diff --git a/utils/raspberrypi/ctt/ctt_alsc.py b/utils/raspberrypi/ctt/ctt_alsc.py
index c0ba7b637..e51d69319 100644
--- a/utils/raspberrypi/ctt/ctt_alsc.py
+++ b/utils/raspberrypi/ctt/ctt_alsc.py
@@ -132,7 +132,7 @@ def alsc(Cam, Img, do_alsc_colour, plot=False):
     """
     average the green channels into one
     """
-    av_ch_g = np.mean((channels[1:2]), axis=0)
+    av_ch_g = np.mean((channels[1:3]), axis=0)
     if do_alsc_colour:
         """
         obtain 16x12 grid of intensities for each channel and subtract black level