diff --git a/peyote/__init__.py b/peyote/__init__.py
index 42985a99f5c2e06a3073c7657bec70a96fa60d5b..6a38b8f632314188902790423d91b8e9d0a002e1 100644
--- a/peyote/__init__.py
+++ b/peyote/__init__.py
@@ -1,7 +1,4 @@
 from __future__ import print_function, division
-import os
-import numpy as np
-
 
 # import local files
 import detector
diff --git a/peyote/detector.py b/peyote/detector.py
index 621906625f8170767079b4c780e3ead3be4d8646..acba61167560353eab2d904f9d49138ed7ffc9b7 100644
--- a/peyote/detector.py
+++ b/peyote/detector.py
@@ -1,3 +1,6 @@
+import os
+import numpy as np
+
 class Interferometer:
     """Class for the Interferometer """
 
@@ -22,33 +25,39 @@ class Interferometer:
 class PowerSpectralDensity:
 
     def __init__(self):
+        self.frequencies = []
+        self.power_spectral_density = []
+        self.amplitude_spectral_density = []
         return None
 
-    def import_spectral_density_file(self, spectral_density_file='aLIGO_ZERO_DET_high_P_psd.txt'):
+    def import_power_spectral_density(self, spectral_density_file='aLIGO_ZERO_DET_high_P_psd.txt'):
         """
         Automagically load one of the power spectral density or amplitude spectral density
         curves contained in the noise_curves directory
         """
         sd_file = os.path.join(os.path.dirname(__file__), 'noise_curves', spectral_density_file)
         spectral_density = np.genfromtxt(sd_file)
-        return spectral_density
+        self.frequencies = spectral_density[:, 0]
+        self.power_spectral_density = spectral_density[:, 1]
+
+    def import_amplitude_spectral_density(self, spectral_density_file='aLIGO_ZERO_DET_high_P_asd.txt'):
+        """
+        Automagically load one of the p]amplitude spectral density
+        curves contained in the noise_curves directory
+        """
+        sd_file = os.path.join(os.path.dirname(__file__), 'noise_curves', spectral_density_file)
+        spectral_density = np.genfromtxt(sd_file)
+        self.frequencies = spectral_density[:, 0]
+        self.amplitude_spectral_density = spectral_density[:, 1]
 
-    def convert_psd_to_asd(self, power_spectral_density):
+    def convert_psd_to_asd(self):
         """
         Convert a power spectral density to an amplitude spectral spectral_density
-        Return a two-dimensional array of frequency and amplitude spectral density.
         """
-        frequencies = self.power_spectral_density[:, 0]
-        psd = self.power_spectral_density[:, 1]
-        amplitude_spectral_density = np.sqrt(psd)
-        return np.c_[frequencies, amplitude_spectral_density]
+        self.amplitude_spectral_density = np.sqrt(self.power_spectral_density)
 
-    def convert_asd_to_psd(self, amplitude_spectral_density):
+    def convert_asd_to_psd(self):
         """
         Convert an amplitude spectral density to a power spectral density.
-        Return two dimensional array: frequency, power spectral density
         """
-        frequencies = self.amplitude_spectral_density[:, 0]
-        asd = self.amplitude_spectral_density[:, 1]
-        power_spectral_density = asd**2.
-        return np.c_[frequencies, power_spectral_density]
+        self.power_spectral_density = self.amplitude_spectral_density**2