diff --git a/image_analysis/scale_merge/HKLKey.cpp b/image_analysis/scale_merge/HKLKey.cpp
index ffa28162..3f05db05 100644
--- a/image_analysis/scale_merge/HKLKey.cpp
+++ b/image_analysis/scale_merge/HKLKey.cpp
@@ -6,8 +6,16 @@
 #include "HKLKey.h"
 #include "gemmi/symmetry.hpp"
 
-HKLKeyGenerator::HKLKeyGenerator(bool merge_friedel, const std::optional<gemmi::SpaceGroup> &sg)
-: merge_friedel(merge_friedel), sg(sg) {}
+HKLKeyGenerator::HKLKeyGenerator(bool merge_friedel, int32_t space_group_number)
+    : HKLKeyGenerator(merge_friedel, *gemmi::find_spacegroup_by_number(space_group_number)) {
+}
+
+HKLKeyGenerator::HKLKeyGenerator(bool merge_friedel, const gemmi::SpaceGroup &sg)
+    : merge_friedel(merge_friedel),
+      sg(sg),
+      ops(sg.operations()),
+      asu(&sg) {
+}
 
 HKLKey HKLKeyGenerator::operator()(const MergedReflection &r) const {
     return operator()(r.h, r.k, r.l);
@@ -20,7 +28,7 @@ HKLKey HKLKeyGenerator::operator()(const Reflection &r) const {
 HKLKey HKLKeyGenerator::operator()(int32_t h, int32_t k, int32_t l) const {
     HKLKey key{h, k, l, true};
 
-    if (!sg.has_value()) {
+    if (sg.number == 1) {
         const HKLKey neg{-h, -k, -l, true};
         if (std::tie(key.h, key.k, key.l) < std::tie(neg.h, neg.k, neg.l)) {
             key.h = -key.h;
@@ -29,10 +37,6 @@ HKLKey HKLKeyGenerator::operator()(int32_t h, int32_t k, int32_t l) const {
             key.plus = merge_friedel;
         }
     } else {
-        const auto sg_local = sg.value();
-        const auto ops = sg_local.operations();
-        const gemmi::ReciprocalAsu asu(&sg_local);
-
         const gemmi::Op::Miller in{h, k, l};
         const auto [hkl, sign_plus] = asu.to_asu_sign(in, ops);
 
diff --git a/image_analysis/scale_merge/HKLKey.h b/image_analysis/scale_merge/HKLKey.h
index 93c5d505..8084d8e9 100644
--- a/image_analysis/scale_merge/HKLKey.h
+++ b/image_analysis/scale_merge/HKLKey.h
@@ -23,9 +23,13 @@ struct HKLKey {
 
 class HKLKeyGenerator {
     bool merge_friedel;
-    std::optional<gemmi::SpaceGroup> sg;
+    gemmi::SpaceGroup sg;
+    gemmi::GroupOps ops;
+    gemmi::ReciprocalAsu asu;
+
 public:
-    HKLKeyGenerator(bool merge_friedel, const std::optional<gemmi::SpaceGroup> &sg);
+    HKLKeyGenerator(bool merge_friedel, int32_t space_group_number);
+    HKLKeyGenerator(bool merge_friedel, const gemmi::SpaceGroup &sg);
     HKLKey operator()(const Reflection &r) const;
     HKLKey operator()(const MergedReflection &r) const;
     HKLKey operator()(int32_t h, int32_t k, int32_t l) const;
diff --git a/image_analysis/scale_merge/Merge.cpp b/image_analysis/scale_merge/Merge.cpp
index 186d4c1e..9f049362 100644
--- a/image_analysis/scale_merge/Merge.cpp
+++ b/image_analysis/scale_merge/Merge.cpp
@@ -41,7 +41,7 @@ namespace {
 
         auto scaling_settings = x.GetScalingSettings();
 
-        HKLKeyGenerator key_generator(scaling_settings.GetMergeFriedel(), x.GetGemmiSpaceGroup() );
+        HKLKeyGenerator key_generator(scaling_settings.GetMergeFriedel(), x.GetSpaceGroupNumber().value_or(1) );
 
         for (const auto &image: observations) {
             for (const auto &r: image) {
diff --git a/image_analysis/scale_merge/ScaleOnTheFly.cpp b/image_analysis/scale_merge/ScaleOnTheFly.cpp
index 99c29b7b..d2057fbe 100644
--- a/image_analysis/scale_merge/ScaleOnTheFly.cpp
+++ b/image_analysis/scale_merge/ScaleOnTheFly.cpp
@@ -92,10 +92,10 @@ ScaleOnTheFly::ScaleOnTheFly(const std::vector<MergedReflection> &ref, const Dif
       model(x.GetPartialityModel()),
       s(x.GetScalingSettings()),
       rot_wedge_deg(x.GetRotationWedgeForScaling()),
-      refine_rot_wedge(x.GetRefineRotationWedgeInScaling()) {
-    const HKLKeyGenerator key_generator(s.GetMergeFriedel(), sg);
+      refine_rot_wedge(x.GetRefineRotationWedgeInScaling()),
+      hkl_key_generator(s.GetMergeFriedel(), x.GetSpaceGroupNumber().value_or(1)) {
     for (const auto &r: ref) {
-        const auto key = key_generator(r);
+        const auto key = hkl_key_generator(r);
         reference_data[key] = r.I;
     }
 }
@@ -137,13 +137,12 @@ ScaleOnTheFlyResult ScaleOnTheFly::Scale(std::vector<Reflection> &reflections, s
     }
 
     size_t n_reflections = 0;
-    HKLKeyGenerator key_generator(s.GetMergeFriedel(), sg);
     for (const auto &r: reflections) {
-        const HKLKey key = key_generator(r);
-
         if (!Accept(r))
             continue;
 
+        const HKLKey key = hkl_key_generator(r);
+
         if (!reference_data.contains(key))
             continue;
 
diff --git a/image_analysis/scale_merge/ScaleOnTheFly.h b/image_analysis/scale_merge/ScaleOnTheFly.h
index b037ed16..c491c4a5 100644
--- a/image_analysis/scale_merge/ScaleOnTheFly.h
+++ b/image_analysis/scale_merge/ScaleOnTheFly.h
@@ -28,6 +28,7 @@ class ScaleOnTheFly {
     const ScalingSettings s;
     const std::optional<double> rot_wedge_deg;
     const bool refine_rot_wedge;
+    const HKLKeyGenerator hkl_key_generator;
     std::map<HKLKey, double> reference_data;
 
     bool Accept(const Reflection &r);
diff --git a/tests/MergeScaleTest.cpp b/tests/MergeScaleTest.cpp
index b821b8d8..534871d7 100644
--- a/tests/MergeScaleTest.cpp
+++ b/tests/MergeScaleTest.cpp
@@ -5,14 +5,14 @@
 #include "../image_analysis/scale_merge/HKLKey.h"
 
 TEST_CASE("HKLKey_NoSG_noMergeFriedel") {
-    HKLKeyGenerator hkl_key_gen(false, std::nullopt);
+    HKLKeyGenerator hkl_key_gen(false, 1);
     CHECK(hkl_key_gen(-1, -2, -3) != hkl_key_gen(1,2,3));
     CHECK(hkl_key_gen(-1,-2,-3) == hkl_key_gen(-1,-2,-3));
     CHECK(hkl_key_gen(-1,-2,-3) != hkl_key_gen(1,-2,-3));
 }
 
 TEST_CASE("HKLKey_NoSG_MergeFriedel") {
-    HKLKeyGenerator hkl_key_gen(true, std::nullopt);
+    HKLKeyGenerator hkl_key_gen(true, 1);
     CHECK(hkl_key_gen(-1, -2, -3) == hkl_key_gen(1,2,3));
     CHECK(hkl_key_gen(-1,-2,-3) == hkl_key_gen(-1,-2,-3));
     CHECK(hkl_key_gen(-1,-2,-3) != hkl_key_gen(1,-2,-3));