphic().order();
+                  const int order_b = b.space_group.operations().derive_symmorphic().order();
+                  if (order_a != order_b)
+                      return order_a > order_b;
+
+                  if (a.accepted_operators != b.accepted_operators)
+                      return a.accepted_operators > b.accepted_operators;
+                  if (a.min_cc != b.min_cc)
+                      return a.min_cc > b.min_cc;
+                  if (a.mean_cc != b.mean_cc)
+                      return a.mean_cc > b.mean_cc;
+
+                  return a.space_group.number < b.space_group.number;
+              });
+
+    for (const auto& cand : result.candidates) {
+        if (cand.accepted) {
+            result.best_space_group = cand.space_group;
+            break;
+        }
+    }
+
+    return result;
+}
+
+std::string SearchSpaceGroupResultToText(const SearchSpaceGroupResult& result,
+                                         size_t max_candidates_to_print) {
+    std::ostringstream os;
+
+    os << "Space-group candidates\n";
+    os << "  "
+       << std::setw(10) << "SG"
+       << " "
+       << std::setw(4) << "Acc"
+       << " "
+       << std::setw(8) << "<CC>"
+       << " "
+       << std::setw(8) << "minCC"
+       << " "
+       << std::setw(9) << "compared"
+       << " "
+       << std::setw(7) << "ops"
+       << " "
+       << std::setw(8) << "absent"
+       << " "
+       << std::setw(10) << "<I/sig>abs"
+       << "\n";
+
+    os << "  "
+       << std::setw(10) << "----------"
+       << " "
+       << std::setw(4) << "----"
+       << " "
+       << std::setw(8) << "--------"
+       << " "
+       << std::setw(8) << "--------"
+       << " "
+       << std::setw(9) << "---------"
+       << " "
+       << std::setw(7) << "-------"
+       << " "
+       << std::setw(8) << "--------"
+       << " "
+       << std::setw(10) << "----------"
+       << "\n";
+
+    const size_t n = std::min(max_candidates_to_print, result.candidates.size());
+    for (size_t i = 0; i < n; ++i) {
+        const auto& c = result.candidates[i];
+        os << "  "
+           << std::setw(10) << c.space_group.short_name()
+           << " "
+           << std::setw(4) << (c.accepted ? "yes" : "no")
+           << " "
+           << std::setw(8) << std::fixed << std::setprecision(3) << c.mean_cc
+           << " "
+           << std::setw(8) << std::fixed << std::setprecision(3) << c.min_cc
+           << " "
+           << std::setw(9) << c.compared_total
+           << " "
+           << std::setw(3) << c.accepted_operators << "/" << std::setw(3) << c.tested_operators
+           << " "
+           << std::setw(8) << c.absence_score.violating_reflections
+           << " "
+           << std::setw(10) << std::fixed << std::setprecision(2) << c.absence_score.mean_i_over_sigma
+           << "\n";
+    }
+
+    if (result.best_space_group.has_value())
+        os << "Best space group: " << result.best_space_group->short_name() << "\n";
+    else
+        os << "Best space group: none accepted\n";
+
+    return os.str();
+}
\ No newline at end of file
diff --git a/image_analysis/scale_merge/SearchSpaceGroup.h b/image_analysis/scale_merge/SearchSpaceGroup.h
new file mode 100644
index 00000000..b044869c
--- /dev/null
+++ b/image_analysis/scale_merge/SearchSpaceGroup.h
@@ -0,0 +1,80 @@
+#pragma once
+
+#include <optional>
+#include <string>
+#include <vector>
+
+#include "ScaleAndMerge.h"
+#include "gemmi/symmetry.hpp"
+
+struct SpaceGroupOperatorScore {
+    std::string op_triplet_hkl;
+    double cc = 0.0;
+    int compared = 0;
+    bool accepted = false;
+};
+
+struct SpaceGroupAbsenceScore {
+    int violating_reflections = 0;
+    double mean_i_over_sigma = 0.0;
+    bool accepted = true;
+};
+
+struct SpaceGroupCandidateScore {
+    gemmi::SpaceGroup space_group;
+    double mean_cc = 0.0;
+    double min_cc = 0.0;
+    int compared_total = 0;
+    int accepted_operators = 0;
+    int tested_operators = 0;
+    bool accepted = false;
+    SpaceGroupAbsenceScore absence_score;
+    std::vector<SpaceGroupOperatorScore> operator_scores;
+};
+
+struct SearchSpaceGroupOptions {
+    // If not set, search all crystal systems.
+    std::optional<gemmi::CrystalSystem> crystal_system;
+
+    // '\0' means: search all centerings compatible with the candidate system.
+    char centering = '\0';
+
+    // If true, Friedel mates are treated as equivalent when matching HKLs.
+    bool merge_friedel = true;
+
+    // Ignore reflections beyond this resolution (higher-resolution means smaller d).
+    // Set to 0 to disable the cutoff.
+    double d_min_limit_A = 0.0;
+
+    // Optional I/sigma filter on merged reflections.
+    double min_i_over_sigma = 0.0;
+
+    // Acceptance thresholds for each tested rotational operator.
+    double min_operator_cc = 0.80;
+    int min_pairs_per_operator = 20;
+
+    // Minimum total number of compared reflection pairs for a candidate SG.
+    int min_total_compared = 100;
+
+    // Test systematic absences from centering / screws / glides.
+    bool test_systematic_absences = true;
+
+    // Candidate is rejected if forbidden reflections are too strong on average.
+    double max_mean_absent_i_over_sigma = 1.0;
+
+    // Hard cap on number of observed forbidden reflections.
+    int max_absent_violations = 0;
+};
+
+struct SearchSpaceGroupResult {
+    std::optional<gemmi::SpaceGroup> best_space_group;
+    std::vector<SpaceGroupCandidateScore> candidates;
+};
+
+SearchSpaceGroupResult SearchSpaceGroup(
+    const std::vector<MergedReflection>& merged,
+    const SearchSpaceGroupOptions& opt = {});
+
+std::string SearchSpaceGroupResultToText(
+    const SearchSpaceGroupResult& result,
+    size_t max_candidates_to_print = 20);
\ No newline at end of file
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index e4800f6a..1c4bb8f4 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -66,6 +66,7 @@ ADD_EXECUTABLE(jfjoch_test
         HKLKeyTest.cpp
         TCPImagePusherTest.cpp
         BraggIntegrate2DTest.cpp
+        SearchSpaceGroupTest.cpp
 )
 
 target_link_libraries(jfjoch_test Catch2WithMain JFJochBroker JFJochReceiver JFJochReader JFJochWriter JFJochImageAnalysis JFJochCommon JFJochHLSSimulation JFJochPreview)
diff --git a/tests/SearchSpaceGroupTest.cpp b/tests/SearchSpaceGroupTest.cpp
new file mode 100644
index 00000000..8e7290bd
--- /dev/null
+++ b/tests/SearchSpaceGroupTest.cpp
@@ -0,0 +1,143 @@
+#include <catch2/catch_all.hpp>
+
+#include "../image_analysis/scale_merge/SearchSpaceGroup.h"
+#include "gemmi/symmetry.hpp"
+
+#include <cmath>
+#include <cstdint>
+#include <string>
+#include <tuple>
+#include <unordered_set>
+#include <vector>
+
+namespace {
+    struct HKL {
+        int h = 0;
+        int k = 0;
+        int l = 0;
+
+        bool operator==(const HKL& o) const noexcept {
+            return h == o.h && k == o.k && l == o.l;
+        }
+    };
+
+    struct HKLHash {
+        size_t operator()(const HKL& x) const noexcept {
+            auto mix = [](uint64_t v) {
+                v ^= v >> 33;
+                v *= 0xff51afd7ed558ccdULL;
+                v ^= v >> 33;
+                v *= 0xc4ceb9fe1a85ec53ULL;
+                v ^= v >> 33;
+                return v;
+            };
+            return static_cast<size_t>(
+                mix(static_cast<uint64_t>(x.h)) ^
+                (mix(static_cast<uint64_t>(x.k)) << 1) ^
+                (mix(static_cast<uint64_t>(x.l)) << 2));
+        }
+    };
+
+    double CalcSyntheticD(int h, int k, int l) {
+        const double q2 = static_cast<double>(h * h + k * k + l * l);
+        return 40.0 / std::sqrt(q2 + 1.0);
+    }
+
+    double SyntheticIntensityFromAsu(const gemmi::Op::Miller& asu) {
+        uint64_t x = static_cast<uint64_t>((asu[0] + 31) * 73856093u) ^
+                     static_cast<uint64_t>((asu[1] + 37) * 19349663u) ^
+                     static_cast<uint64_t>((asu[2] + 41) * 83492791u);
+        x ^= x >> 13;
+        x *= 0x9e3779b97f4a7c15ULL;
+        x ^= x >> 17;
+        return 100.0 + static_cast<double>(x % 500);
+    }
+
+    std::vector<MergedReflection> GenerateMergedReflectionsForSpaceGroup(
+        const gemmi::SpaceGroup& sg,
+        int hmax = 8) {
+
+        std::vector<MergedReflection> merged;
+        std::unordered_set<HKL, HKLHash> added;
+
+        const gemmi::GroupOps gops = sg.operations();
+        const gemmi::ReciprocalAsu rasu(&sg);
+
+        for (int h = -hmax; h <= hmax; ++h) {
+            for (int k = -hmax; k <= hmax; ++k) {
+                for (int l = -hmax; l <= hmax; ++l) {
+                    if (h == 0 && k == 0 && l == 0)
+                        continue;
+
+                    gemmi::Op::Miller hkl{{h, k, l}};
+                    if (gops.is_systematically_absent(hkl))
+                        continue;
+
+                    const auto [asu, sign_plus] = rasu.to_asu_sign(hkl, gops);
+                    if (!sign_plus)
+                        continue;
+
+                    const HKL key{h, k, l};
+                    if (added.find(key) != added.end())
+                        continue;
+                    added.insert(key);
+
+                    merged.push_back(MergedReflection{
+                        .h = h,
+                        .k = k,
+                        .l = l,
+                        .I = SyntheticIntensityFromAsu(asu),
+                        .sigma = 1.0,
+                        .d = CalcSyntheticD(h, k, l)
+                    });
+                }
+            }
+        }
+
+        return merged;
+    }
+}
+
+TEST_CASE("SearchSpaceGroup detects synthetic space groups") {
+    struct Case {
+        std::string input_name;
+        std::string expected_short_name;
+    };
+
+    const std::vector<Case> cases = {
+        {"P 1", "P1"},
+        {"P 1 2 1", "P2"},
+        {"P 3 2 1", "P321"},
+        {"P 4 2 2", "P422"},
+        {"P 4 3 2", "P432"},
+        {"P 6 2 2", "P622"},
+        {"C 1 2 1", "C2"},
+        {"C 2 2 2", "C222"},
+        {"I 4 3 2", "I432"},
+    };
+
+    for (const auto& tc : cases) {
+        DYNAMIC_SECTION(tc.expected_short_name) {
+            const gemmi::SpaceGroup& sg = gemmi::get_spacegroup_by_name(tc.input_name);
+            const auto merged = GenerateMergedReflectionsForSpaceGroup(sg);
+
+            SearchSpaceGroupOptions opt;
+            opt.crystal_system.reset();
+            opt.centering = '\0';
+            opt.merge_friedel = true;
+            opt.d_min_limit_A = 2.0;
+            opt.min_operator_cc = 0.99;
+            opt.min_pairs_per_operator = 20;
+            opt.min_total_compared = 80;
+            opt.test_systematic_absences = true;
+            opt.max_mean_absent_i_over_sigma = 0.1;
+            opt.max_absent_violations = 0;
+
+            const auto result = SearchSpaceGroup(merged, opt);
+
+            INFO(SearchSpaceGroupResultToText(result));
+            REQUIRE(result.best_space_group.has_value());
+            CHECK(result.best_space_group->short_name() == tc.expected_short_name);
+        }
+    }
+}
\ No newline at end of file
diff --git a/tools/jfjoch_process.cpp b/tools/jfjoch_process.cpp
index 102a9d7e..7aef052d 100644
--- a/tools/jfjoch_process.cpp
+++ b/tools/jfjoch_process.cpp
@@ -10,6 +10,7 @@
 #include <atomic>
 #include <chrono>
 #include <fstream>
+#include <sstream>
 
 #include "../reader/JFJochHDF5Reader.h"
 #include "../common/Logger.h"
@@ -25,7 +26,8 @@
 #include "../receiver/JFJochReceiverPlots.h"
 #include "../compression/JFJochCompressor.h"
 #include "../image_analysis/scale_merge/FrenchWilson.h"
-#include "../image_analysis/WriteMmcif.h"  // (actually include at top of file)
+#include "../image_analysis/scale_merge/SearchSpaceGroup.h"
+#include "../image_analysis/WriteMmcif.h"
 
 void print_usage(Logger &logger) {
     logger.Info("Usage ./jfjoch_analysis {<options>} <input.h5>");
@@ -517,6 +519,7 @@ int main(int argc, char **argv) {
         logger.Info("Rotation Indexing found lattice");
     }
 
+    // --- Optional: run scaling (mosaicity refinement) on accumulated reflections ---
     // --- Optional: run scaling (mosaicity refinement) on accumulated reflections ---
     if (run_scaling) {
         logger.Info("Running scaling (mosaicity refinement) ...");
@@ -528,6 +531,8 @@ int main(int argc, char **argv) {
         scale_opts.merge_friedel = !anomalous_mode;
         scale_opts.d_min_limit_A = d_min_scale_merge.value_or(0.0);
 
+        const bool fixed_space_group = space_group || experiment.GetGemmiSpaceGroup().has_value();
+
         if (space_group)
             scale_opts.space_group = *space_group;
         else
@@ -538,46 +543,91 @@ int main(int argc, char **argv) {
         auto scale_end = std::chrono::steady_clock::now();
         double scale_time = std::chrono::duration<double>(scale_end - scale_start).count();
 
+        if (scale_result && !fixed_space_group) {
+            logger.Info("Searching for space group from P1-merged reflections ...");
+
+            SearchSpaceGroupOptions sg_opts;
+            sg_opts.crystal_system.reset();
+            sg_opts.centering = '\0';
+            sg_opts.merge_friedel = !anomalous_mode;
+            sg_opts.d_min_limit_A = d_min_scale_merge.value_or(0.0);
+            sg_opts.min_i_over_sigma = 0.0;
+            sg_opts.min_operator_cc = 0.80;
+            sg_opts.min_pairs_per_operator = 20;
+            sg_opts.min_total_compared = 100;
+            sg_opts.test_systematic_absences = true;
+            sg_opts.max_mean_absent_i_over_sigma = 1.5;
+            sg_opts.max_absent_violations = 3;
+
+            const auto sg_search = SearchSpaceGroup(scale_result->merged, sg_opts);
+
+            logger.Info("");
+            {
+                std::istringstream iss(SearchSpaceGroupResultToText(sg_search));
+                std::string line;
+                while (std::getline(iss, line)) {
+                    if (!line.empty())
+                        logger.Info("{}", line);
+                }
+            }
+            logger.Info("");
+
+            if (sg_search.best_space_group.has_value()) {
+                logger.Info("Re-running scaling in detected space group {}", sg_search.best_space_group->short_name());
+
+                scale_opts.space_group = *sg_search.best_space_group;
+
+                auto rescale_start = std::chrono::steady_clock::now();
+                auto refined_scale_result = indexer.ScaleRotationData(scale_opts);
+                auto rescale_end = std::chrono::steady_clock::now();
+
+                if (refined_scale_result) {
+                    scale_result = std::move(refined_scale_result);
+                    scale_time += std::chrono::duration<double>(rescale_end - rescale_start).count();
+                }
+            } else {
+                logger.Warning("No space group accepted; keeping P1-merged result");
+            }
+        }
+
         if (scale_result) {
             end_msg.scale_factor = scale_result->image_scale_g;
 
-                logger.Info("Scaling completed in {:.2f} s  ({} unique reflections)",
-                            scale_time, scale_result->merged.size());
+            logger.Info("Scaling completed in {:.2f} s  ({} unique reflections)",
+                        scale_time, scale_result->merged.size());
 
-                // Print resolution-shell statistics table
+            // Print resolution-shell statistics table
+            {
+                const auto &stats = scale_result->statistics;
+                logger.Info("");
+                logger.Info("  {:>8s} {:>8s} {:>8s} {:>8s} {:>8s} {:>10s}",
+                            "d_min", "N_obs", "N_uniq", "Rmeas", "<I/sig>", "Complete");
+                logger.Info("  {:->8s} {:->8s} {:->8s} {:->8s} {:->8s} {:->10s}",
+                            "", "", "", "", "", "");
+                for (const auto &sh: stats.shells) {
+                    if (sh.unique_reflections == 0)
+                        continue;
+                    std::string compl_str = (sh.completeness > 0.0)
+                                                ? fmt::format("{:8.1f}%", sh.completeness * 100.0)
+                                                : "      N/A";
+                    logger.Info("  {:8.2f} {:8d} {:8d} {:8.3f}% {:8.1f} {:>10s}",
+                                sh.d_min, sh.total_observations, sh.unique_reflections,
+                                sh.rmeas * 100, sh.mean_i_over_sigma, compl_str);
+                }
                 {
-                    const auto& stats = scale_result->statistics;
-                    logger.Info("");
-                    logger.Info("  {:>8s} {:>8s} {:>8s} {:>8s} {:>8s} {:>10s}",
-                                "d_min", "N_obs", "N_uniq", "Rmeas", "<I/sig>", "Complete");
+                    const auto &ov = stats.overall;
                     logger.Info("  {:->8s} {:->8s} {:->8s} {:->8s} {:->8s} {:->10s}",
                                 "", "", "", "", "", "");
-                    for (const auto& sh : stats.shells) {
-                        if (sh.unique_reflections == 0)
-                            continue;
-                        std::string compl_str = (sh.completeness > 0.0)
-                            ? fmt::format("{:8.1f}%", sh.completeness * 100.0)
-                            : "      N/A";
-                        logger.Info("  {:8.2f} {:8d} {:8d} {:8.3f}% {:8.1f} {:>10s}",
-                                    sh.d_min, sh.total_observations, sh.unique_reflections,
-                                    sh.rmeas * 100, sh.mean_i_over_sigma, compl_str);
-                    }
-                    // Overall
-                    {
-                        const auto& ov = stats.overall;
-                        logger.Info("  {:->8s} {:->8s} {:->8s} {:->8s} {:->8s} {:->10s}",
-                                    "", "", "", "", "", "");
-                        std::string compl_str = (ov.completeness > 0.0)
-                            ? fmt::format("{:8.1f}%", ov.completeness * 100.0)
-                            : "      N/A";
-                        logger.Info("  {:>8s} {:8d} {:8d} {:8.3f}% {:8.1f} {:>10s}",
-                                    "Overall", ov.total_observations, ov.unique_reflections,
-                                    ov.rmeas * 100, ov.mean_i_over_sigma, compl_str);
-                    }
-                    logger.Info("");
+                    std::string compl_str = (ov.completeness > 0.0)
+                                                ? fmt::format("{:8.1f}%", ov.completeness * 100.0)
+                                                : "      N/A";
+                    logger.Info("  {:>8s} {:8d} {:8d} {:8.3f}% {:8.1f} {:>10s}",
+                                "Overall", ov.total_observations, ov.unique_reflections,
+                                ov.rmeas * 100, ov.mean_i_over_sigma, compl_str);
                 }
+                logger.Info("");
+            }
 
-            // Write image.dat  (image_id  mosaicity_deg  K)
             {
                 const std::string img_path = output_prefix + "_image.dat";
                 std::ofstream img_file(img_path);
@@ -586,13 +636,13 @@ int main(int argc, char **argv) {
                 } else {
                     img_file << "# image_id  mosaicity_deg  K\n";
                     for (size_t i = 0; i < scale_result->mosaicity_deg.size(); ++i) {
-                        img_file << i << " " << scale_result->mosaicity_deg[i] << " " << scale_result->image_scale_g[i] << "\n";
+                        img_file << i << " " << scale_result->mosaicity_deg[i] << " " << scale_result->image_scale_g[i]
+                                << "\n";
                     }
                     img_file.close();
                 }
             }
 
-            // --- French-Wilson: convert I → F ---
             {
                 FrenchWilsonOptions fw_opts;
                 fw_opts.acentric = true; // typical for MX
@@ -601,7 +651,6 @@ int main(int argc, char **argv) {
                 auto fw = FrenchWilson(scale_result->merged, fw_opts);
                 {
                     {
-                        // Write scaled.hkl  (h  k  l  I  sigma)
                         const std::string hkl_path = output_prefix + "_amplitudes.hkl";
                         std::ofstream hkl_file(hkl_path);
                         if (!hkl_file) {
@@ -619,23 +668,20 @@ int main(int argc, char **argv) {
                     }
                     MmcifMetadata cif_meta;
 
-                    // Unit cell — from rotation indexing result or experiment setting
                     if (rotation_indexer_ret.has_value()) {
                         cif_meta.unit_cell = rotation_indexer_ret->lattice.GetUnitCell();
                     } else if (experiment.GetUnitCell().has_value()) {
                         cif_meta.unit_cell = experiment.GetUnitCell().value();
                     }
 
-                    // Space group
-                    if (auto sg = experiment.GetGemmiSpaceGroup(); sg.has_value()) {
+                    if (scale_opts.space_group.has_value()) {
+                        cif_meta.space_group_name = scale_opts.space_group->hm;
+                        cif_meta.space_group_number = scale_opts.space_group->number;
+                    } else if (auto sg = experiment.GetGemmiSpaceGroup(); sg.has_value()) {
                         cif_meta.space_group_name = sg->hm;
                         cif_meta.space_group_number = sg->number;
-                    } else if (space_group) {
-                        cif_meta.space_group_name = space_group->hm;
-                        cif_meta.space_group_number = space_group->number;
                     }
 
-                    // Detector & experiment info
                     cif_meta.detector_name = experiment.GetDetectorDescription();
                     cif_meta.wavelength_A = experiment.GetWavelength_A();
                     cif_meta.detector_distance_mm = experiment.GetDetectorDistance_mm();