diff --git a/common/src/aaredaqlib/diffraction_geometry.py b/common/src/aaredaqlib/diffraction_geometry.py
index 3c541306..d91e9846 100644
--- a/common/src/aaredaqlib/diffraction_geometry.py
+++ b/common/src/aaredaqlib/diffraction_geometry.py
@@ -38,7 +38,7 @@ class DiffractionGeometry(BaseModel):
         if exp_dtz_mm <= 0:
             raise ValueError("Detector distance must be positive")
 
-        theta = math.atan(self.detector_radius_mm / exp_dtz_mm)
+        theta = math.atan(self.detector_radius_mm / exp_dtz_mm)*0.5
         return self.wavelength_angstrom / (2 * math.sin(theta))
 
     def calc_dtz_mm(self, exp_resolution_angstrom: float) -> float:
@@ -48,4 +48,4 @@ class DiffractionGeometry(BaseModel):
         if x >= 1.0 or x <= -1.0:
             return 0.0
         theta = math.asin(x)
-        return self.detector_radius_mm / math.tan(theta)
\ No newline at end of file
+        return self.detector_radius_mm / math.tan(2*theta)
\ No newline at end of file