diff --git a/.gitignore b/.gitignore
index 56d24dd..ddcc79f 100644
--- a/.gitignore
+++ b/.gitignore
@@ -38,7 +38,7 @@ Thumbs.db
 # Claude Code - local settings excluded, hooks tracked
 .claude/settings.local.json
 
-# Original NAAP Flash-based reference simulation (not tracked)
+# Local decompile output (and any accidental NAAP re-clones). Upstream sources: ../Baseline/Astronomy/
 NAAP/
 
 # Flash decompilation tooling (npm run decompile) — downloaded FFDec
diff --git a/CLAUDE.md b/CLAUDE.md
index d406db6..ad9c534 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -77,5 +77,5 @@ npm run lint && npm run check && npm run build && npm test
 ## Development notes
 
 - **`VariableStarPhotometryConstants.ts`** uses nested frozen `as const` groups (`FIELD`, `APERTURE`, `TIME`, `LAYOUT`, `PDM`) instead of a flat namespace — intentional for five distinct concerns; still no magic numbers in model/view code.
-- **`npm run decompile`** extracts NAAP Flash ActionScript via JPEXS FFDec into gitignored `NAAP/decompiled/` — read-only reference.
+- **`npm run decompile`** extracts NAAP Flash ActionScript via JPEXS FFDec from `../Baseline/Astronomy/flash-animations` into gitignored `NAAP/decompiled/`.
 - After `npm run build`, the sim is installable offline via Workbox (`dist/manifest.webmanifest`).
diff --git a/README.md b/README.md
index 68fbbe5..f9c2391 100644
--- a/README.md
+++ b/README.md
@@ -16,6 +16,21 @@ A SceneryStack port of the [NAAP Variable Star Photometry Lab](https://astro.unl
 - English, French, and Spanish localization
 - Progressive Web App (installable, offline-capable)
 
+### NAAP reference sources
+
+Upstream Flash / AIR / React NAAP sources live in the sibling
+[`Baseline`](https://github.com/OpenPhysics/Baseline) repo under `Astronomy/`
+(see `baselines.json`). Clone Baseline with the fleet bootstrap, then:
+
+```bash
+(cd ../Baseline && ./scripts/fetch-baselines.sh)
+```
+
+`npm run decompile` reads `.swf` files from
+`../Baseline/Astronomy/flash-animations` and writes ActionScript into the
+sim-local gitignored `NAAP/decompiled/` (requires Java; one-time
+`npm run decompile -- --setup`).
+
 ## Quick Start
 
 ```bash
diff --git a/scripts/decompile-flash.ts b/scripts/decompile-flash.ts
index b982e3f..6b6b2a9 100644
--- a/scripts/decompile-flash.ts
+++ b/scripts/decompile-flash.ts
@@ -2,7 +2,7 @@
  * decompile-flash.ts
  *
  * Extract readable ActionScript (and optionally assets) from the original NAAP
- * Flash sources under `NAAP/`, so the ported Variable Star Photometry sim can be
+ * Flash sources under `../Baseline/Astronomy/flash-animations/` (sibling Baseline repo), so the ported Variable Star Photometry sim can be
  * checked against the real implementation. See PORTING_PLAN.md → "Flash simulator
  * inventory".
  *
@@ -36,7 +36,18 @@ import { inflateRawSync } from "node:zlib";
 
 const here = dirname(fileURLToPath(import.meta.url));
 const repoRoot = resolve(here, "..");
-const flashRoot = join(repoRoot, "NAAP", "flash-animations", "flashdev2");
+const flashRoot = resolve(repoRoot, "..", "Baseline", "Astronomy", "flash-animations", "flashdev2");
+
+/** Fail fast when the Baseline sibling checkout (or fetch) is missing. */
+function ensureFlashRoot(): void {
+  if (!existsSync(flashRoot)) {
+    fail(
+      `NAAP Flash sources not found at:\n  ${flashRoot}\n` +
+        `Clone OpenPhysics/Baseline as a sibling of this sim, then:\n` +
+        `  (cd ../Baseline && ./scripts/fetch-baselines.sh --only Astronomy/flash-animations)`,
+    );
+  }
+}
 
 /**
  * Latest canonical SWFs whose ActionScript informs the port (used by --all).
@@ -438,6 +449,8 @@ async function main(): Promise<void> {
     return;
   }
 
+  ensureFlashRoot();
+
   const movies = resolveTargets(opts);
   if (movies.length === 0) {
     fail("No .swf targets resolved. Check the paths, or run with --help.");