modified: ../config/sidebar.yml

	modified:   part1/simple_detector.md
	modified:   ../../static/hit_position.png

src/config/sidebar.yml

@@ -14,7 +14,7 @@
       #      link: '/getting-started/eic_container'
       #    - label: 'Environment'
       #      link: '/getting-started/environment'
-- label: Tutorials
+- label: Tutorial
   items:
     - label: 'Quick-start '
       link: '/getting-started/quickstart'

src/docs/part1/simple_detector.md

@@ -80,7 +80,29 @@ https://eic.phy.anl.gov/eicd/
 root -b -q scripts/tutorial1_hit_position.cxx
 # note if this fails because of ACLiC, run it again. This is a root bug.
 ```
-Look at the generated file in results
+Look at the generated file in results:
 ![local hit position](../hit_position.png)
 
+This is the local position in the segmentation (i.e. pixel, strip, readout pad, etc.).
+
+### Cell ID (channel) to Position
+
+At the top of the script you can see these lines:
+```cpp
+  dd4hep::Detector& detector = dd4hep::Detector::getInstance();
+  detector.fromCompact("gem_tracker.xml");
+  dd4hep::rec::CellIDPositionConverter cellid_converter(detector);
+```
+This 
+  - gets the DD4hep instance
+  - loads the compact detector file
+  - initializes the [position converter tool](https://dd4hep.web.cern.ch/dd4hep/reference/classdd4hep_1_1rec_1_1CellIDPositionConverter.html) (which provides thread safe access)
+
+Later on you can see its use:
+```cpp
+      auto pos1 = cellid_converter.position(h.cellID);
+```
+
+
+