diff --git a/esbo_etc/classes/SpectralQty.py b/esbo_etc/classes/SpectralQty.py
index 07e8c8d..2102244 100644
--- a/esbo_etc/classes/SpectralQty.py
+++ b/esbo_etc/classes/SpectralQty.py
@@ -110,7 +110,8 @@ class SpectralQty:
             all([math.isclose(x, y, rel_tol=1e-5) for x, y in zip(self.wl.value, other.wl.to(self.wl.unit).value)]) and\
             all([math.isclose(x, y, rel_tol=1e-5) for x, y in zip(self.qty.value, other.qty.to(self.qty.unit).value)])
 
-    def __add__(self, other: Union[int, float, u.Quantity, "SpectralQty", Callable]) -> "SpectralQty":
+    def __add__(self, other: Union[int, float, u.Quantity, "SpectralQty", Callable[[u.Quantity], u.Quantity]]) ->\
+            "SpectralQty":
         """
         Calculate the sum with another object
 
@@ -157,7 +158,8 @@ class SpectralQty:
 
     __radd__ = __add__
 
-    def __sub__(self, other: Union[int, float, u.Quantity, "SpectralQty", Callable]) -> "SpectralQty":
+    def __sub__(self, other: Union[int, float, u.Quantity, "SpectralQty", Callable[[u.Quantity], u.Quantity]]) ->\
+            "SpectralQty":
         """
         Calculate the difference to another object
 
@@ -202,7 +204,8 @@ class SpectralQty:
             else:
                 error("Units are not matching for substraction.")
 
-    def __mul__(self, other: Union[int, float, u.Quantity, "SpectralQty", Callable]) -> "SpectralQty":
+    def __mul__(self, other: Union[int, float, u.Quantity, "SpectralQty", Callable[[u.Quantity], u.Quantity]]) ->\
+            "SpectralQty":
         """
         Calculate the product with another object
 
diff --git a/esbo_etc/classes/config.py b/esbo_etc/classes/config.py
index 7444460..3bdb4e8 100644
--- a/esbo_etc/classes/config.py
+++ b/esbo_etc/classes/config.py
@@ -75,13 +75,13 @@ class Configuration(object):
 
         self.calc_metaoptions()
 
-    def parser(self, parent):
+    def parser(self, parent: eT.Element):
         """
         Parse a XML element tree to an Entry-tree
 
         Parameters
         ----------
-        parent : ElementTree
+        parent : xml.etree.ElementTree.Element
             The parent XML tree to be parsed
 
         Returns
diff --git a/esbo_etc/classes/optical_component/AHotOpticalComponent.py b/esbo_etc/classes/optical_component/AHotOpticalComponent.py
index 033bf3c..f52e294 100644
--- a/esbo_etc/classes/optical_component/AHotOpticalComponent.py
+++ b/esbo_etc/classes/optical_component/AHotOpticalComponent.py
@@ -42,32 +42,32 @@ class AHotOpticalComponent(AOpticalComponent):
         if temp > 0 * u.K:
             # Create noise from black body model
             if isinstance(emissivity, SpectralQty):
-                bb = self._gb_factory(temp)
-                self._noise = SpectralQty(emissivity.wl, bb(emissivity.wl)) * emissivity
+                bb = self.__gb_factory(temp)
+                self.__noise = SpectralQty(emissivity.wl, bb(emissivity.wl)) * emissivity
             elif isinstance(emissivity, str):
                 em = SpectralQty.fromFile(emissivity, u.nm, u.dimensionless_unscaled)
-                bb = self._gb_factory(temp)
-                self._noise = SpectralQty(em.wl, bb(em.wl)) * em
+                bb = self.__gb_factory(temp)
+                self.__noise = SpectralQty(em.wl, bb(em.wl)) * em
             else:
-                bb = self._gb_factory(temp, emissivity)
-                self._noise = bb
+                bb = self.__gb_factory(temp, emissivity)
+                self.__noise = bb
         else:
-            self._noise = 0
+            self.__noise = 0
 
-    def ownNoise(self) -> Union[SpectralQty, Callable, int, float]:
+    def _ownNoise(self) -> Union[SpectralQty, Callable[[u.Quantity], u.Quantity], int, float]:
         """
         Calculate the noise created by the optical component
 
         Returns
         -------
-        noise : Union[SpectralQty, Callable, int, float]
+        noise : Union[SpectralQty, Callable[[u.Quantity], u.Quantity], int, float]
             The noise created by the optical component
         """
-        return self._noise
+        return self.__noise
 
     @staticmethod
     @u.quantity_input(temp=[u.Kelvin, u.Celsius])
-    def _gb_factory(temp: u.Quantity, em: Union[int, float] = 1):
+    def __gb_factory(temp: u.Quantity, em: Union[int, float] = 1):
         """
         Factory for a grey body lambda-function.
 
diff --git a/esbo_etc/classes/optical_component/AOpticalComponent.py b/esbo_etc/classes/optical_component/AOpticalComponent.py
index 9874428..d7f64a5 100644
--- a/esbo_etc/classes/optical_component/AOpticalComponent.py
+++ b/esbo_etc/classes/optical_component/AOpticalComponent.py
@@ -41,14 +41,14 @@ class AOpticalComponent(IRadiant):
         obstructor_emissivity : float
             Emissivity of the obstructing component.
         """
-        self._parent = parent
+        self.__parent = parent
         if transreflectivity:
-            self._transreflectivity = transreflectivity
+            self.__transreflectivity = transreflectivity
         if noise:
-            self._noise = noise
-        self._obstruction = obstruction
-        self._obstructor_temp = obstructor_temp
-        self._obstructor_emissivity = obstructor_emissivity
+            self.__noise = noise
+        self.__obstruction = obstruction
+        self.__obstructor_temp = obstructor_temp
+        self.__obstructor_emissivity = obstructor_emissivity
 
     def calcSignal(self) -> SpectralQty:
         """
@@ -59,7 +59,7 @@ class AOpticalComponent(IRadiant):
         signal : SpectralQty
             The spectral flux density of the target's signal
         """
-        return self.propagate(self._parent.calcSignal()) * (1 - self._obstruction)
+        return self._propagate(self.__parent.calcSignal()) * (1 - self.__obstruction)
 
     def calcNoise(self) -> SpectralQty:
         """
@@ -70,44 +70,44 @@ class AOpticalComponent(IRadiant):
         noise : SpectralQty
             The spectral radiance of the target's noise
         """
-        parent = self.propagate(self._parent.calcNoise())
-        if self._obstructor_temp > 0 * u.K:
-            bb = BlackBody(temperature=self._obstructor_temp, scale=1. * u.W / (u.m ** 2 * u.nm * u.sr))
-            obstructor = bb(parent.wl) * self._obstructor_emissivity
-            noise = parent * (1. - self._obstruction) + obstructor * self._obstruction
+        parent = self._propagate(self.__parent.calcNoise())
+        if self.__obstructor_temp > 0 * u.K:
+            bb = BlackBody(temperature=self.__obstructor_temp, scale=1. * u.W / (u.m ** 2 * u.nm * u.sr))
+            obstructor = bb(parent.wl) * self.__obstructor_emissivity
+            noise = parent * (1. - self.__obstruction) + obstructor * self.__obstruction
         else:
-            noise = parent * (1. - self._obstruction)
-        return noise + self.ownNoise()
+            noise = parent * (1. - self.__obstruction)
+        return noise + self._ownNoise()
 
-    def propagate(self, sqty: SpectralQty) -> SpectralQty:
+    def _propagate(self, rad: SpectralQty) -> SpectralQty:
         """
         Propagate incoming radiation through the optical component
 
         Parameters
         ----------
-        sqty : SpectralQty
+        rad : SpectralQty
             The incoming radiation
 
         Returns
         -------
-        sqty : SpectralQty
+        rad : SpectralQty
             Manipulated incoming radiation
         """
-        if hasattr(self, "_transreflectivity"):
-            return sqty * self._transreflectivity
-        else:
+        try:
+            return rad * self.__transreflectivity
+        except AttributeError:
             error("Transreflectivity not given. Method propagate() needs to be implemented.")
 
-    def ownNoise(self) -> Union[SpectralQty, Callable, int, float]:
+    def _ownNoise(self) -> Union[SpectralQty, Callable[[u.Quantity], u.Quantity], int, float]:
         """
         Calculate the noise created by the optical component
 
         Returns
         -------
-        noise : Union[SpectralQty, Callable, int, float]
+        noise : Union[SpectralQty, Callable[[u.Quantity], u.Quantity], int, float]
             The noise created by the optical component
         """
-        if hasattr(self, "_noise"):
-            return self._noise
-        else:
+        try:
+            return self.__noise
+        except AttributeError:
             error("noise not given. Method ownNoise() needs to be implemented.")
diff --git a/esbo_etc/classes/optical_component/BeamSplitter.py b/esbo_etc/classes/optical_component/BeamSplitter.py
index 4ee9269..bd9df0c 100644
--- a/esbo_etc/classes/optical_component/BeamSplitter.py
+++ b/esbo_etc/classes/optical_component/BeamSplitter.py
@@ -39,18 +39,18 @@ class BeamSplitter(AHotOpticalComponent):
         self._transmittance = SpectralQty.fromFile(transmittance, u.nm, u.dimensionless_unscaled)
         super().__init__(parent, emissivity, temp, obstruction, obstructor_temp, obstructor_emissivity)
 
-    def propagate(self, sqty: SpectralQty) -> SpectralQty:
+    def _propagate(self, rad: SpectralQty) -> SpectralQty:
         """
         Propagate incoming radiation through the optical component
 
         Parameters
         ----------
-        sqty : SpectralQty
+        rad : SpectralQty
             The incoming radiation
 
         Returns
         -------
-        sqty : SpectralQty
+        rad : SpectralQty
             Manipulated incoming radiation
         """
-        return sqty * self._transmittance
+        return rad * self._transmittance
diff --git a/esbo_etc/classes/optical_component/Filter.py b/esbo_etc/classes/optical_component/Filter.py
index 0576b41..925357d 100644
--- a/esbo_etc/classes/optical_component/Filter.py
+++ b/esbo_etc/classes/optical_component/Filter.py
@@ -17,7 +17,7 @@ class Filter(AHotOpticalComponent):
                  H=dict(cwl=1630 * u.nm, bw=400 * u.nm), K=dict(cwl=2190 * u.nm, bw=600 * u.nm))
 
     @u.quantity_input(temp=[u.Kelvin, u.Celsius], obstructor_temp=[u.Kelvin, u.Celsius])
-    def __init__(self, parent: IRadiant, transmittance: Union[SpectralQty, Callable],
+    def __init__(self, parent: IRadiant, transmittance: Union[SpectralQty, Callable[[u.Quantity], u.Quantity]],
                  emissivity: Union[int, float, str] = 1, temp: u.Quantity = 0 * u.K,
                  obstruction: float = 0, obstructor_temp: u.Quantity = 0 * u.K, obstructor_emissivity: float = 1):
         """
@@ -159,10 +159,10 @@ class Filter(AHotOpticalComponent):
         filter : Filter
             The instantiated filter object.
         """
-        return cls(parent, cls._filter_factory(start, end), emissivity, temp,
+        return cls(parent, cls.__filter_factory(start, end), emissivity, temp,
                    obstruction, obstructor_temp, obstructor_emissivity)
 
-    def propagate(self, sqty: SpectralQty) -> SpectralQty:
+    def _propagate(self, sqty: SpectralQty) -> SpectralQty:
         """
         Propagate incoming radiation through the optical component
 
@@ -179,8 +179,8 @@ class Filter(AHotOpticalComponent):
         return sqty * self._transmittance
 
     @staticmethod
-    # @u.quantity_input(start="length", end="length")
-    def _filter_factory(start: u.Quantity, end: u.Quantity):
+    @u.quantity_input(start="length", end="length")
+    def __filter_factory(start: u.Quantity, end: u.Quantity):
         """
         Create a infinite order bandpass filter
 
@@ -193,7 +193,7 @@ class Filter(AHotOpticalComponent):
 
         Returns
         -------
-        lambda : Callable
+        lambda : Callable[[u.Quantity], u.Quantity]
             The filter function
         """
         return lambda wl: 1 * u.dimensionless_unscaled if start <= wl <= end else 0 * u.dimensionless_unscaled
diff --git a/esbo_etc/classes/optical_component/Lens.py b/esbo_etc/classes/optical_component/Lens.py
index e76c688..944072b 100644
--- a/esbo_etc/classes/optical_component/Lens.py
+++ b/esbo_etc/classes/optical_component/Lens.py
@@ -39,18 +39,18 @@ class Lens(AHotOpticalComponent):
         self._transmittance = SpectralQty.fromFile(transmittance, u.nm, u.dimensionless_unscaled)
         super().__init__(parent, emissivity, temp, obstruction, obstructor_temp, obstructor_emissivity)
 
-    def propagate(self, sqty: SpectralQty) -> SpectralQty:
+    def _propagate(self, rad: SpectralQty) -> SpectralQty:
         """
         Propagate incoming radiation through the optical component
 
         Parameters
         ----------
-        sqty : SpectralQty
+        rad : SpectralQty
             The incoming radiation
 
         Returns
         -------
-        sqty : SpectralQty
+        rad : SpectralQty
             Manipulated incoming radiation
         """
-        return sqty * self._transmittance
+        return rad * self._transmittance
diff --git a/esbo_etc/classes/optical_component/Mirror.py b/esbo_etc/classes/optical_component/Mirror.py
index e931635..90d723c 100644
--- a/esbo_etc/classes/optical_component/Mirror.py
+++ b/esbo_etc/classes/optical_component/Mirror.py
@@ -39,18 +39,18 @@ class Mirror(AHotOpticalComponent):
         self._reflectance = SpectralQty.fromFile(reflectance, u.nm, u.dimensionless_unscaled)
         super().__init__(parent, emissivity, temp, obstruction, obstructor_temp, obstructor_emissivity)
 
-    def propagate(self, sqty: SpectralQty) -> SpectralQty:
+    def _propagate(self, rad: SpectralQty) -> SpectralQty:
         """
         Propagate incoming radiation through the optical component
 
         Parameters
         ----------
-        sqty : SpectralQty
+        rad : SpectralQty
             The incoming radiation
 
         Returns
         -------
-        sqty : SpectralQty
+        rad : SpectralQty
             Manipulated incoming radiation
         """
-        return sqty * self._reflectance
+        return rad * self._reflectance
diff --git a/esbo_etc/classes/optical_component/StrayLight.py b/esbo_etc/classes/optical_component/StrayLight.py
index b6cd36b..82a290e 100644
--- a/esbo_etc/classes/optical_component/StrayLight.py
+++ b/esbo_etc/classes/optical_component/StrayLight.py
@@ -9,7 +9,7 @@ class StrayLight(AOpticalComponent):
     A class to model additional stray light sources e.g. zodiacal light
     """
 
-    def __init__(self, parent: IRadiant, emission: str = None):
+    def __init__(self, parent: IRadiant, emission: str):
         """
         Initialize a new stray light source
 
diff --git a/esbo_etc/classes/target/ATarget.py b/esbo_etc/classes/target/ATarget.py
index da2c451..73f0191 100644
--- a/esbo_etc/classes/target/ATarget.py
+++ b/esbo_etc/classes/target/ATarget.py
@@ -20,7 +20,7 @@ class ATarget(IRadiant):
         sfd: SpectralQty
             The spectral flux density of the target
         """
-        self._sfd = sfd
+        self.__sfd = sfd
 
     def calcNoise(self) -> SpectralQty:
         """
@@ -31,7 +31,7 @@ class ATarget(IRadiant):
         noise : SpectralQty
             The spectral radiance of the target's noise
         """
-        return SpectralQty(self._sfd.wl, np.repeat(0, len(self._sfd.wl)) << u.W / (u.m**2 * u.nm * u.sr))
+        return SpectralQty(self.__sfd.wl, np.repeat(0, len(self.__sfd.wl)) << u.W / (u.m**2 * u.nm * u.sr))
 
     def calcSignal(self) -> SpectralQty:
         """
@@ -42,4 +42,4 @@ class ATarget(IRadiant):
         signal : SpectralQty
             The spectral flux density of the target's signal
         """
-        return self._sfd
+        return self.__sfd
diff --git a/tests/optical_component/test_AHotOpticalComponent.py b/tests/optical_component/test_AHotOpticalComponent.py
index 1d45c84..b7e3ec4 100644
--- a/tests/optical_component/test_AHotOpticalComponent.py
+++ b/tests/optical_component/test_AHotOpticalComponent.py
@@ -13,7 +13,7 @@ class HotOpticalComponent(AHotOpticalComponent):
                  obstruction: float = 0, obstructor_temp: u.Quantity = 0 * u.K, obstructor_emissivity: float = 1):
         super().__init__(parent, emissivity, temp, obstruction, obstructor_temp, obstructor_emissivity)
 
-    def propagate(self, sqty: SpectralQty) -> SpectralQty:
+    def _propagate(self, sqty: SpectralQty) -> SpectralQty:
         return sqty