Check if tel_id exists when looking up TelescopeParameters

ctapipe/core/telescope_component.py

@@ -217,8 +217,10 @@ def __getitem__(self, tel: Optional[Union[int, str]]):
             try:
                 return self._value_for_tel_id[tel]
             except KeyError:
+                if tel not in self._subarray.tel:
+                    raise KeyError(f"No telescope with id {tel} in subarray")
                 raise KeyError(
-                    f"TelescopeParameterLookup: no "
+                    "TelescopeParameterLookup: no "
                     f"parameter value was set for telescope with tel_id="
                     f"{tel}. Please set it explicitly, "
                     f"or by telescope type or '*'."

ctapipe/core/tests/test_telescope_component.py

@@ -342,3 +342,16 @@ class Foo(TelescopeComponent):
 
     f = Foo(mock_subarray, bar=[("type", "*", 1), ("id", 1, None)])
     assert f.bar.tel[1] is None
+
+
+def test_telescope_parameter_nonexistent_telescope(mock_subarray):
+    class Foo(TelescopeComponent):
+        bar = IntTelescopeParameter(
+            default_value=None,
+            allow_none=True,
+        ).tag(config=True)
+
+    foo = Foo(subarray=mock_subarray)
+
+    with pytest.raises(KeyError, match="No telescope with id 0"):
+        foo.bar.tel[0]

docs/changes/2429.feature.rst

@@ -0,0 +1,3 @@
+In case no configuration is found for a telescope in ``TelescopeParameter``,
+it is now checked whether the telescope exists at all to provide a better
+error message.