diff --git a/qpms/constants.py b/qpms/constants.py
index bbdcec6..7874890 100644
--- a/qpms/constants.py
+++ b/qpms/constants.py
@@ -1,7 +1,26 @@
-# unit conversions, mostly for standalone usage
-# TODO avoid importing the "heavy" qpms parts
-from scipy.constants import epsilon_0 as ε_0, c, pi as π, e as eV, hbar, hbar as ℏ, mu_0 as μ_0
-pi = π
+"""
+Constants and unit conversion, mostly for standalode usage.
+
+Previously, QPMS used scipy.constants in the python parts.
+Now the relevant ones are set explicitly here in order
+to avoid dependency on the whole scipy (which needs a fortran
+compiler to build.
+
+The values are taken from scipy / "2018 CODATA recommended values".
+They slightly differ from the constants in GSL that are used
+in the C code. It would be desirable to use the same source,
+hence the values here might be subject to change in future versions.
+"""
+
+epsilon_0 = ε_0 = 8.8541878128e-12  # ± 0.0000000013e-12      F m^-1
+c = speed_of_light = 299792458.
+eV = e = elementary_charge = 1.602176487e-19  # ± 0000000040e-19    C
+hbar = ℏ = 1.054571800e-34  #  ± 0.000000013e-34    J s
+mu_0 = μ_0 = 1.25663706212e-6  #  ± 0.00000000019 e-6     N A^-2
+
+from math import pi
+π = pi
+
 μm = 1e-6
 nm = 1e-9
 # "SCUFF FREQUENCY UNIT"
diff --git a/qpms/cymaterials.pyx b/qpms/cymaterials.pyx
index 1e3915b..330c8ba 100644
--- a/qpms/cymaterials.pyx
+++ b/qpms/cymaterials.pyx
@@ -9,7 +9,7 @@ cimport cython
 import enum
 import warnings
 import os
-from scipy.constants import e as eV, hbar, c
+from .constants import e as eV, hbar, c
 from libc.stdlib cimport malloc, free, calloc, abort
 
 class EpsMuGeneratorType(enum.Enum):
diff --git a/qpms/qpms_p.py b/qpms/qpms_p.py
index f1a0202..0b39711 100644
--- a/qpms/qpms_p.py
+++ b/qpms/qpms_p.py
@@ -6,7 +6,7 @@ import numpy as np
 from .qpms_c import *
 ň = np.newaxis
 import scipy
-from scipy.constants import epsilon_0 as ε_0, c, pi as π, e, hbar as ℏ, mu_0 as μ_0
+from .constants import ε_0, c, pi, π, e, ℏ, μ_0
 eV = e
 from scipy.special import lpmn, lpmv, spherical_jn, spherical_yn, poch, gammaln, factorial
 import math
diff --git a/qpms/tmatrices.py b/qpms/tmatrices.py
index 1c9064c..a74a41f 100644
--- a/qpms/tmatrices.py
+++ b/qpms/tmatrices.py
@@ -12,8 +12,7 @@ except ImportError:
     use_moble_quaternion = False
 
 import re
-from scipy import interpolate
-from scipy.constants import hbar, e as eV, pi, c
+from .constants import hbar, eV, pi, c
 from .cycommon import get_mn_y, get_nelem
 from .cyquaternions import CQuat
 ň = np.newaxis
@@ -440,6 +439,7 @@ class TMatrix(TMatrixSpec):
     TODO support for different/multiple interpolators
     '''
     def __init__(self, tmatrix_spec):
+        from scipy import interpolate
         #self.specification = tmatrix_spec
         self.lMax_override = tmatrix_spec.lMax_override
         self.tmatrix_path = tmatrix_spec.tmatrix_path