diff --git a/js/focusanddirectrix/model/FocusAndDirectrixModel.ts b/js/focusanddirectrix/model/FocusAndDirectrixModel.ts
index 70fdf7f6..f6173bb7 100644
--- a/js/focusanddirectrix/model/FocusAndDirectrixModel.ts
+++ b/js/focusanddirectrix/model/FocusAndDirectrixModel.ts
@@ -94,6 +94,7 @@ export default class FocusAndDirectrixModel extends GQModel {
     this.pointOnParabolaProperty = new Vector2Property( initialPoint, {
       tandem: tandem.createTandem( 'pointOnParabolaProperty' ),
       phetioFeatured: true,
+      phetioReadOnly: true,
       phetioDocumentation: 'the interactive point on the parabola'
     } );