diff --git a/csst_ifs_gehong/config.py b/csst_ifs_gehong/config.py
index 46da374b365c903beca5f2022628add5a5a46541..451798c4b66ad37402d5ed7a4aa7bb5846a73d7a 100644
--- a/csst_ifs_gehong/config.py
+++ b/csst_ifs_gehong/config.py
@@ -33,3 +33,4 @@ class config():
         self.dpix = dpix
         self.fov_x = nx * dpix
         self.fov_y = ny * dpix
+        self.inst_fwhm = 2.5