diff --git a/dx2/detector.cxx b/dx2/detector.cxx
index cb8737e..6e4fc54 100644
--- a/dx2/detector.cxx
+++ b/dx2/detector.cxx
@@ -74,23 +74,54 @@ void Panel::update(Matrix3d d) {
 }
 
 Panel::Panel(json panel_data) {
+  // We can either get an explicit origin or beam centre and distance
+  std::vector<std::string> required_keys = {
+      "fast_axis",        "slow_axis", "pixel_size", "image_size",
+      "trusted_range",    "type",      "name",       "thickness",
+      "raw_image_offset", "gain",      "pedestal",   "px_mm_strategy"};
+  for (const auto &key : required_keys) {
+    if (panel_data.find(key) == panel_data.end()) {
+      throw std::invalid_argument(
+          "Key " + key + " is missing from the input detector panel JSON");
+    }
+  }
   Vector3d fast{{panel_data["fast_axis"][0], panel_data["fast_axis"][1],
                  panel_data["fast_axis"][2]}};
   Vector3d slow{{panel_data["slow_axis"][0], panel_data["slow_axis"][1],
                  panel_data["slow_axis"][2]}};
-  Vector3d origin{{panel_data["origin"][0], panel_data["origin"][1],
-                   panel_data["origin"][2]}};
-  Matrix3d d_matrix{{fast[0], slow[0], origin[0]},
-                    {fast[1], slow[1], origin[1]},
-                    {fast[2], slow[2], origin[2]}};
-  origin_ = origin;
+  fast.normalize();
+  slow.normalize();
   fast_axis_ = fast;
   slow_axis_ = slow;
-  normal_ = fast_axis_.cross(slow_axis_);
-  d_ = d_matrix;
-  D_ = d_.inverse();
   pixel_size_ = {{panel_data["pixel_size"][0], panel_data["pixel_size"][1]}};
   image_size_ = {{panel_data["image_size"][0], panel_data["image_size"][1]}};
+  Matrix3d d_matrix;
+
+  if (panel_data.contains("beam_center") && panel_data.contains("distance")) {
+    double distance = panel_data["distance"];
+    std::array<double, 2> beam_center = panel_data["beam_center"];
+    origin_ = {0., 0., -1.0 * distance};
+    origin_ -= beam_center[0] * pixel_size_[0] * fast_axis_;
+    origin_ -= beam_center[1] * pixel_size_[1] * slow_axis_;
+    normal_ = fast_axis_.cross(slow_axis_);
+    d_matrix << fast_axis_[0], slow_axis_[0], origin_[0], fast_axis_[1],
+        slow_axis_[1], origin_[1], fast_axis_[2], slow_axis_[2], origin_[2];
+  } else {
+    if (!panel_data.contains("origin")) {
+      throw std::invalid_argument("Detector panel JSON must contain either "
+                                  "origin or beam_center + distance");
+    }
+    origin_ = Vector3d{{panel_data["origin"][0], panel_data["origin"][1],
+                        panel_data["origin"][2]}};
+    d_matrix << fast[0], slow[0], origin_[0], fast[1], slow[1], origin_[1],
+        fast[2], slow[2], origin_[2];
+    // origin_ = origin;
+    normal_ = fast_axis_.cross(slow_axis_);
+  }
+
+  d_ = d_matrix;
+  D_ = d_.inverse();
+
   image_size_mm_ = {
       {image_size_[0] * pixel_size_[0], image_size_[1] * pixel_size_[1]}};
   trusted_range_ = {
@@ -200,10 +231,12 @@ const std::map<std::string, Vector3d> axis_map = {
     {"-x", Vector3d(-1.0, 0.0, 0.0)},
     {"y", Vector3d(0.0, 1.0, 0.0)},
     {"-y", Vector3d(0.0, -1.0, 0.0)}};
-Panel::Panel(double distance, std::array<double, 2> beam_center,
-             std::array<double, 2> pixel_size, std::array<int, 2> image_size,
-             const std::string &fast_axis, const std::string &slow_axis,
-             double thickness, double mu)
+
+Panel::Panel(double distance,                   // units mm
+             std::array<double, 2> beam_center, // units px
+             std::array<double, 2> pixel_size,  // units mm
+             std::array<int, 2> image_size, const std::string &fast_axis,
+             const std::string &slow_axis, double thickness, double mu)
     : pixel_size_(pixel_size), image_size_(image_size), thickness_(thickness),
       mu_(mu) {
   if (valid_axes.find(fast_axis) == valid_axes.end()) {
diff --git a/include/dx2/detector.hpp b/include/dx2/detector.hpp
index 0fea7b4..2ad16ec 100644
--- a/include/dx2/detector.hpp
+++ b/include/dx2/detector.hpp
@@ -70,14 +70,14 @@ class Panel {
 
 protected:
   // panel_frame items
-  Vector3d origin_{{0.0, 0.0, 100.0}}; // needs to be set
+  Vector3d origin_{{0.0, 0.0, 100.0}}; // needs to be set // units mm
   Vector3d fast_axis_{{1.0, 0.0, 0.0}};
   Vector3d slow_axis_{{0.0, 1.0, 0.0}};
   Vector3d normal_{{0.0, 0.0, 1.0}};
   Matrix3d d_{{1, 0, 0}, {0, 1, 0}, {0, 0, 100.0}};
   Matrix3d D_{{1, 0, 0}, {0, 1, 0}, {0, 0, 0.01}};
   //  panel data
-  std::array<double, 2> pixel_size_{{0.075, 0.075}};
+  std::array<double, 2> pixel_size_{{0.075, 0.075}}; // units mm
   std::array<int, 2> image_size_{{0, 0}};
   std::array<double, 2> image_size_mm_{{0, 0}};
   std::array<double, 2> trusted_range_{0.0, 65536.0};