diff --git a/megabeast/mbmodel.py b/megabeast/mbmodel.py
index 0ade923..fe86704 100644
--- a/megabeast/mbmodel.py
+++ b/megabeast/mbmodel.py
@@ -68,11 +68,13 @@ def __init__(self, stellar_model, dust_model):
                         self.physics_model[cparam]
                     )
                 elif cparam == "distance":
-                     self.physics_model[cparam]["model"] = PhysDistanceModel(
+                    self.physics_model[cparam]["model"] = PhysDistanceModel(
                         self.physics_model[cparam]
-                    )                   
+                    )
                 else:
-                    raise NotImplementedError(f"{cparam} is not an allowed stellar model parameter")
+                    raise NotImplementedError(
+                        f"{cparam} is not an allowed stellar model parameter"
+                    )
             elif cparam in self.dust_model.keys():
                 self.physics_model[cparam]["nsubvars"] = 1
                 self.physics_model[cparam]["model"] = PhysDustModel(
diff --git a/megabeast/tools/simulate_obs.py b/megabeast/tools/simulate_obs.py
index 2a879ee..a3edcc6 100644
--- a/megabeast/tools/simulate_obs.py
+++ b/megabeast/tools/simulate_obs.py
@@ -275,7 +275,7 @@ def gen_SimObs_from_sedgrid(
 def simulate_obs(
     physgrid_list,
     noise_model_list,
-    output_catalog,
+    output_catalog=None,
     beastinfo_list=None,
     mbmodel=None,
     nsim=0,
@@ -443,7 +443,11 @@ def simulate_obs(
         simtable_list.append(simtable)
 
     # stack all the tables into one and write it out
-    vstack(simtable_list).write(output_catalog, overwrite=True)
+    fincat = vstack(simtable_list)
+    if output_catalog is not None:
+        fincat.write(output_catalog, overwrite=True)
+
+    return fincat
 
 
 def main():
@@ -522,7 +526,7 @@ def main():
     simulate_obs(
         args.physgrid_list,
         args.noise_model_list,
-        args.output_catalog,
+        output_catalog=args.output_catalog,
         beastinfo_list=args.beastinfo_list,
         mbmodel=mbmodel,
         nsim=args.nsim,