fitFocusCurve: fix 'scale' computation for robust fitting

focus_utils.py

@@ -121,11 +121,20 @@ def fitFocusCurve(
             np.inf,
         )  # parabola with minimum (2nd derivative must be positive)
 
-        if coeffs[2] < 0:
-            coeffs[2] = 0
+        if coeffs[2] < 0:  # it seems the non-robust fit failed!
+            coeffs[2] = 0  # so, guess curve is just constant!
+            coeffs[1] = 0
+            coeffs[0] = np.nanmedian(flux_rad)
+            scale = np.nanmedian(np.abs(flux_rad - coeffs[0]))
+        else:
+            nr_model = nr_res(np.asarray(focus_value))
+            scale = np.nanmedian(np.abs(flux_rad - nr_model))
+
+        # if coeffs[2] < 0:
+        #     coeffs[2] = 0
+        # nr_model = nr_res(np.asarray(focus_value))
+        # scale = np.abs(np.nanmedian(flux_rad - nr_model))
 
-        nr_model = nr_res(np.asarray(focus_value))
-        scale = np.abs(np.nanmedian(flux_rad - nr_model))
         # robust fitting
         res = least_squares(
             polyModelResid,