diff --git a/focus_app.py b/focus_app.py
index 03ffd36..c2851b7 100644
--- a/focus_app.py
+++ b/focus_app.py
@@ -145,7 +145,7 @@ def focussing_app(parser, init_seq_func, set_focus_func, get_image_func):
             return result["ret_code"]
 
         # compute precise range start and stop values
-        r = (args.num-1) * focus_step / 2.0
+        r = (args.num - 1) * focus_step / 2.0
         focus_start = result["focus_value"] - r
         focus_stop = result["focus_value"] + r
 
@@ -177,16 +177,34 @@ def focussing_app(parser, init_seq_func, set_focus_func, get_image_func):
             **seq_kwds
         )
 
+    if result["ret_code"]:
+        if args.verbose:
+            print(
+                "\tFocussing sequence returned error code: {:g}".format(
+                    result["ret_code"]
+                )
+            )
+            print("\tCannot set best focus value!")
+
+        return result["ret_code"]
+
     if args.verbose:
         print("\tThe best focus value: {:g}".format(result["focus_value"]))
 
     if not args.do_not_set:
         if args.verbose:
             print("\n\tSet focus to the best value ...", end="")
-            set_focus_func(result["focus_value"])
-            print("\tOK")
+
+        ret_code = set_focus_func(result["focus_value"])
+
+        if ret_code:
+            if args.verbose:
+                print("\tFAILED")
+        else:
+            if args.verbose:
+                print("\tOK")
 
     if args.verbose:
         print("DONE.")
 
-    return 0
+    return ret_code
diff --git a/focus_seq_FLI.py b/focus_seq_FLI.py
index c00d86b..5907ac9 100755
--- a/focus_seq_FLI.py
+++ b/focus_seq_FLI.py
@@ -21,7 +21,7 @@ import pathlib as pl
 
 
 def init_seq(seq_kwds):
-    # replace extension as Eddy's 'fli_control' add hardcoded '.fit'
+    # replace extension since Eddy's 'fli_control' adds hardcoded '.fit'
     pt = pl.Path(seq_kwds["root_filename"]).with_suffix(".fit")
     seq_kwds["root_filename"] = str(pt)
 
@@ -47,6 +47,10 @@ def init_seq(seq_kwds):
 
 
 def set_focus(foc_val):
+    if foc_val < 0:
+        print("INVALID FLI-FOCUSER VALUE! IT MUST BE NON-NEGATIVE VALUE!")
+        return -1
+
     cmd = ["fli_control", "-g", str(int(foc_val))]
     ret = sp.run(cmd, stdout=sp.PIPE, stderr=sp.PIPE)
     return ret.returncode
diff --git a/focus_sequence_FLI.py b/focus_sequence_FLI.py
index ec1e34d..dc827ae 100755
--- a/focus_sequence_FLI.py
+++ b/focus_sequence_FLI.py
@@ -20,6 +20,9 @@ import subprocess as sp
 
 
 def set_focus(foc_val):
+    if foc_val < 0:
+        print("INVALID FLI-FOCUSER VALUE! IT MUST BE NON-NEGATIVE VALUE!")
+        return -1
     cmd = ["fli_control", "-g", str(int(foc_val))]
     ret = sp.run(cmd, stdout=sp.PIPE, stderr=sp.PIPE)
     return ret.returncode
@@ -159,8 +162,11 @@ if __name__ == "__main__":
         focus_stop = result["focus_value"] + r
 
         if args.verbose:
-            print("\testimate guess focus position in range [{:g},{:g}]".format(focus_start, focus_stop))
-
+            print(
+                "\testimate guess focus position in range [{:g},{:g}]".format(
+                    focus_start, focus_stop
+                )
+            )
 
     if np.isfinite(args.focus_step):
         result = obsutil.focussingSequence(