diff --git a/swissmx.py b/swissmx.py
index cd6a688..d23ed72 100755
--- a/swissmx.py
+++ b/swissmx.py
@@ -879,6 +879,8 @@ class WndSwissMx(QMainWindow, Ui_MainWindow):
   def cb_move_backlight_safe(self, pos):
     app=QApplication.instance()
     bl=app._backlight
+    # any move of backlight requires detector move first
+    self.move_detector("out")       
     # any move of backlight requires post sample tube out
     self.move_post_tube("out")
     bl.move(pos)