diff --git a/pyproject.toml b/pyproject.toml
index 2bbc7a8..3646bec 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -42,3 +42,7 @@ docstring-code-format = true
 
 [tool.basedpyright]
 typeCheckingMode = "all"
+reportUnknown = false
+reportUnknownMemberType = false
+reportUnknownArgumentType = false
+reportUnknownVariableType = false
diff --git a/src/backscattering_analyzer/__init__.py b/src/backscattering_analyzer/__init__.py
index adb422c..04c6029 100644
--- a/src/backscattering_analyzer/__init__.py
+++ b/src/backscattering_analyzer/__init__.py
@@ -98,4 +98,39 @@ def opt_compute_light(
     )
 
 
+def fit_compute_light(
+    scatter_factor: float,
+    factor_n: Signal,
+    coupling_n: Signal,
+    factor_d: Signal,
+    coupling_d: Signal,
+    power_in: float,
+    power_out: float,
+    data: Signal,
+    reference: Signal,
+) -> float:
+    """
+    Scalar function used to find the right scattering factor
+    """
+    return sum(
+        abs(
+            Signal(
+                factor_n.x,
+                opt_compute_light(
+                    scatter_factor=scatter_factor,
+                    factor_n=factor_n,
+                    coupling_n=coupling_n,
+                    factor_d=factor_d,
+                    coupling_d=coupling_d,
+                    power_in=power_in,
+                    power_out=power_out,
+                ),
+                factor_n.settings,
+            )
+            + reference
+            - data
+        ).y
+    )
+
+
 from backscattering_analyzer.signal import Signal  # no circular import
diff --git a/src/backscattering_analyzer/analyzer.py b/src/backscattering_analyzer/analyzer.py
index c674e35..c345bc7 100644
--- a/src/backscattering_analyzer/analyzer.py
+++ b/src/backscattering_analyzer/analyzer.py
@@ -1,14 +1,15 @@
-# utils
 from sys import argv
 from backscattering_analyzer.settings import Settings
 from backscattering_analyzer.signal import Signal
 from backscattering_analyzer import (
     compute_light,
+    fit_compute_light,
     opt_compute_light,
     interpolate,
 )
-from numpy import loadtxt, logspace, where, zeros, argmin, intp, pi
+from numpy import loadtxt, pi
 from scipy.io.matlab import loadmat
+from scipy.optimize import Bounds, minimize
 
 
 class Analyzer:
@@ -149,24 +150,22 @@ class Analyzer:
             power_out=self.settings.power_out,
         )
 
-    def fit_scatter_factor(
-        self, start: int, stop: int, number: int
-    ) -> tuple[intp, float]:
+    def fit_scatter_factor(self, guess: None | float = None) -> float:
         """
         Find the best scatter factor (first order only) in the given
         range
         """
-        import matplotlib.pyplot as plt
-        factors = logspace(start, stop, number)
-        sums = zeros(number)
-
+        if guess is None:
+            guess = self.settings.scattering_factor[0]
         phase = 4 * pi / self.settings.wavelength
         factor_n = (self.movement * phase).sin().psd().sqrt()
         coupling_n = self.coupling[0].abs()
         factor_d = (self.movement * phase).cos().psd().sqrt()
         coupling_d = self.coupling[1].abs()
 
-        coupling_d.cut(5, 40) # cut signal between 5 and 40 Hz
+        coupling_d = coupling_d.cut_x(
+            10, 200
+        )  # cut signal between 10 and 200 Hz
 
         factor_n, coupling_n, factor_d, coupling_d, data, reference = (
             interpolate(
@@ -181,25 +180,56 @@ class Analyzer:
             )
         )
 
-        reference = reference.y
-        data = data.y
+        bounds = Bounds(0, 1)
+        min_result = minimize(
+            fit_compute_light,
+            guess,
+            (
+                factor_n,
+                coupling_n,
+                factor_d,
+                coupling_d,
+                self.settings.power_in,
+                self.settings.power_out,
+                data,
+                reference,
+            ),
+            method = 'TNC',
+            bounds=bounds,
+        )
 
-        for index in range(number):
-            self.settings.log("{}".format(index))
-            projection = opt_compute_light(
-                scatter_factor=factors[index],
+        if not min_result.success:
+            raise Exception(min_result.message)
+
+        self.settings.log(
+            "found the best scattering factor in {} iterations".format(
+                min_result.nit
+            )
+        )
+
+        import matplotlib.pyplot as plt
+
+        projection = Signal(
+            factor_n.x,
+            opt_compute_light(
+                scatter_factor=min_result.x,
                 factor_n=factor_n,
                 coupling_n=coupling_n,
                 factor_d=factor_d,
                 coupling_d=coupling_d,
                 power_in=self.settings.power_in,
                 power_out=self.settings.power_out,
-            )
-            diff = abs(projection + reference - data)
-            _ = plt.loglog(projection + reference)
-            _ = plt.loglog(data)
-            _ = plt.show()
-            sums[index] = sum(diff)
-        min_index = argmin(sums)
+            ),
+            self.settings,
+        )
 
-        return min_index, factors[min_index]
+        _ = plt.loglog(projection.x, projection.y, label="projection")
+        _ = plt.loglog(reference.x, reference.y, label="référence")
+        _ = plt.loglog(data.x, data.y, label="data")
+        _ = plt.loglog(
+            reference.x, projection.y + reference.y, label="somme"
+        )
+        _ = plt.legend()
+        _ = plt.show()
+
+        return min_result.x
diff --git a/src/backscattering_analyzer/settings.py b/src/backscattering_analyzer/settings.py
index 3797ece..2f591d7 100644
--- a/src/backscattering_analyzer/settings.py
+++ b/src/backscattering_analyzer/settings.py
@@ -14,6 +14,7 @@ class Settings:
                 "option": "grey50 italic",
                 "argument": "red",
                 "description": "green italic",
+                "warning": "bold red",
             }
         )
         self.console = Console(theme=self.theme)
diff --git a/src/backscattering_analyzer/signal.py b/src/backscattering_analyzer/signal.py
index 1829df8..de84f19 100644
--- a/src/backscattering_analyzer/signal.py
+++ b/src/backscattering_analyzer/signal.py
@@ -6,7 +6,7 @@ from backscattering_analyzer import interpolate
 from scipy.signal import welch, detrend
 from scipy.fft import irfft, rfft, rfftfreq
 from scipy.interpolate import CubicSpline
-from numpy import where, sin, cos, array, sqrt, float64
+from numpy import logical_and, where, sin, cos, array, sqrt, float64
 
 
 class Signal:
@@ -21,7 +21,11 @@ class Signal:
         settings: Settings,
     ) -> None:
         if x.shape != value.shape:
-            raise Exception("x and y does not have the same dimension")
+            raise Exception(
+                "x and y does not have the same dimension ({x} and {y})".format(
+                    x=x.shape, y=value.shape
+                )
+            )
         self.sampling = x[1] - x[0]
         self.rate = 1 / self.sampling
         self.x = x
@@ -66,6 +70,17 @@ class Signal:
             self.settings,
         )
 
+    def cut_x(self, start: float, end: float) -> Signal:
+        """
+        Cut signal from a start value to an end value
+        """
+        indexes = where(logical_and(self.x > start, self.x < end))
+        return Signal(
+            self.x[indexes],
+            self.y[indexes],
+            self.settings,
+        )
+
     def low_pass_filter(self, cutoff: float) -> Signal:
         """
         Cut higher frequencies than cutoff for this signal
@@ -76,9 +91,10 @@ class Signal:
         freq_y = rfft(self.y)
         index_to_remove = where(abs(freq_x) > cutoff)
         freq_y[index_to_remove] = 0
+        y = irfft(freq_y)
         signal = Signal(
-            self.x,
-            irfft(freq_y),
+            self.x[: len(y)],
+            y[: len(self.x)],
             self.settings,
         )
         return signal