Add comment about ddof

cta-observatory · Jun 18, 2018 · e289b11 · e289b11
1 parent 07bffe6
commit e289b11
Showing 1 changed file with 3 additions and 0 deletions.
diff --git a/ctapipe/image/hillas.py b/ctapipe/image/hillas.py
@@ -704,6 +704,9 @@ def hillas_parameters_5(geom: CameraGeometry, image):
     delta_x = pix_x - cog_x
     delta_y = pix_y - cog_y
 
+    # The ddof=0 makes this comparable to the other methods,
+    # but ddof=1 should be more correct, mostly affects small showers
+    # on a percent level
     cov = np.cov(delta_x, delta_y, aweights=image, ddof=0)
     eig_vals, eig_vecs = np.linalg.eigh(cov)