diff --git a/csst_common/data_manager.py b/csst_common/data_manager.py
index a1f90895cf38bd9f0822872d02c1978e52ce55cb..dc2f8b3d3f3cc90fce577f355009e8168591cce9 100644
--- a/csst_common/data_manager.py
+++ b/csst_common/data_manager.py
@@ -1064,6 +1064,15 @@ class CsstMsFile(dict):
                  f"{self.obs_id}_{self.detector}_L1_V{self.version}_{post}"
         return os.path.join(self.dir_out, fn)
 
+    def fno(self, post=".fits"):
+        if post.startswith("."):
+            fn = f"{self.telescope}_{self.instrument}_{self.project}_{self.obs_type}_{self.exp_start}_{self.exp_stop}_" \
+                 f"{self.obs_id}_{self.detector}_L1_V{self.version}{post}"
+        else:
+            fn = f"{self.telescope}_{self.instrument}_{self.project}_{self.obs_type}_{self.exp_start}_{self.exp_stop}_" \
+                 f"{self.obs_id}_{self.detector}_L1_V{self.version}_{post}"
+        return fn
+
     def fpi(self):
         return os.path.join(self.dir_in, self.filename)