diff --git a/.gitignore b/.gitignore
index 3682231..421daf1 100644
--- a/.gitignore
+++ b/.gitignore
@@ -42,7 +42,6 @@ Thumbs.db
 .claude/settings.local.json
 
 # Optics simulation template
-optics-template/
 
 # CI / local npm audit artifact (must not be linted)
 audit.json
diff --git a/CLAUDE.md b/CLAUDE.md
index 9eb7e9c..ad7a066 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -11,7 +11,7 @@ Geometric optics simulation: ray tracing through lenses, mirrors, beam splitters
 - **Presets** — curated demonstration scenes
 - **Diffraction** — gratings-focused carousel
 
-**Ignore `optics-template/`** — reference material, not part of the shipped sim.
+**Upstream reference:** `../Baseline/OpticsLab/ray-optics` (OpenPhysics/Baseline); not part of the shipped sim.
 
 Physics for educators: `doc/model.md`. Architecture: `doc/implementation-notes.md`.