From 55f74dea2bbfff05c4f79536ec3557c7fcb094a8 Mon Sep 17 00:00:00 2001
From: Maximilian Linhoff <maximilian.linhoff@tu-dortmund.de>
Date: Mon, 29 Jul 2024 12:59:48 +0200
Subject: [PATCH] Better error message for invalid tel_id

---
 src/ctapipe_io_lst/__init__.py | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/src/ctapipe_io_lst/__init__.py b/src/ctapipe_io_lst/__init__.py
index a60128da..37596281 100644
--- a/src/ctapipe_io_lst/__init__.py
+++ b/src/ctapipe_io_lst/__init__.py
@@ -454,12 +454,15 @@ def create_subarray(tel_id=1, reference_location=None):
 
         tel_descriptions = {tel_id: lst_tel_descr}
 
-        xyz = ground_frame_from_earth_location(
-            LST_LOCATIONS[tel_id],
-            reference_location,
-        ).cartesian.xyz
-        tel_positions = {tel_id: xyz}
-
+        try:
+            location = LST_LOCATIONS[tel_id]
+        except KeyError:
+            known = list(LST_LOCATIONS.keys())
+            msg = f"Location missing for tel_id={tel_id}. Known tel_ids: {known}. Is this LST data?"
+            raise KeyError(msg) from None
+
+        ground_frame = ground_frame_from_earth_location(location, reference_location)
+        tel_positions = {tel_id: ground_frame.cartesian.xyz}
         subarray = SubarrayDescription(
             name=f"LST-{tel_id} subarray",
             tel_descriptions=tel_descriptions,