diff --git a/data-access/servlet/src/main/java/cutout/SodaImpl.java b/data-access/servlet/src/main/java/cutout/SodaImpl.java
index 959ef54f68dd38df638f08b607c11e42c539fae5..5501c876b48caab2b6acee49aa0098a15b89c70a 100644
--- a/data-access/servlet/src/main/java/cutout/SodaImpl.java
+++ b/data-access/servlet/src/main/java/cutout/SodaImpl.java
@@ -78,7 +78,7 @@ class SodaImpl implements Soda
 		cmd[0] = "/usr/local/bin/vlkb";
 		cmd[1] = "cutout";
 		cmd[2] = fitsPaths.surveys() + "/" + relPathname;
-		cmd[3] = String.valueOf(hdunum);
+		cmd[3] = String.valueOf(hdunum - 1); // extnum = hdunum - 1
 		cmd[4] = coordString;
 		LOGGER.finest(String.join(" ", cmd));