diff --git a/CMakeLists.txt b/CMakeLists.txt
index dddb44b..fa9838e 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -398,7 +398,6 @@ set(SourceFiles
     ${CMAKE_CURRENT_SOURCE_DIR}/src/RawFile.cpp
     ${CMAKE_CURRENT_SOURCE_DIR}/src/RawSubFile.cpp
     ${CMAKE_CURRENT_SOURCE_DIR}/src/RawMasterFile.cpp
-
     ${CMAKE_CURRENT_SOURCE_DIR}/src/utils/task.cpp
     ${CMAKE_CURRENT_SOURCE_DIR}/src/utils/ifstream_helpers.cpp
 )
diff --git a/python/tests/test_RawSubFile.py b/python/tests/test_RawSubFile.py
index cdde248..aa4721a 100644
--- a/python/tests/test_RawSubFile.py
+++ b/python/tests/test_RawSubFile.py
@@ -36,4 +36,4 @@ def test_iterate_over_a_jungfrau_RawSubFile(test_data_path):
             assert np.all(frame == data[i])
             i += 1
         assert i == 10
-        assert header["frameNumber"] == 10
\ No newline at end of file
+        assert header["frameNumber"] == 10
diff --git a/src/ClusterFile.cpp b/src/ClusterFile.cpp
new file mode 100644
index 0000000..d24e803
--- /dev/null
+++ b/src/ClusterFile.cpp
@@ -0,0 +1,402 @@
+#include "aare/ClusterFile.hpp"
+
+#include <algorithm>
+
+namespace aare {
+
+ClusterFile::ClusterFile(const std::filesystem::path &fname, size_t chunk_size,
+                         const std::string &mode)
+    : m_chunk_size(chunk_size), m_mode(mode) {
+
+    if (mode == "r") {
+        fp = fopen(fname.c_str(), "rb");
+        if (!fp) {
+            throw std::runtime_error("Could not open file for reading: " +
+                                     fname.string());
+        }
+    } else if (mode == "w") {
+        fp = fopen(fname.c_str(), "wb");
+        if (!fp) {
+            throw std::runtime_error("Could not open file for writing: " +
+                                     fname.string());
+        }
+    } else if (mode == "a") {
+        fp = fopen(fname.c_str(), "ab");
+        if (!fp) {
+            throw std::runtime_error("Could not open file for appending: " +
+                                     fname.string());
+        }
+    } else {
+        throw std::runtime_error("Unsupported mode: " + mode);
+    }
+}
+
+void ClusterFile::set_roi(ROI roi){
+    m_roi = roi;
+}
+
+void ClusterFile::set_noise_map(const NDView<int32_t, 2> noise_map){
+    m_noise_map = NDArray<int32_t, 2>(noise_map);
+}
+
+void ClusterFile::set_gain_map(const NDView<double, 2> gain_map){
+    m_gain_map = NDArray<double, 2>(gain_map);
+    
+    // Gain map is passed as ADU/keV to avoid dividing in when applying the gain
+    // map we invert it here
+    for (auto &item : m_gain_map->view()) {
+        item = 1.0 / item;
+    }
+}
+
+ClusterFile::~ClusterFile() { close(); }
+
+void ClusterFile::close() {
+    if (fp) {
+        fclose(fp);
+        fp = nullptr;
+    }
+}
+
+void ClusterFile::write_frame(const ClusterVector<int32_t> &clusters) {
+    if (m_mode != "w" && m_mode != "a") {
+        throw std::runtime_error("File not opened for writing");
+    }
+    if (!(clusters.cluster_size_x() == 3) &&
+        !(clusters.cluster_size_y() == 3)) {
+        throw std::runtime_error("Only 3x3 clusters are supported");
+    }
+    //First write the frame number - 4 bytes
+    int32_t frame_number = clusters.frame_number();
+    if(fwrite(&frame_number, sizeof(frame_number), 1, fp)!=1){
+        throw std::runtime_error(LOCATION + "Could not write frame number");
+    }
+
+    //Then write the number of clusters - 4 bytes
+    uint32_t n_clusters = clusters.size();
+    if(fwrite(&n_clusters, sizeof(n_clusters), 1, fp)!=1){
+        throw std::runtime_error(LOCATION + "Could not write number of clusters");
+    }
+
+    //Now write the clusters in the frame
+    if(fwrite(clusters.data(), clusters.item_size(), clusters.size(), fp)!=clusters.size()){
+        throw std::runtime_error(LOCATION + "Could not write clusters");
+    }
+}
+
+
+ClusterVector<int32_t> ClusterFile::read_clusters(size_t n_clusters){
+    if (m_mode != "r") {
+        throw std::runtime_error("File not opened for reading");
+    }
+    if (m_noise_map || m_roi){
+        return read_clusters_with_cut(n_clusters);
+    }else{
+        return read_clusters_without_cut(n_clusters);
+    }
+}
+
+ClusterVector<int32_t> ClusterFile::read_clusters_without_cut(size_t n_clusters) {
+    if (m_mode != "r") {
+        throw std::runtime_error("File not opened for reading");
+    }
+    
+    ClusterVector<int32_t> clusters(3,3, n_clusters);
+
+    int32_t iframe = 0; // frame number needs to be 4 bytes!
+    size_t nph_read = 0;
+    uint32_t nn = m_num_left;
+    uint32_t nph = m_num_left; // number of clusters in frame needs to be 4
+
+    // auto buf = reinterpret_cast<Cluster3x3 *>(clusters.data());
+    auto buf = clusters.data();
+    // if there are photons left from previous frame read them first
+    if (nph) {
+        if (nph > n_clusters) {
+            // if we have more photons left in the frame then photons to read we
+            // read directly the requested number
+            nn = n_clusters;
+        } else {
+            nn = nph;
+        }
+        nph_read += fread((buf + nph_read*clusters.item_size()),
+                          clusters.item_size(), nn, fp);
+        m_num_left = nph - nn; // write back the number of photons left
+    }
+
+    if (nph_read < n_clusters) {
+        // keep on reading frames and photons until reaching n_clusters
+        while (fread(&iframe, sizeof(iframe), 1, fp)) {
+            clusters.set_frame_number(iframe);
+            // read number of clusters in frame
+            if (fread(&nph, sizeof(nph), 1, fp)) {
+                if (nph > (n_clusters - nph_read))
+                    nn = n_clusters - nph_read;
+                else
+                    nn = nph;
+
+                nph_read += fread((buf + nph_read*clusters.item_size()),
+                                  clusters.item_size(), nn, fp);
+                m_num_left = nph - nn;
+            }
+            if (nph_read >= n_clusters)
+                break;
+        }
+    }
+
+    // Resize the vector to the number of clusters.
+    // No new allocation, only change bounds.
+    clusters.resize(nph_read);
+    if(m_gain_map)
+        clusters.apply_gain_map(m_gain_map->view());
+    return clusters;
+}
+
+
+ClusterVector<int32_t> ClusterFile::read_clusters_with_cut(size_t n_clusters) {
+    ClusterVector<int32_t> clusters(3,3);
+    clusters.reserve(n_clusters);
+
+    // if there are photons left from previous frame read them first
+    if (m_num_left) {
+        while(m_num_left && clusters.size() < n_clusters){
+            Cluster3x3 c = read_one_cluster();
+            if(is_selected(c)){
+                clusters.push_back(c.x, c.y, reinterpret_cast<std::byte*>(c.data));
+            }
+        }
+    }
+
+    // we did not have enough clusters left in the previous frame
+    // keep on reading frames until reaching n_clusters
+    if (clusters.size() < n_clusters) {
+        // sanity check
+        if (m_num_left) {
+            throw std::runtime_error(LOCATION + "Entered second loop with clusters left\n");
+        }
+        
+        int32_t frame_number = 0; // frame number needs to be 4 bytes!
+        while (fread(&frame_number, sizeof(frame_number), 1, fp)) {
+            if (fread(&m_num_left, sizeof(m_num_left), 1, fp)) {
+                clusters.set_frame_number(frame_number); //cluster vector will hold the last frame number
+                while(m_num_left && clusters.size() < n_clusters){
+                    Cluster3x3 c = read_one_cluster();
+                    if(is_selected(c)){
+                        clusters.push_back(c.x, c.y, reinterpret_cast<std::byte*>(c.data));
+                    }
+                }
+            }
+
+            // we have enough clusters, break out of the outer while loop
+            if (clusters.size() >= n_clusters)
+                break;
+        }
+
+    }
+    if(m_gain_map)
+        clusters.apply_gain_map(m_gain_map->view());
+
+    return clusters;
+}
+
+Cluster3x3 ClusterFile::read_one_cluster(){
+    Cluster3x3 c;
+    auto rc = fread(&c, sizeof(c), 1, fp);
+    if (rc != 1) {
+        throw std::runtime_error(LOCATION + "Could not read cluster");
+    }
+    --m_num_left;
+    return c;
+}
+
+ClusterVector<int32_t> ClusterFile::read_frame(){
+    if (m_mode != "r") {
+        throw std::runtime_error(LOCATION + "File not opened for reading");
+    }
+    if (m_noise_map || m_roi){
+        return read_frame_with_cut();
+    }else{
+        return read_frame_without_cut();
+    }
+}
+
+ClusterVector<int32_t> ClusterFile::read_frame_without_cut() {
+    if (m_mode != "r") {
+        throw std::runtime_error("File not opened for reading");
+    }
+    if (m_num_left) {
+        throw std::runtime_error(
+            "There are still photons left in the last frame");
+    }
+    int32_t frame_number;
+    if (fread(&frame_number, sizeof(frame_number), 1, fp) != 1) {
+        throw std::runtime_error(LOCATION + "Could not read frame number");
+    }
+
+    int32_t n_clusters; // Saved as 32bit integer in the cluster file
+    if (fread(&n_clusters, sizeof(n_clusters), 1, fp) != 1) {
+        throw std::runtime_error(LOCATION + "Could not read number of clusters");
+    }
+
+    ClusterVector<int32_t> clusters(3, 3, n_clusters);
+    clusters.set_frame_number(frame_number);
+
+    if (fread(clusters.data(), clusters.item_size(), n_clusters, fp) !=
+        static_cast<size_t>(n_clusters)) {
+        throw std::runtime_error(LOCATION + "Could not read clusters");
+    }
+    clusters.resize(n_clusters);
+    if (m_gain_map)
+        clusters.apply_gain_map(m_gain_map->view());
+    return clusters;
+}
+
+ClusterVector<int32_t> ClusterFile::read_frame_with_cut() {
+    if (m_mode != "r") {
+        throw std::runtime_error("File not opened for reading");
+    }
+    if (m_num_left) {
+        throw std::runtime_error(
+            "There are still photons left in the last frame");
+    }
+    int32_t frame_number;
+    if (fread(&frame_number, sizeof(frame_number), 1, fp) != 1) {
+        throw std::runtime_error("Could not read frame number");
+    }
+
+
+    if (fread(&m_num_left, sizeof(m_num_left), 1, fp) != 1) {
+        throw std::runtime_error("Could not read number of clusters");
+    }
+    
+    ClusterVector<int32_t> clusters(3, 3);
+    clusters.reserve(m_num_left);
+    clusters.set_frame_number(frame_number);
+    while(m_num_left){
+        Cluster3x3 c = read_one_cluster();
+        if(is_selected(c)){
+            clusters.push_back(c.x, c.y, reinterpret_cast<std::byte*>(c.data));
+        }
+    }
+    if (m_gain_map)
+        clusters.apply_gain_map(m_gain_map->view());
+    return clusters;
+}
+
+
+
+bool ClusterFile::is_selected(Cluster3x3 &cl) {
+    //Should fail fast
+    if (m_roi) {
+        if (!(m_roi->contains(cl.x, cl.y))) {
+            return false;
+        }
+    }
+    if (m_noise_map){
+        int32_t sum_1x1 = cl.data[4]; // central pixel
+        int32_t sum_2x2 = cl.sum_2x2(); // highest sum of 2x2 subclusters
+        int32_t sum_3x3 = cl.sum(); // sum of all pixels
+
+        auto noise = (*m_noise_map)(cl.y, cl.x); //TODO! check if this is correct
+        if (sum_1x1 <= noise || sum_2x2 <= 2 * noise || sum_3x3 <= 3 * noise) {
+            return false;
+        }
+    }
+    //we passed all checks
+    return true;
+}
+
+NDArray<double, 2> calculate_eta2(ClusterVector<int> &clusters) {
+    //TOTO! make work with 2x2 clusters
+    NDArray<double, 2> eta2({static_cast<int64_t>(clusters.size()), 2});
+    
+    if (clusters.cluster_size_x() == 3 || clusters.cluster_size_y() == 3) {
+        for (size_t i = 0; i < clusters.size(); i++) {
+            auto e = calculate_eta2(clusters.at<Cluster3x3>(i));
+            eta2(i, 0) = e.x;
+            eta2(i, 1) = e.y;
+        }
+    }else if(clusters.cluster_size_x() == 2 || clusters.cluster_size_y() == 2){
+        for (size_t i = 0; i < clusters.size(); i++) {
+            auto e = calculate_eta2(clusters.at<Cluster2x2>(i));
+            eta2(i, 0) = e.x;
+            eta2(i, 1) = e.y;
+        }
+    }else{
+        throw std::runtime_error("Only 3x3 and 2x2 clusters are supported");
+    }
+    
+    return eta2;
+}
+
+/** 
+ * @brief Calculate the eta2 values for a 3x3 cluster and return them in a Eta2 struct
+ * containing etay, etax and the corner of the cluster. 
+*/
+Eta2 calculate_eta2(Cluster3x3 &cl) {
+    Eta2 eta{};
+
+    std::array<int32_t, 4> tot2;
+    tot2[0] = cl.data[0] + cl.data[1] + cl.data[3] + cl.data[4];
+    tot2[1] = cl.data[1] + cl.data[2] + cl.data[4] + cl.data[5];
+    tot2[2] = cl.data[3] + cl.data[4] + cl.data[6] + cl.data[7];
+    tot2[3] = cl.data[4] + cl.data[5] + cl.data[7] + cl.data[8];
+
+    auto c = std::max_element(tot2.begin(), tot2.end()) - tot2.begin();
+    eta.sum = tot2[c];
+    switch (c) {
+    case cBottomLeft:
+        if ((cl.data[3] + cl.data[4]) != 0)
+            eta.x =
+                static_cast<double>(cl.data[4]) / (cl.data[3] + cl.data[4]);
+        if ((cl.data[1] + cl.data[4]) != 0)
+            eta.y =
+                static_cast<double>(cl.data[4]) / (cl.data[1] + cl.data[4]);
+        eta.c = cBottomLeft;
+        break;
+    case cBottomRight:
+        if ((cl.data[2] + cl.data[5]) != 0)
+            eta.x =
+                static_cast<double>(cl.data[5]) / (cl.data[4] + cl.data[5]);
+        if ((cl.data[1] + cl.data[4]) != 0)
+            eta.y =
+                static_cast<double>(cl.data[4]) / (cl.data[1] + cl.data[4]);
+        eta.c = cBottomRight;
+        break;
+    case cTopLeft:
+        if ((cl.data[7] + cl.data[4]) != 0)
+            eta.x =
+                static_cast<double>(cl.data[4]) / (cl.data[3] + cl.data[4]);
+        if ((cl.data[7] + cl.data[4]) != 0)
+            eta.y =
+                static_cast<double>(cl.data[7]) / (cl.data[7] + cl.data[4]);
+        eta.c = cTopLeft;
+        break;
+    case cTopRight:
+        if ((cl.data[5] + cl.data[4]) != 0)
+            eta.x =
+                static_cast<double>(cl.data[5]) / (cl.data[5] + cl.data[4]);
+        if ((cl.data[7] + cl.data[4]) != 0)
+            eta.y =
+                static_cast<double>(cl.data[7]) / (cl.data[7] + cl.data[4]);
+        eta.c = cTopRight;
+        break;
+    // no default to allow compiler to warn about missing cases
+    }
+    return eta;
+}
+
+
+Eta2 calculate_eta2(Cluster2x2 &cl) {
+    Eta2 eta{};
+    if ((cl.data[0] + cl.data[1]) != 0)
+        eta.x = static_cast<double>(cl.data[1]) / (cl.data[0] + cl.data[1]);
+    if ((cl.data[0] + cl.data[2]) != 0)
+        eta.y = static_cast<double>(cl.data[2]) / (cl.data[0] + cl.data[2]);
+    eta.sum = cl.data[0] + cl.data[1] + cl.data[2]+ cl.data[3];
+    eta.c = cBottomLeft; //TODO! This is not correct, but need to put something
+    return eta;
+}
+
+
+} // namespace aare
\ No newline at end of file
diff --git a/src/RawSubFile.cpp b/src/RawSubFile.cpp
index 01ef48c..a8d29ce 100644
--- a/src/RawSubFile.cpp
+++ b/src/RawSubFile.cpp
@@ -11,6 +11,8 @@
 #include <regex>
 
 
+
+
 namespace aare {
 
 RawSubFile::RawSubFile(const std::filesystem::path &fname,
diff --git a/src/algorithm.test.cpp b/src/algorithm.test.cpp
index 6bd707b..bf49c52 100644
--- a/src/algorithm.test.cpp
+++ b/src/algorithm.test.cpp
@@ -192,4 +192,4 @@ TEST_CASE("All equal on a vector with two different  elements is false", "[algor
 TEST_CASE("Last element is different", "[algorithm]") {
     std::vector<int> vec = {1, 1, 1, 1, 2};
     REQUIRE(aare::all_equal(vec) == false);
-}
\ No newline at end of file
+}