diff --git a/src/mcstastox/ReadNeXus.py b/src/mcstastox/ReadNeXus.py
index 621f922..9b928b6 100644
--- a/src/mcstastox/ReadNeXus.py
+++ b/src/mcstastox/ReadNeXus.py
@@ -3,7 +3,9 @@
 import re
 from dataclasses import dataclass
 from types import MappingProxyType
+from typing import cast
 
+import h5py
 import numpy as np
 
 
@@ -43,6 +45,35 @@ def _get_mcstas_version_settings(
     raise ValueError(f"McStas version {version} not supported by this tool.")
 
 
+def _validate_file(file_handle: h5py.File) -> None:
+    # Check file is formatted as expected
+    if "entry1" not in list(file_handle.keys()):
+        raise ValueError("h5 file not formatted as expected, lacks 'entry1'.")
+
+    entry_obj = cast(h5py.Group, file_handle["entry1"])
+    entry_keys = tuple(entry_obj.keys())
+
+    mandatory_entry_keys = ("data", "simulation", "instrument")
+    missing_keys = tuple(key for key in mandatory_entry_keys if key not in entry_keys)
+    if any(missing_keys):
+        raise ValueError(
+            "'entry1' not formatted as expected, lacks keys: "
+            f"[{', '.join(missing_keys)}]."
+        ) from None
+
+    simluation_keys = tuple(entry_obj["simulation"].keys())
+    if "Param" not in simluation_keys:
+        raise ValueError(
+            "'entry1/simulation' not formatted as expected, lacks 'Param'."
+        )
+
+    instrument_keys = tuple(entry_obj["instrument"].keys())
+    if "components" not in instrument_keys:
+        raise ValueError(
+            "'entry1/instrument' not formatted as expected, lacks 'components'."
+        )
+
+
 class McStasNeXus:
     """
     Reads a McStas NeXus files and provides methods to retrieve data or entries
@@ -57,63 +88,43 @@ def __init__(
         mcstas_setting_registry: _McStasVersionSettingTp = _MCSTAS_VERSION_SETTINGS,
     ):
         self.file_handle = file_handle
-        f = self.file_handle
-
-        self.mcstas_version = mcstas_version or self.read_mcstas_version()
+        # Check file is formatted as expected
+        _validate_file(self.file_handle)
+        self.mcstas_version = mcstas_version or self._read_mcstas_version()
 
         # Load settings appropriate for this McStas version
         self.settings = _get_mcstas_version_settings(
             self.mcstas_version, mcstas_setting_registry
         )
 
-        # Check file is formatted as expected
-        if "entry1" not in list(f.keys()):
-            raise ValueError("h5 file not formatted as expected, lacks 'entry1'.")
-
-        if "data" not in list(f["entry1"].keys()):
-            raise ValueError("h5 file not formatted as expected, lacks 'data'.")
-
-        if "simulation" not in list(f["entry1"].keys()):
-            raise ValueError("h5 file not formatted as expected, lacks 'simulation'.")
-
-        if "Param" not in list(f["entry1"]["simulation"].keys()):
-            raise ValueError("h5 file not formatted as expected, lacks 'Param'.")
-
-        if "instrument" not in list(f["entry1"].keys()):
-            raise ValueError("h5 file not formatted as expected, lacks 'instrument'.")
-
-        if "components" not in list(f["entry1"]["instrument"].keys()):
-            raise ValueError("h5 file not formatted as expected, lacks 'components'.")
-
         # Grab basic information
+        self.component_names: list
+        self.component_path_names: dict
+        self._read_component_name_and_path()
+
+    def _read_component_name_and_path(self) -> None:
         if self.settings.component_numbers is None:
-            self.component_names = f["entry1"]["instrument"]["components"].keys()
+            self.component_names = list(
+                self.file_handle["entry1"]["instrument"]["components"].keys()
+            )
             self.component_path_names = {name: name for name in self.component_names}
         else:
             comp_name_start_index = self.settings.component_numbers + 1
-            self.component_names = []
-            self.component_path_names = {}
-            full_comp_names = f["entry1"]["instrument"]["components"].keys()
-            for name in full_comp_names:
-                component_name = name[comp_name_start_index:]
-                self.component_names.append(component_name)
-                self.component_path_names[component_name] = name
-
-    def read_mcstas_version(self) -> tuple[int, int, int]:
-        f = self.file_handle
-
-        # First attempt at reading version
-        if "entry1" not in list(f.keys()):
-            raise ValueError("h5 file not formatted as expected, lacks 'entry1'")
-
-        if "simulation" not in list(f["entry1"].keys()):
-            raise ValueError(
-                "h5 file not formatted as expected, lacks 'entry1/simulation'"
+            components = self.file_handle["entry1"]["instrument"]["components"].keys()
+            component_paths = list(components)
+            self.component_names = [
+                name[comp_name_start_index:] for name in component_paths
+            ]
+            self.component_path_names = dict(
+                zip(self.component_names, component_paths, strict=True)
             )
 
+    def _read_mcstas_version(self) -> tuple[int, int, int]:
+        f = self.file_handle
         if "program" not in list(f["entry1"]["simulation"].attrs):
             raise ValueError(
-                "h5 file not formatted as expected, lacks 'entry1/simulation/program'"
+                "h5 file not formatted as expected, "
+                "lacks 'program' attribute in 'entry1/simulation/program'"
             )
 
         version_string = f["entry1"]["simulation"].attrs["program"].decode("utf-8")