diff --git a/ObservationSim/sim_steps/readout_output.py b/ObservationSim/sim_steps/readout_output.py
index 380d445ec22c9d1dd552322b3ba8f82b6f263ba2..bd5a51952adc0ff1d3adee056a96bdbe000e6080 100644
--- a/ObservationSim/sim_steps/readout_output.py
+++ b/ObservationSim/sim_steps/readout_output.py
@@ -35,6 +35,9 @@ def apply_gain(self, chip, filt, tel, pointing, catalog, obs_param):
 
 def quantization_and_output(self, chip, filt, tel, pointing, catalog, obs_param):
 
+    if not hasattr(self, 'h_ext'):
+        _, _ = self.prepare_headers(chip=chip, pointing=pointing)
+
     if obs_param["format_output"] == True:
         self.chip_output.Log_info("  Apply 1*16 format")
         chip.img = chip_utils.formatOutput(GSImage=chip.img)